# AC 00-45H CHG 2: Aviation Weather Services - Type: Advisory circular (guidance, not mandatory unless a rule or AD requires it) - Status: Historical - Approved: March 6, 2019 - Family: AC 00-45 - Pages: 348 - FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/DRSDOCID105165973720240913200717.0001 - Page: https://openairdocs.org/ac/00-45 ## Versions - AC 00-45H CHG 2 (Historical), approved Mar 6, 2019 - AC 00-45H CHG 1 (Historical), approved Jan 8, 2018 - AC 00-45H (Historical), approved Nov 14, 2016 - AC 00-45G CHG 2 (Historical), approved Oct 1, 2014 - AC 00-45G CHG 1 (Historical), approved Jul 29, 2010 - AC 00-45G (Historical), approved Feb 11, 2010 - AC 00-45F CHG 2 (Historical), approved Mar 12, 2009 - AC 00-45F CHG 1 (Historical), approved Jul 1, 2008 - AC 00-45F (Historical), approved Oct 1, 2007 - AC 00-45E (Historical), approved Dec 1, 1999 ## Text ### Page 1 U. S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Aviation Weather Services Date: 3/6/19 AC No: 00-45H Initiated by: AFS-400 Change: 2 1 PURPOSE OF THIS ADVISORY CIRCULAR (AC). This AC explains U.S. aviation weather products and services. It provides details when necessary for interpretation and to aid usage. This publication supplements its companion manual, AC 00-6, Aviation Weather, which documents weather theory and its application to aviation. The objective of this AC is to bring the pilot and operator up-to-date on new and evolving weather information and capabilities to help plan a safe and efficient flight, while also describing the traditional weather products that remain. 2 PRINCIPAL CHANGES. This change adds guidance and information on new Flight Information Service-Broadcast (FIS-B) weather products/tables and updates the GS/GR coding elements, the Convective significant meteorological information (SIGMET) description, and changes Helicopter Emergency Management Service (HEMS) to helicopter air ambulance (HAA). ### Page 2 PAGE CONTROL CHART Remove Pages Dated Insert Pages Dated Pages iii thru vi 1/8/18 Pages iii thru vi 3/6/19 Pages viii thru ix 1/8/18 Pages viii thru ix 3/6/19 Page xii 1/8/18 Page xii 3/6/19 Page 1-8 1/8/18 Pages 1-9 thru 1-12 11/14/16 Pages 1-8 thru 1-13 3/6/19 Page 2-2 11/14/16 Page 2-2 3/6/19 Page 3-3 11/14/16 Page 3-3 3/6/19 Pages 3-10 thru 3-12 11/14/16 Pages 3-10 thru 3-12 3/6/19 Page 3-61 1/8/18 Page 3-61 3/6/19 Page 5-7 11/14/16 Page 5-7 3/6/19 Page 5-63 1/8/18 Page 5-63 3/6/19 Page 5-172 11/14/16 Page 5-173 3/6/19 Page 6-1 11/14/16 Page 6-1 3/6/19 Page 6-13 11/14/16 Page 6-13 3/6/19 Page D-1 1/8/18 Page D-1 3/6/19 ORIGINAL SIGNED by /s/ Robert C. Carty Deputy Executive Director, Flight Standards Service ### Page 3 U.S . Department of Transportation Federal Aviation Administration Advisory Circular Subject: Aviation Weather Services Date: 11/14/16 AC No: 00-45H Initiated by: AFS-400 Change: The Federal Aviation Administration (FAA) publishes Advisory Circular (AC) 00-45, Aviation Weather Services. This publication supplements its companion manual, AC 00-6, Aviation Weather, which documents weather theory and its application to aviation. Revision H of AC 00-45 (AC 00-45H) provides an improved organization of aviation weather information. The document is organized using the FAA’s three distinct types of weather information: observations, analyses, and forecasts. Within this construct, AC 00-45H explains U.S. aviation weather products and services. It provides details when necessary for interpretation and to aid usage. In the past decade, access to aviation weather products has greatly improved with the increase of flight planning services and weather websites. The experience of listening to a weather briefing over a phone while trying to write down pertinent weather information becomes less tolerable when the reports are easily obtainable on a home computer, tablet computer, or even a smart phone. To see weather along your route using a graphic of plotted weather reports combined with radar and satellite is preferable to trying to mentally visualize a picture from verbalized reports. Although most of the traditional weather products, which rolled off the teletype and facsimile machines decades ago, are still available, some are being phased out by the National Weather Service (NWS) in favor of new, Web-based weather information. It is the objective of AC 00-45H to bring the pilot and operator up to date on new and evolving weather information and capabilities to help plan a safe and efficient flight, while also describing the traditional weather products that remain. On line aviation weather information is easy to access, and so are references explaining the information. That is why AC 00-45H contains fewer illustrations and less detail for products available online. This AC will give an overview and direct the pilot where to find more weather information and explanatory details. Product examples and explanations are taken primarily from the National Oceanic and Atmospheric Administration (NOAA) NWS Aviation Weather Center’s (AWC) website (https://www.aviationweather.gov) and other pertinent NWS websites. Due to the fluid nature of Web addresses, this AC minimizes the inclusion of website links. Ins tead, it provides the name of the website which can be easily found using Internet search tools. ### Page 4 ii An onli ne version of this document (including digital images) can be found at http://rgl.faa.gov/. AC 00-45H cancels AC 00-45G, Change 2, published October 2014. If you have suggestions for improving this AC, you may use the Advisory Circular Feedback Form at the end of this AC. ### Page 5 iii CONTENTS Paragraph Page Chapter 1. Aviation Weather Service Program … 1-1 1.1 National Oceanic and Atmospheric Administration (NOAA) … 1-1 1.1.1 National Environmental Satellite, Data, and Information Service (NESDIS) … 1-1 1.1.2 National Weather Service (NWS) … 1-1 1.2 Federal Aviation Administration (FAA) … 1-4 1.2.1 Air Traffic Control Systems Command Center (ATCSCC) … 1-4 1.2.2 Air Route Traffic Control Center (ARTCC) … 1-4 1.2.3 Airport Traffic Control Tower (ATCT) and Terminal Radar Approach Control (TRACON) … 1-5 1.2.4 Flight Service Station (FSS) … 1-5 1.3 Dissemination of Aviation Weather Products … 1-5 1.3.1 Weather Briefings … 1-6 1.3.2 Pilot Briefing via the Internet … 1-8 1.3.3 Hazardous In-Flight Weather Advisory Service (HIWAS) … 1-8 1.3.4 Flight Information Service-Broadcast (FIS-B) … 1-9 1.3.5 Operational Use of FIS-B Products … 1-13 Chapter 2. Aviation Weather Product Policy … 2-1 2.1 Use of Aviation Weather Products … 2-1 2.2 Types of Aviation Weather Information … 2-1 2.2.1 Observations … 2-1 2.2.2 Analyses … 2-1 2.2.3 Forecasts … 2-2 2.3 Categorizing Aviation Weather Sources … 2-2 2.3.1 Federal Government … 2-2 2.4 Commercial Weather Information Providers … 2-2 Chapter 3. Observations … 3-1 3.1 Aviation Routine Weather Reports (METAR) and Special Weather Reports (SPECI) … 3-1 3.1.1 Aviation Routine Weather Report (METAR) … 3-1 ### Page 6 iv 3.1.2 Special Weather Report (SPECI) … 3-1 3.1.3 General Types of Observations … 3-3 3.1.4 Recency of Observed Elements at Automated Stations … 3-3 3.1.5 Format … 3-4 3.2 Aircraft Observations and Reports … 3-26 3.2.1 Pilot Weather Report (PIREP) … 3-26 3.2.2 Air-Reports (AIREP) … 3-36 3.3 Radar Observations … 3-37 3.3.1 Description … 3-37 3.3.2 Issuance … 3-37 3.3.3 Modes of Operation … 3-39 3.3.4 Echo Intensities … 3-40 3.3.5 Radar Products … 3-41 3.3.6 Precipitation … 3-48 3.3.7 Limitations … 3-50 3.3.8 Use … 3-56 3.3.9 Future Products … 3-56 3.4 Terminal Doppler Weather Radar (TDWR) … 3-57 3.4.1 History … 3-57 3.4.2 Data Access … 3-58 3.4.3 Products … 3-58 3.4.4 How Does the Radar Collect Data? … 3-59 3.4.5 Volume Coverage Pattern (VCP) 90 … 3-59 3.4.6 Volume Coverage Pattern (VCP) 80 … 3-60 3.4.7 Where Can I Get Details on TDWR-SPG? … 3-60 3.4.8 TDWR Level-II (Base) Data … 3-60 3.4.9 TDWR Level-III Products … 3-60 3.5 Satellite … 3-65 3.5.1 Description … 3-65 3.5.2 Imagery Types … 3-65 3.5.3 Polar Orbiting Environment Satellites (POES) … 3-70 3.6 Upper-Air Observations … 3-70 3.6.1 Radiosonde Observations … 3-70 ### Page 7 v 3.7 Graphical Depiction of Surface Observations … 3-72 3.7.1 AAWU Surface Observations … 3-72 3.7.2 METARs on Aviationweather.gov … 3-77 3.8 Aviation Weather Cameras … 3-82 3.8.1 FAA Aviation Weather Cameras … 3-82 3.8.2 Issuance … 3-82 3.8.3 Use … 3-84 Chapter 4. Analysis … 4-1 4.1 Surface Analysis Charts … 4-1 4.1.1 WPC Surface Analysis Charts … 4-2 4.1.2 Unified Surface Analysis Chart … 4-11 4.1.3 AAWU Surface Chart … 4-12 4.2 Ceiling and Visibility … 4-13 4.2.1 Ceiling and Visibility Analysis (CVA) … 4-13 4.2.2 Weather Depiction Chart … 4-19 4.3 Upper-Air Analyses … 4-20 4.3.1 Issuance … 4-20 4.3.2 Radiosonde Observation Analysis … 4-23 Chapter 5. Forecasts … 5-1 5.1 Significant Meteorological Information (SIGMET) … 5-1 5.1.1 SIGMET Issuance … 5-1 5.1.2 SIGMET Standardization … 5-4 5.1.3 SIGMET (Non-Convective)—CONUS … 5-4 5.1.4 Convective SIGMET … 5-7 5.1.5 SIGMET—Outside the CONUS … 5-11 5.2 Airmen’s Meteorological Information (AIRMET) … 5-16 5.2.1 AIRMET Issuance … 5-17 5.2.2 AIRMET Issuance Criteria … 5-19 5.2.3 AIRMET Standardization … 5-19 5.2.4 AIRMET Bulletins, Issuance Times, and Valid Period … 5-20 5.2.5 AIRMET Format and Example … 5-20 ### Page 8 vi 5.3 Graphical Airman’s Meteorological Advisory (G-AIRMET) … 5-23 5.3.1 G-AIRMET Issuance … 5-24 5.3.2 G-AIRMET Content … 5-24 5.3.3 Use … 5-30 5.4 Center Weather Advisory (CWA) … 5-30 5.4.1 CWA Issuance … 5-30 5.4.2 CWA Criteria … 5-31 5.4.3 CWA Format and Example … 5-32 5.5 Meteorological Impact Statement (MIS) … 5-33 5.5.1 Valid Period … 5-34 5.5.2 Content … 5-34 5.5.3 Format … 5-34 5.6 Additional Products for Convection … 5-35 5.6.1 Convective Outlooks (AC) … 5-35 5.6.2 Watch Notification Messages … 5-38 5.6.3 Traffic Flow Management (TFM) Convective Forecast (TCF) … 5-46 5.6.4 Extended Convective Forecast Product (ECFP) … 5-50 5.6.5 National Convective Weather Forecast (NCWF) … 5-51 5.7 Products for Tropical Cyclones … 5-56 5.7.1 Tropical Cyclone Advisory (TCA) … 5-56 5.8 Volcanic Ash Forecasts … 5-58 5.8.1 Volcanic Ash Advisory Center (VAAC) … 5-58 5.8.2 Volcanic Ash Advisory (VAA) … 5-59 5.9 Graphical Forecast for Aviation (GFA) … 5-61 5.9.1 Time Selection and Slider … 5-62 5.9.2 GFA Static Images … 5-64 5.9.3 Strengths and Limitations (GFA) … 5-66 5.9.4 Recommended Action for Users … 5-67 5.10 Localized Aviation Model Output Statistics (MOS) Program (LAMP) … 5-67 5.10.1 Strengths and Limitations (LAMP) … 5-67 5.10.2 Recommended Action for Users … 5-67 5.11 Area Forecasts (FA) … 5-68 5.11.1 FA—Gulf of Mexico, Caribbean, Hawaii, and Alaska … 5-68 ### Page 9 vii 5.12 Alaskan Graphical Area Forecasts (FA) … 5-78 5.12.1 Issuance … 5-79 5.12.2 Content … 5-80 5.12.3 Use … 5-84 5.13 Terminal Aerodrome Forecast (TAF) … 5-84 5.13.1 Responsibility … 5-84 5.13.2 Generic Format of the Forecast Text of an NWS-Prepared TAF … 5-85 5.13.3 Issuance … 5-105 5.14 Route Forecasts (ROFOR) … 5-107 5.14.1 Issuance … 5-107 5.14.2 ROFOR Amendments … 5-107 5.14.3 ROFOR Corrections … 5-107 5.14.4 ROFOR Content … 5-108 5.14.5 Example … 5-108 5.15 Wind and Temperature Aloft … 5-109 5.15.1 Wind and Temperature Aloft Forecast (FB) … 5-109 5.16 Freezing-Level Graphics … 5-113 5.16.1 Issuance … 5-114 5.16.2 Format … 5-114 5.16.3 Use … 5-114 5.17 Upper-Air Forecasts … 5-115 5.17.1 Constant Pressure Levels Forecasts … 5-116 5.18 Short-Range Surface Prognostic (Prog) Charts … 5-118 5.18.1 Content … 5-119 5.18.2 Issuance … 5-121 5.18.3 Use … 5-121 5.19 Significant Weather (SIGWX) Forecast … 5-121 5.19.1 Low-Level Significant Weather (SIGWX) Charts … 5-121 5.19.2 Mid-Level Significant Weather (SIGWX) Chart … 5-127 5.19.3 High-Level Significant Weather (SIGWX) Charts … 5-129 5.19.4 Alaska Significant Weather (SIGWX) Charts … 5-135 5.20 World Area Forecast System (WAFS) … 5-137 5.20.1 WAFS Forecasts … 5-137 ### Page 10 viii 5.21 Additional Products for Icing and Turbulence … 5-146 5.21.1 Current and Forecast Icing Products (CIP/FIP) … 5-146 5.21.2 Graphical Turbulence Guidance (GTG) … 5-155 5.22 Additional Products for Clouds, Visibility, Weather, and Surface Wind Forecasts … 5-160 5.22.1 NWS Aviation Forecast Discussion … 5-160 5.22.2 NWS National Digital Forecast Database Graphic Products … 5-162 5.22.3 Cloud Top Heights Product … 5-173 5.22.4 Cloud Layer Product … 5-175 5.23 Airport Weather Warning (AWW) … 5-175 5.23.1 Issuance … 5-175 5.23.2 Airport Weather Warning (AWW) Format and Example … 5-176 5.23.3 Use … 5-176 5.24 Space Weather … 5-176 5.24.1 NOAA Space Weather Scales … 5-176 5.24.2 Space Weather Products … 5-177 5.24.3 Use … 5-182 5.25 Soaring Forecast … 5-183 5.25.1 Issuance … 5-183 5.25.2 Content and Format … 5-183 5.25.3 Example … 5-183 Chapter 6. Aviation Weather Tools … 6-1 6.1 Helicopter Emergency Medical Services (HEMS) Tool … 6-1 6.1.1 Availability … 6-1 6.1.2 Content … 6-1 6.1.3 Strengths and Limitations … 6-13 6.1.4 Use … 6-14 6.2 Flight Path Tool … 6-14 6.2.1 Description … 6-14 Appendix A. Definition of Common Terms Used in En Route Forecasts and Advisories … A-1 Appendix B. Standard Conversion Chart … B-1 Appendix C. Density Altitude Calculation … C-1 ### Page 11 ix Appendix D. Internet Links … D-1 Appendix E. WSR-88D Weather Radar Network … E-1 L ist of Figures Figure 3-1. METAR/SPECI Coding Format … 3-4 Figure 3-2. ICAO Continental Codes … 3-5 Figure 3-3. Obscuration Effects on Slant Range Visibility .................................................................................... 3- 14 Figure 3-4. METAR/SPECI Sky Condition Coding ............................................................................................... 3- 16 Figure 3-5. Pilot Weather Report (PIREP) Coding Format .................................................................................... 3- 27 Figure 3-6. Location of WSR-88D Weather Radar in the CONUS and Their Respective Coverage at 4,000 Feet AGL, 6,000 Feet AGL, and 10,000 Feet AGL ................................................................... 3- 38 Figure 3-7. Location of WSR-88D Weather Radar Outside of the CONUS .......................................................... 3- 39 Figure 3-8. WSR-88D (NEXRAD) Weather Radar Echo Intensity Legend........................................................... 3- 40 Figure 3-9. NWS National Radar Mosaic Example, Which Utilizes NEXRAD Base Reflectivity … 3-42 Figure 3-10. NWS Regional Radar Mosaic Sector Example, Which Utilizes NEXRAD Base Reflectivity … 3-43 Figure 3-11. Alaska Radar Mosaic Example .......................................................................................................... 3- 43 Figure 3-12. WSR-88D Weather Radar Composite Reflectivity, Single-Site Product Example … 3-44 Figure 3-13. Creation of a Composite Reflectivity, Single-Site Product ................................................................ 3- 45 Figure 3-14. Weather Radar 0.5º Base Reflectivity (left) versus Composite Reflectivity (right) Comparison … 3-45 Figure 3-15. WSR-88D Weather Radar Short-Range (124 NM) Base Reflectivity, Single-Site Product Example ............................................................................................................................................. 3- 46 Figure 3-16. WSR-88D Weather Radar Long-Range (248 NM) Base Reflectivity, Single-Site Product Example ............................................................................................................................................. 3- 47 Figure 3-17. Radar Coded Message (RCM) Display .............................................................................................. 3- 48 Figure 3-18. WSR-88D Weather Radar Convective Precipitation on the 0.5º Base Reflectivity Product Example ............................................................................................................................................. 3- 49 Figure 3-19. WSR-88D Weather Radar Stratiform Precipitation on the 0.5º Base Reflectivity Product Example ............................................................................................................................................. 3- 50 Figure 3-20. Cone of Silence .................................................................................................................................. 3- 51 Figure 3-21. WSR-88D Weather Radar Beam Blockage on Base Reflectivity Product Example … 3-52 Figure 3-22. WSR-88D Weather Radar Ground Clutter Example .......................................................................... 3- 52 Figure 3-23. WSR-88D Weather Radar Ghost Example ........................................................................................ 3- 53 Figure 3-24. WSR-88D Weather Radar Angel Example ........................................................................................ 3- 54 Figure 3-25. WSR-88D Weather Radar Anomalous Propagation (AP) Example … 3-55 Figure 3-26. Wind Farms Have Been Known to Make Benign Echoes Appear as Small Storms … 3-56 Figure 3-27. WSR-88D vs. TDWR ......................................................................................................................... 3- 63 Figure 3-28. TDWR Site Locations ........................................................................................................................ 3- 64 Figure 3-29. Thunderstorm Producing a Microburst .............................................................................................. 3- 64 Figure 3-30. Visible Satellite Image—U.S. Example ............................................................................................. 3- 66 Figure 3-31. Visible Satellite Data Legend ............................................................................................................. 3- 66 Figure 3-32. Infrared (Color) Satellite Image—U.S. Example ............................................................................... 3- 67 Figure 3-33. Unenhanced Infrared (Black and White) Satellite Image—U.S. Example … 3-68 Figure 3-34. Infrared (Color) Satellite Image Data Legend .................................................................................... 3- 68 Figure 3-35. Unenhanced Infrared (Black and White) Satellite Image Data Legend … 3-68 Figure 3-36. W ater Vapor Satellite Image—U.S. Example .................................................................................... 3- 69 Figure 3-37. Water Vapor Satellite Image Data Legend ......................................................................................... 3- 70 Figure 3-38. U.S. Radiosonde Network .................................................................................................................. 3- 71 Figure 3-39. AAWU Surface Observations Example ............................................................................................. 3- 72 Figure 3-40. AAWU Surface Observation Plot Legend ......................................................................................... 3- 73 Figure 3-41. Surface Observations—Aviationweather.gov METAR Interactive Display Example … 3-77 Figure 3-42. Interactive Display Station Plot Example .......................................................................................... 3- 78 Figure 3-43. Station Plot Sky Symbols ................................................................................................................... 3- 80 Figure 3-44. Surface Observations—Aviationweather.gov METAR Static Display Example … 3-81 Figure 3-45. Static Display Station Plot Example .................................................................................................. 3- 81 ### Page 12 x Figure 3-46. Map of FAA’s Aviation Weather Camera Network in Alaska........................................................... 3- 83 Figure 3-47. Sectional Chart Depicting Shungnak (SHG), Alaska Camera Orientations … 3-83 Figure 3-48. “Clear Day” Image from Aviation Weather Camera Image at Shungnak (SHG), Alaska … 3-84 Figure 3-49. Aviation Weather Camera Image at Shungnak (SHG), Alaska .......................................................... 3- 84 Figure 4-1. NWS Surface Analysis Chart Example … 4-1 Figure 4-2. NWS Surface Analysis and Radar Composite Example … 4-2 Figure 4-3. NWS Surface Analysis Chart Symbols … 4-3 Figure 4-4. NWS Surface Analysis Chart Station Plot Model … 4-4 Figure 4-5. NWS Surface Analysis Chart Ship/Buoy Plot Model … 4-4 Figure 4-6. NWS Surface Analysis Chart for Aviation Interests Station Plot Model … 4-5 Figure 4-7. NWS Surface Analysis Chart Common Weather Symbols … 4-6 Figure 4-8. NWS Surface Analysis Chart Wind Plotting Model … 4-7 Figure 4-9. NWS Surface Analysis Chart Pressure Trends … 4-8 Figure 4-10. NWS Surface Analysis Chart Sky Cover Symbols … 4-9 Figure 4-11. Unified Surface Analysis Chart Example .......................................................................................... 4- 11 Figure 4-12. Unified Surface Analysis Chart Example .......................................................................................... 4- 12 Figure 4-13. Unified Surface Analysis Chart Example With Fixed Area Coverage Over Alaska … 4-13 Figure 4-14. CONUS Display of CVA Ceiling Analysis........................................................................................ 4- 15 Figure 4-15. Regional Display of CVA Ceiling Analysis ....................................................................................... 4- 16 Figure 4-16. Regional Display of CVA Visibility Analysis ................................................................................... 4- 17 Figure 4-17. Regional Display of CVA Flight Category Analysis ......................................................................... 4- 18 Figure 4-18. Weather Depiction Chart—Example.................................................................................................. 4- 20 Figure 4-19. NWS NCEP Central Operations Model Analyses and Guidance Website … 4-21 Figure 4-20. 500 MB Analysis Chart—Example .................................................................................................... 4- 22 Figure 4-21. 500 MB Height, Temperature, and Wind, 00-hour Forecast (i.e., Model’s Analyses from the NWS’s North American Model (NAM) Example) ............................................................................ 4- 22 Figure 4-22. Skew-T Diagram—Example .............................................................................................................. 4- 24 Figure 4-23. Skew-T Diagram—Multiple Freezing Level Example ...................................................................... 4- 25 Figure 4-24. Skew-T Diagram—Cloud Top Example ............................................................................................ 4- 25 Figure 5-1. AWC SIGMET Areas of Responsibility—Continental U.S … 5-2 Figure 5-2. AWC SIGMET Areas of Responsibility—Atlantic Basin … 5-2 Figure 5-3. SIGMET Areas of Responsibility—Pacific Basin … 5-3 Figure 5-4. SIGMET for the Continental U.S.—Example … 5-5 Figure 5-5. AWC Convective SIGMET Areas of Responsibility … 5-8 Figure 5-6. Convective SIGMET Decoding—Example … 5-9 Figure 5-7. SIGMET Outside the Continental U.S. Decoding Example ................................................................ 5- 13 Figure 5-8. AWC AIRMET Areas of Responsibility—Continental U.S. ............................................................... 5- 17 Figure 5-9. AAWU Flight Advisory and Area Forecast (FA) Zones—Alaska … 5-17 Figure 5-10. WFO Honolulu AIRMET Areas of Responsibility—Hawaii … 5-18 Figure 5-11. AIRMET Bulletin Decoding—Example ............................................................................................ 5- 21 Figure 5-12. G- AIRMET—Ceiling and Visibility (IFR) Snapshot Example … 5-25 Figure 5-13. G- AIRMET—Mountain Obscuration Snapshot Example .................................................................. 5- 25 Figure 5-14. G- AIRMET—Icing Snapshot Example ............................................................................................. 5- 26 Figure 5-15. G- AIRMET—Freezing Level Snapshot Example .............................................................................. 5- 27 Figure 5-16. G- AIRMET—Turbulence-High Snapshot Example .......................................................................... 5- 28 Figure 5-17. G- AIRMET—Turbulence-Low Snapshot Example ........................................................................... 5- 28 Figure 5-18. G- AIRMET—Low-Level Wind Shear (LLWS) Snapshot Example … 5-29 Figure 5-19. G- AIRMET—Strong Surface Winds Snapshot Example .................................................................. 5- 30 Figure 5-20. Center Weather Service Unit (CWSU) Areas of Responsibility … 5-31 Figure 5-21. Center Weather Advisory (CWA) Decoding Example ...................................................................... 5- 32 Figure 5-22. Day 1 Categorical Convective Outlook Graphic Example ................................................................. 5- 36 Figure 5-23. Categorical Outlook Legend for Days 1-3 Convective Outlook Graphic Example … 5-36 Figure 5-24. Aviation Watch (polygon) Compared to Public Watch (shaded) Example … 5-39 Figure 5-25. TFM Convective Forecast (TCF)—Example ..................................................................................... 5- 47 Figure 5-26. TFM Convective Forecast (TCF)—Example ..................................................................................... 5- 48 Figure 5-27. TCF Product Legend .......................................................................................................................... 5- 48 ### Page 13 xi Figure 5-28. ECFP Example ................................................................................................................................... 5- 50 Figure 5-29. NCWF Example ................................................................................................................................. 5- 52 Figure 5-30. NCWF Hazard Scale .......................................................................................................................... 5- 53 Figure 5-31. NCWF One-Hour Extrapolated Forecast Polygon, Forecast Movement Velocity and Echo Tops—Example.................................................................................................................................. 5- 54 Figure 5-32. NCWF Past Performance Polygons—Example ................................................................................. 5- 55 Figure 5-33. Volcanic Ash Advisory Centers (VAACs)......................................................................................... 5- 59 Figure 5-34. Volcanic Ash Advisory in Graphical Format ..................................................................................... 5- 61 Figure 5-35. Aviation Forecast for Clouds—Example ........................................................................................... 5- 62 Figure 5-36. Time Selection and Slider .................................................................................................................. 5- 62 Figure 5-37. Forecast Weather Products Tabs ........................................................................................................ 5- 63 Figure 5-38. Observations/Warnings Tab ............................................................................................................... 5- 64 Figure 5-39. GFA Static Cloud Forecast ................................................................................................................ 5- 65 Figure 5-40. GFA Surface Forecast ....................................................................................................................... 5- 66 Figure 5-41. AWC Area Forecast (FA) Region and WMO Header—Gulf of Mexico … 5-69 Figure 5-42. AWC Area Forecast (FA) Region and WMO Header—Caribbean … 5-70 Figure 5-43. WFO Honolulu Area Forecast (FA) Region and WMO Header—Hawaii … 5-70 Figure 5-44. AAWU Flight Advisory and Area Forecast Zones—Alaska ............................................................. 5- 71 Figure 5-45. Alaskan Graphical Area Forecast (FA)—Flying Weather Example … 5-80 Figure 5-46. Alaskan Graphical Area Forecast (FA)—Surface Chart Example … 5-81 Figure 5-47. Alaskan Graphical Area Forecast (FA)—Icing Forecast Example … 5-82 Figure 5-48. Alaskan Graphical Area Forecast (FA)—Turbulence Forecast Example … 5-83 Figure 5-49. Alaskan Graphical Area Forecast (FA)—Convective Outlook .......................................................... 5- 84 Figure 5-50. Microburst Effect on Landing Aircraft .............................................................................................. 5- 95 Figure 5-51. FB Wind and Temperature Aloft Interactive Display Example ....................................................... 5- 110 Figure 5-52. Aviationweather.gov Freezing Level Graphic—Example ............................................................... 5- 115 Figure 5-53. NWS NCEP Central Operations Model Analyses and Guidance Website … 5-116 Figure 5-54. 300 MB Constant Pressure Forecast—Example .............................................................................. 5- 117 Figure 5-55. NDFD Surface Prog Forecast—Example ......................................................................................... 5- 119 Figure 5-56. Surface Prog Forecast Symbols ........................................................................................................ 5- 120 Figure 5-57. 12-Hour Low-Level SIGWX Chart—Example ................................................................................ 5- 122 Figure 5-58. Low-Level SIGWX Chart Symbols ................................................................................................. 5- 123 Figure 5-59. Low-Level SIGWX Chart Flying Categories—Example ................................................................. 5- 123 Figure 5-60. Low-Level SIGWX Chart Turbulence Forecast—Example............................................................. 5- 124 Figure 5-61. Low-Level SIGWX Chart Freezing Level Forecast—Example ....................................................... 5- 125 Figure 5-62. Low-Level SIGWX Chart Multiple Freezing Levels—Example … 5-126 Figure 5-63. Mid-Level SIGWX Chart—Example ............................................................................................... 5- 127 Figure 5-64. High-Level SIGWX Chart—Example ............................................................................................. 5- 130 Figure 5-65. High-Level SIGWX Chart Turbulence—Examples ......................................................................... 5- 132 Figure 5-66. High-Level SIGWX Chart Jet Stream—Example ............................................................................ 5- 133 Figure 5-67. High-Level SIGWX Chart Tropopause Height—Examples ............................................................ 5- 133 Figure 5-68. High-Level SIGWX Chart Tropical Cyclone—Examples ............................................................... 5- 134 Figure 5-69. High-Level SIGWX Chart Volcanic Eruption Site—Example ........................................................ 5- 134 Figure 5-70. High-Level SIGWX Chart Widespread Sandstorm and Duststorm—Example … 5-134 Figure 5-71. Alaska Significant Weather Chart—Example .................................................................................. 5- 136 Figure 5-72. WAFS Wind and Temperature 6-Hour Forecast at FL390—Example … 5-139 Figure 5-73. WAFS CAT Forecast—Example ..................................................................................................... 5- 142 Figure 5-74. WAFS Turbulence Forecast—Example ........................................................................................... 5- 143 Figure 5-75. WAFS Icing Forecast—Example ..................................................................................................... 5- 144 Figure 5-76. WAFS CB Cloud Forecast—Example ............................................................................................. 5- 145 Figure 5-77. C IP/FIP Icing Probability (3,000 Feet MSL)—Example ................................................................. 5- 148 Figure 5-78. CIP/FIP Icing Probability—Max Example ...................................................................................... 5- 149 Figure 5-79. CIP/FIP Icing Severity (3,000 Feet MSL)—Example...................................................................... 5- 150 Figure 5-80. CIP/FIP Icing Severity—Max Example ........................................................................................... 5- 151 Figure 5-81. CIP/FIP Icing Severity Plus Supercooled Large Drops (SLD)—Example … 5-153 Figure 5-82. CIP/FIP Icing Severity Plus Supercooled Large Droplets (SLD)—Max Example … 5-154 ### Page 14 xii Figure 5-83. GTG—Max Intensity Example CAT+MWT All Levels .................................................................. 5- 157 Figure 5-84. CAT+MWT Low Levels Example (GA Users) ............................................................................... 5- 158 Figure 5-85. Explicit MWT Forecasts Example ................................................................................................... 5- 159 Figure 5-86. Map of NWS Weather Forecast Office’s Area of Responsibility … 5-161 Figure 5-87. NDFD Sky Cover Graphic Product—Example ................................................................................ 5- 164 Figure 5-88. NDFD Sky Cover Graphic Product—Example ................................................................................ 5- 165 Figure 5-89. NDFD Weather Graphic Product—Example ................................................................................... 5- 166 Figure 5-90. NDFD Weather Graphic Product—Example ................................................................................... 5- 167 Figure 5-91. Legend for NDFD Weather Graphic ................................................................................................ 5- 168 Figure 5-92. NDFD Weather Graphic Product—Example ................................................................................... 5- 169 Figure 5-93. NDFD Weather Graphic Product—Example ................................................................................... 5- 170 Figure 5-94. NDFD Weather Graphic Product—Example ................................................................................... 5- 170 Figure 5-95. NDFD Surface Wind Speed and Direction—Example .................................................................... 5- 171 Figure 5-96. NDFD Wind Speed and Direction Graphic Product Plot Model ...................................................... 5- 172 Figure 5-97. NDFD Surface Wind Gust Graphic—Example ............................................................................... 5- 172 Figure 5-98. Cloud Top Heights Product—Example ............................................................................................ 5- 174 Figure 6-1. HEMS Ceiling with Flight Category Overlay—Example … 6-3 Figure 6-2. HEMS Visibility with Flight Category Overlay—Example … 6-3 Figure 6-3. HEMS Flight Category with Flight Category Overlay—Example … 6-4 Figure 6-4. HEMS Radar Display—Example … 6-5 Figure 6-5. HEMS Satellite Imagery—Example … 6-6 Figure 6-6. HEMS Icing Severity—Example … 6-7 Figure 6-7. HEMS Icing Probability—Example … 6-7 Figure 6-8. HEMS Temperature—Example … 6-8 Figure 6-9. HEMS Relative Humidity—Example … 6-9 Figure 6-10. HEMS Wind Speed with Windbarb—Example … 6-9 Figure 6-11. HEMS Data Overlays with Text METAR—Example ....................................................................... 6- 12 Figure 6-12. Configure Menu—Example ............................................................................................................... 6- 13 Figure 6-13. Flight Path Tool—Icing Severity Example (Image Credit: AWC) … 6-15 Figure 6-14. Flight Path Tool—Windbarbs Example (Image Credit: AWC) ......................................................... 6- 16 Figure 6-15. Flight Path Tool—Cross Section Example (Image Credit: AWC) … 6-17 List of Tables Table 1-1. FIS-B Over UAT Product Update and Transmission Intervals.............................................................. 1- 11 Table 1-2. Product Parameters for Low/Medium/High Altitude Tier Radios ......................................................... 1- 12 Table 3-1. SPECI Criteria … 3-2 Table 3-2. METAR/SPECI Notations for Reporting Present Weather1 .................................................................. 3- 10 Table 3-3. METAR/SPECI Contractions for Sky Cover ......................................................................................... 3- 15 Table 3-4. METAR/SPECI Increments of Reportable Values of Sky Cover Height … 3-15 Table 3-5. METAR/SPECI Order of Remarks ........................................................................................................ 3- 18 Table 3-6. METAR/SPECI Type and Frequency of Lightning ............................................................................... 3- 21 Table 3-7. WSR-88D Weather Radar Precipitation Intensity Terminology ............................................................ 3- 41 Table 3-8. Technical Specifications: WSR-88D vs. TDWR ................................................................................... 3- 59 Table 3-9. AAWU Surface Observations Flying Categories and Criteria ............................................................... 3- 73 Table 3-10. AAWU Surface Observation Precipitation Symbols ........................................................................... 3- 75 Table 3-11. AAWU Surface Observation Obstruction to Visibility Symbols … 3-76 Table 3-12. AAWU Surface Observation Precipitation Intensity Symbols ............................................................ 3- 76 Table 3-13. Rounding of Actual METAR Visibility Values for Depiction on Aviationweather.gov/AWC METAR Display .................................................................................................................................. 3- 79 Table 3-14. Station Plot Weather Categories .......................................................................................................... 3- 80 Table 5-1. Decoding a SIGMET (Non-Convective) for the Continental U.S … 5-6 Table 5-2. Decoding a Convective SIGMET .......................................................................................................... 5- 10 Table 5-3. Decoding a SIGMET Outside of the Continental U.S. .......................................................................... 5- 13 Table 5-4. AAWU Flight Advisory and Area Forecast (FA) Zones—Alaska … 5-18 Table 5-5. AIRMET Issuance Schedule .................................................................................................................. 5- 20 ### Page 15 xiii Table 5-6. D ecoding an AIRMET Bulletin ............................................................................................................. 5- 22 Table 5-7. G- AIRMET Issuance Schedule.............................................................................................................. 5- 24 Table 5-8. Decoding a Center Weather Advisory (CWA) ...................................................................................... 5- 33 Table 5-9. Convective Outlook Issuance Schedule ................................................................................................. 5- 35 Table 5-10. Decoding an Aviation Weather Watch Notification Message ............................................................. 5- 41 Table 5-11. Weather Elements on Forecast Product Tabs ....................................................................................... 5- 63 Table 5-12. Products Displayed on Observations/Warnings Weather Products Tab … 5-64 Table 5-13. AAWU Area Forecast (FA) Zones—Alaska ....................................................................................... 5- 71 Table 5-14. Area Forecast (FA) Issuance Schedule ................................................................................................ 5- 72 Table 5-15. Generic Format of NWS TAFs ............................................................................................................ 5- 85 Table 5-16. TAF Fog Terms ................................................................................................................................... 5- 92 Table 5-17. TAF Use of Vicinity (VC) ................................................................................................................... 5- 93 Table 5-18. TAF Sky Cover .................................................................................................................................... 5- 93 Table 5-19. TAF Issuance Schedule ...................................................................................................................... 5- 105 Table 5-20. Wind and Temperature Aloft Forecast Decoding Examples .............................................................. 5- 113 Table 5-21. Wind and Temperature Aloft Forecast (FB) Periods ......................................................................... 5- 113 Table 5-22. Select Constant Pressure Levels From the GFS Model ..................................................................... 5- 118 Table 5-23. Short-Range Surface Prog Forecast Schedule .................................................................................... 5- 121 Table 5-24. Low-Level SIGWX Chart Issuance Schedule .................................................................................... 5- 126 Table 5-25. Icing and Turbulence Intensity Symbols ............................................................................................ 5- 128 Table 5-26. Mid-Level SIGWX Chart Issuance Schedule .................................................................................... 5- 129 Table 5-27. High-Level SIGWX FORECAST Issuance Schedule ....................................................................... 5- 135 Table 5-28. WAFS Clear Air Turbulence Forecasts ............................................................................................. 5- 141 Table 5-29. WAFS In-Cloud Turbulence Forecasts .............................................................................................. 5- 143 Table 5-30. WAFS Global Icing Forecasts ........................................................................................................... 5- 144 Table 6-1. Flight Categories … 6-2 ### Page 16 1-1 CHAPTER 1. AVIATION WEATHER SERVICE PROGRAM The aviation weather service program is a joint effort of the National Weather Service (NWS), the Federal Aviation Administration (FAA), the Department of Defense (DOD), and other aviation-oriented groups and individuals. This chapter discusses the civilian agencies of the U.S. Government and their observation, communication, and forecast services to the aviation community. 1.1 National Oceanic and Atmospheric Administration (NOAA). NOAA is an agency of the Department of Commerce (DOC). NOAA conducts research and gathers data about the global oceans, atmosphere, space, and sun, and applies this knowledge to science and service, which touches the lives of all Americans. Among its six major divisions are the National Environmental Satellite, Data, and Information Service (NESDIS) and the NWS. 1.1.1 National Environmental Satellite, Data, and Information Service (NESDIS). The NESDIS manages the U.S. civil operational remote-sensing satellite systems, as well as other global information for meteorology, oceanography, solid-earth geophysics, and solar-terrestrial sciences. NESDIS provides this data to NWS meteorologists and a wide range of other users for operational weather forecasting. 1.1.1.1 Satellite Analysis Branch (SAB). NESDIS’ SAB serves as the operational focal point for real-time imagery products and multi-disciplinary environmental analyses. The SAB’s primary mission is to support disaster mitigation and warning services for U.S. Federal agencies and the international community. Routine environmental analyses are provided to forecasters and other environmental users, and are used in the numerical models of the NWS. The SAB schedules and distributes real-time satellite imagery products from global geostationary and polar orbiting satellites to environmental users. The SAB coordinates the satellite and other information for the NOAA Volcanic Hazards Alert Program, under an agreement with the FAA, and works with the NWS as part of the Washington, D.C. Volcanic Ash Advisory Center (VAAC). The Washington, D.C. VAAC Area of Responsibility (AOR) includes the continental United States (CONUS), the Gulf of Mexico, the Oakland Flight Information Region (FIR), and the New York FIR. 1.1.2 National Weather Service (NWS). NWS provides weather data, forecasts, and warnings for the United States, its territories, adjacent waters, and ocean areas for the protection of life and property and the enhancement of the national economy. NWS data and products form a national information database and infrastructure that can be used by other government agencies, the private sector, the public, and the global community. The following is a description of NWS offices associated with aviation weather: 1.1.2.1 National Centers for Environmental Prediction (NCEP). NCEP is where virtually all global meteorological data is collected and ### Page 17 1-2 analyzed. NCEP then provides a wide variety of national and international weather guidance products to NWS field offices, government agencies, emergency managers, private sector meteorologists, and meteorological organizations and societies throughout the world. NCEP is a critical resource in national and global weather prediction and is the starting point for nearly all weather forecasts in the United States. NCEP is comprised of nine distinct centers and the Office of the Director. Each center has its own specific mission. The following NCEP centers provide aviation weather products and services: 1.1.2.1.1 NCEP Central Operations (NCO). The NCO in College Park, MD, sustains and executes the operational suite of the numerical analysis and forecast models and prepares NCEP products for dissemination. It also links all nine of the national centers together via computer and communications-related services. 1.1.2.1.2 Aviation Weather Center (AWC). The AWC in Kansas City, MO, issues a suite of aviation weather forecasts in support of the National Airspace System (NAS) including: Airman’s Meteorological Information (AIRMET), significant meteorological information (SIGMET), Convective SIGMETs, Area Forecasts (FA), Significant Weather Prognostic Charts (low, middle, and high), National Convective Weather Forecast (NCWF), Current Icing Product (CIP), Forecast Icing Product (FIP), Graphical Turbulence Guidance (GTG), and Ceiling and Visibility Analysis (CVA) product. The AWC is a Meteorological Watch Office (MWO) for the International Civil Aviation Organization (ICAO). The website for the AWC is https://www.aviationweather.gov. The AWC’s website provides the aviation community w ith textual, digital, and graphical forecasts, analyses, and observations of aviation-related weather variables. Additionally, the website provides information for international flights through the World Area Forecast System (WAFS) Internet File Service (WIFS). The AWC’s website also provides a flight path tool that allows the user to view data along a specific route of flight. Using the flight path tool, a user can view icing, turbulence, temperature, winds, humidity, AIRMETs/SIGMETs, Aviation Routine Weather Report (METAR)/Special Weather Report (SPECI), Terminal Aerodrome Forecast (TAF), etc. both horizontally and vertically. The flight path tool also allows many overlay options, including Air Route Traffic Control Center (ARTCC) boundaries, counties, highways, and rivers. Product animation is also possible on the AWC JavaScript image. 1.1.2.1.3 Weather Prediction Center (WPC). The WPC in College Park, MD, provides analysis and forecast products specializing in multi-day, quantitative precipitation forecasts and weather ### Page 18 1-3 forecast guidance, weather model diagnostics discussions, and surface pressure and frontal analyses. 1.1.2.1.4 Storm Prediction Center (SPC). The SPC in Norman, OK, provides tornado and severe weather watches for the CONUS along with a suite of hazardous weather forecasts. 1.1.2.1.5 National Hurricane Center (NHC). The NHC in Miami, FL, provides official NWS forecasts of the movement and strength of tropical weather systems and issues the appropriate watches and warnings for the CONUS and surrounding areas. It also issues a suite of marine products covering the tropical Atlantic, Caribbean, Gulf of Mexico, and tropical eastern Pacific. In support of ICAO/World Meteorological Organization (WMO), the NHC is also referred to as the Tropical Cyclone Advisory Center (TCAC). 1.1.2.1.6 Space Weather Prediction Center (SWPC). The SWPC in Boulder, CO, provides space weather information (e.g., current activity and forecasts) to a wide variety of users. SWPC issues alerts, watches, and warnings for space weather events affecting, or expected to affect, Earth’s environment. 1.1.2.2 Alaska Aviation Weather Unit (AAWU). The AAWU, located in Anchorage, AK, is an MWO for ICAO. The AAWU is responsible for the entire Anchorage FIR. They issue a suite of aviation weather products for the airspace over Alaska and adjacent coastal waters, including: AIRMETs, SIGMETs, FAs, Graphic FAs, and Significant Weather Prognostic Charts. The AAWU is also designated as the Anchorage VAAC. The VAAC AOR includes the Anchorage FIR and Far Eastern Russia and is responsible for the issuance of Volcanic Ash Advisories (VAA). 1.1.2.3 Center Weather Service Unit (CWSU). CWSUs are units of NWS meteorologists under contract with the FAA that are stationed at, and support, the FAA’s ARTCC. CWSUs provide timely weather consultation, forecasts, and advice to managers within ARTCCs and to other supported FAA facilities. This information is based on monitoring, analysis, and interpretation of real-time weather data at the ARTCC through the use of all available data sources including radar, satellite, Pilot Weather Reports (PIREP), and various NWS products, such as TAFs and inflight advisories. Special emphasis is given to those weather conditions that are hazardous to aviation or which could impede the flow of air traffic within the NAS. CWSU meteorologists issue the following products in support of their respective ### Page 19 1-4 ARTCC: Center Weather Advisories (CWA) and Meteorological Impact Statements (MIS). 1.1.2.4 Weather Forecast Office (WFO). An NWS WFO is a multi-purpose, local weather forecast center that produces, among its suite of services, aviation-related products. In support of aviation, WFOs issue TAFs, with some offices issuing Airport Weather Warnings and Soaring Forecasts. The Honolulu WFO is unique among NWS WFOs in that it provides multiple services beyond the typical WFO. WFO Honolulu is also designated as an MWO for ICAO. As a result of this unique designation, WFO Honolulu is the only WFO to issue the following text products: AIRMETs, SIGMETs, and Route Forecasts (ROFOR). WFO Honolulu is co-located with the Central Pacific Hurricane Center (CPHC). CPHC provides official NWS forecast of the movement and strength of tropical weather systems and issues the appropriate watches and warnings for the central Pacific, including the state of Hawaii. WFO Honolulu also issues a suite of marine products covering a large portion of the Pacific Ocean. In support of ICAO/WMO, the NHC is also referred to as the TCAC. 1.2 Federal Aviation Administration (FAA). The FAA, a part of the Department of Transportation (DOT), provides a safe, secure, and efficient airspace system that contributes to national security and the promotion of U.S. aerospace safety. As the leading authority in the international aerospace community, the FAA is responsive to the dynamic nature of user needs, economic conditions, and environmental concerns. The FAA provides a wide range of services to the aviation community. The following is a description of those FAA facilities that are involved with aviation weather and pilot services: 1.2.1 Air Traffic Control Systems Command Center (ATCSCC). The ATCSCC is located in Vint Hill, VA. The ATCSCC has the mission of balancing air traffic demand with system capacity. This ensures maximum safety and efficiency for the NAS, while minimizing delays. The ATCSCC utilizes the Traffic Management System (TMS), aircraft situation display, monitor alert, the follow on functions, and direct contact with ARTCC, and Terminal Radar Approach Control (TRACON) facility Traffic Management Units (TMU) to manage flow on a national level. Because weather is the most common reason for air traffic delays and re-routings, NWS meteorologists support the ATCSCC. These meteorologists, called National Aviation Meteorologists (NAM), coordinate NWS operations in support of traffic flow management within the NAS. 1.2.2 Air Route Traffic Control Center (ARTCC). An ARTCC is a facility established to provide air traffic control (ATC) service to aircraft ### Page 20 1-5 operating on instrument flight rules (IFR) flight plans within controlled airspace, principally during the en route phase of flight. When equipment capabilities and controller workload permit, certain advisory/assistance services may be provided to visual flight rules (VFR) aircraft. En route controllers become familiar with pertinent weather information and stay aware of current weather information needed to perform ATC duties. En route controllers advise pilots of hazardous weather that may impact operations within 150 nautical miles (NM) of the controller’s assigned sector(s), and may solicit PIREPs from pilots. 1.2.3 Airport Traffic Control Tower (ATCT) and Terminal Radar Approach Control (TRACON). An ATCT is a terminal facility that uses air/ground communications, visual signaling, and other devices to provide ATC services to aircraft operating in the vicinity of an airport or on the movement area. Terminal controllers become familiar with pertinent weather information and stay aware of current weather information needed to perform ATC duties. Terminal controllers advise pilots of hazardous weather that may impact operations within 150 NM of the controller’s assigned sector or area of jurisdiction and may solicit PIREPs from pilots. ATCTs and TRACONs may opt to broadcast hazardous weather information alerts only when any part of the area described is within 50 NM of the airspace under the ATCT’s jurisdiction. The tower controllers are also properly certified and act as official weather observers, as required. An automated terminal information service (ATIS) is a continuous broadcast of recorded information in selected terminal areas. Its purpose is to improve controller effectiveness and to relieve frequency congestion by automating the repetitive transmission of non-controlled airport/terminal area and meteorological information. 1.2.4 Flight Service Station (FSS). FSSs provide pilot weather briefings, en route weather, receive and process IFR and VFR flight plans, solicit and disseminate pilot reports and urgent pilot reports, relay ATC clearances, and issue Notices to Airmen (NOTAM). They also provide assistance to lost aircraft and aircraft in emergency situations, as well as conduct VFR search and rescue services. 1.3 Dissemination of Aviation Weather Products. The ultimate users of aviation weather services are pilots, aircraft dispatchers, and air traffic management (ATM) and air traffic controllers. Maintenance personnel may use the service to keep informed of weather that could cause possible damage to unprotected aircraft. Pilots contribute to and use aviation weather services. PIREPs help other pilots, dispatchers, briefers, and forecasters as an observation of current conditions. ### Page 21 1-6 In the interest of safety and in compliance with Title 14 of the Code of Federal Regulations (14 CFR), all pilots should get a complete weather briefing before each flight. The pilot is responsible for ensuring he or she has all information needed to make a safe flight. 1.3.1 Weather Briefings. Prior to every flight, pilots should gather all information vital to the nature of the flight. This includes a weather briefing obtained by the pilot from an approved weather source, via the Internet, and/or from an FSS specialist. The FSS’ purpose is to serve the aviation community. Pilots should not hesitate to ask questions and discuss factors they do not fully understand. The briefing should be considered complete only when the pilot has a clear picture of what weather to expect. Pilots should also make a final weather check immediately before departure, when possible. To provide an appropriate weather briefing, specialists need to know which of the three types of briefings is needed—standard, abbreviated, or outlook. Other necessary information includes whether the flight will be conducted with VFR or IFR, aircraft identification and type, departure point, estimated time of departure, flight altitude, route of flight, destination, and estimated time en route. If the briefing updates previously received information, the time of the last briefing is also important. This allows the briefer to provide only pertinent data. The briefer enters this information into the FAA’s flight plan system. The briefer also notes the type of weather briefing provided. If necessary, the information can be referenced later to file or amend a flight plan. It is also used when an aircraft is overdue or is reported missing. Internet data is time-stamped and archived for 15 days. Voice recordings are retained for 45 days. 1.3.1.1 Standard Briefing. A standard briefing provides a complete weather picture and is the most detailed of all briefings. This type of briefing should be obtained prior to the departure of any flight and should be used during flight planning. A standard briefing provides the following information (if applicable to the route of flight) in sequential order: • Adverse Conditions. This includes information about adverse conditions that may influence a decision to cancel or alter the route of flight. Adverse conditions include significant weather (e.g., thunderstorms, aircraft icing, turbulence, windshear, mountain obscuration, and areas of current and forecasted IFR conditions) and other important items, such as airport/runway closings, air traffic delays, and temporary flight restrictions (TFR). • VFR Flight NOT RECOMMENDED (VNR). If the weather for the route of flight is below VFR minimums, or if it is doubtful the flight can be made under VFR conditions due to the forecasted weather, the briefer ### Page 22 1-7 may state that VFR is not recommended. The pilot can then decide whether or not to continue the flight under VFR, but this advisory should be weighed carefully. This advisory is not provided via the Internet. • Synopsis. The synopsis is an overview of the larger weather picture. Fronts and major weather systems along or near the route of flight and weather that may affect the flight are provided. • Current Conditions. This portion of the briefing contains the current surface weather observations, PIREPs, and satellite and radar data along the route of flight. If the departure time is more than 2 hours away, current conditions will not be included in the briefing. • En Route Forecast. The en route forecast is a summary of the weather forecast for the proposed route of flight. • Destination Forecast. The destination forecast is a summary of the expected weather for the destination airport at the estimated time of arrival (ETA). • Winds and Temperatures Aloft. Winds and temperatures aloft is a forecast of the winds at specific altitudes along the route of flight. However, the temperature information is provided only on request. • NOTAMs. This portion supplies NOTAM information that has not been published in the NOTAM publication, but is pertinent to the route of flight. Published NOTAM information is provided during the briefing only on request. • Prohibited Areas and Special Flight Rules Areas (SFRA). Information on Prohibited Areas P-40 and P-56, and the SFRA for Washington, D.C. are given when appropriate to the route of flight. • ATC Delays. This is an advisory of any known ATC delays that may affect the flight. • Other Information. Any additional information requested is also provided at this time. 1.3.1.2 Abbreviated Briefing. An abbreviated briefing is a shortened version of the standard briefing. It should be requested when a departure has been delayed or when specific weather information is needed to update a previous standard briefing. When this is the case, the weather specialist needs to know the time and source of the previous briefing so he or she does not inadvertently omit the necessary weather information. 1.3.1.3 Outlook Briefing. An outlook briefing should be requested when a planned departure is 6 or more hours away. It provides initial forecast information that is limited in scope due to the timeframe of the planned flight. This type of briefing is a ### Page 23 1-8 good source of flight planning information that can influence decisions regarding route of flight, altitude, and ultimately the “go, no-go” decision. A follow-up standard briefing prior to departure is advisable, since an outlook briefing generally only contains information based on weather trends and existing weather in geographical areas at or near the departure airport. 1.3.2 Pilot Briefing via the Internet. 1.3.2.1 Aviation Digital Data Service (ADDS). ADDS is a joint effort of the FAA, NOAA, and the National Center for Atmospheric Research (NCAR). ADDS provides text, digital, and graphical forecasts, analyses, and observations of aviation-related weather variables. 1.3.3 Hazardous In-Flight Weather Advisory Service (HIWAS). HIWAS is a national program for broadcasting hazardous weather information continuously over selected Navigational Aids (NAVAID). The broadcasts include advisories such as AIRMETs, SIGMETS, Convective SIGMETs, and urgent PIREPs. These broadcasts are only a summary of the information, and pilots should contact an FSS for detailed information. The HIWAS broadcast area is defined as the area within 150 NM of HIWAS outlets. HIWAS broadcasts are not interrupted or delayed, except for emergency situations, when an aircraft requires immediate attention, or for reasonable use of the voice override capability on specific HIWAS outlets in order to use the limited remote communications outlet (RCO) to maintain en route communications. The service is provided 24 hours a day. An announcement is made for no hazardous weather advisories. Hazardous weather information is recorded if it is occurring within the HIWAS broadcast area. The broadcast includes the following elements: • A statement of introduction including the appropriate area(s) and a recording time. • A summary of Convective SIGMETs, SIGMETs, AIRMETs, Urgent PIREPs, Aviation Watch Notification Messages (SAW), Center Weather Advisories, and any other weather, such as isolated thunderstorms that are rapidly developing and increasing in intensity, or low ceilings and visibilities that are becoming widespread, which are considered significant and are not included in a current hazardous weather advisory. • A request for PIREPs, if applicable. • A recommendation to contact FSS for additional details concerning hazardous weather. Once the HIWAS broadcast is updated, an announcement will be made once on all communications/NAVAID frequencies, except emergency and navigational frequencies already dedicated to continuous broadcast services. In the event a HIWAS broadcast area is out of service, an announcement is made on all communications/NAVAID frequencies, ### Page 24 1-9 except on emergency and navigational frequencies already dedicated to continuous broadcast services. 1.3.4 Flight Information Service-Broadcast (FIS-B). FIS-B over universal access transceiver (UAT) data-link service provides Aeronautical Information (AI) to the flightdeck for aircraft operating in the NAS. These products are broadcast over the Automatic Dependent Surveillance-Broadcast (ADS-B) UAT link so pilots have timely information of regional weather and NAS status/changes that might affect flight. It is critical that pilots understand that FIS-B meteorological information and AI products provide strategic information to the flight deck that enhances a preflight briefing. FIS-B AI products do not include all NOTAMs. FIS-B NOTAM information is limited to the past 30 days. As a result, FIS-B AI may not include all NOTAMs that a preflight briefing includes. The pilot in command (PIC) is responsible for reviewing all necessary information prior to flight. Therefore, AI information obtained via FIS-B may not be relied upon for a thorough preflight briefing. For additional information on standard briefing, refer to the Aeronautical Information Manual (AIM), Chapter 7, Safety of Flight, Subparagraph 7- 1-4b, Weather Products. The following list represents the initial suite of textual and graphical products available through FIS-B and provided free-of-charge. This advisory circular (AC) and AC 00-63, Use of Cockpit Displays of Digital Weather and Aeronautical Information, contain det ailed information concerning FIS-B meteorological products. AIM Chapter 3, Airspace; Chapter 4, Air Traffic Control; and Chapter 5, Air Traffic Procedures contain information on Special Use Airspace (SUA), TFR, and NOTAM products. • Text: METAR and SPECI; • Text: PIREP; • Text: Winds and Temperatures Aloft; • Text: TAF; • Text: NOTAM Distant and Flight Data Center; • Text/Graphic: AIRMET; • Text/Graphic: SIGMET; • Text/Graphic: Convective SIGMET; • Text/Graphic: SUA; • Text/Graphic: TFR NOTAM; • Graphic: Next generation weather radar (NEXRAD) Composite Reflectivity Products (Regional and National); • Lightning; • Turbulence; • Cloud Tops; ### Page 25 1-10 • Icing; • Graphical AIRMETs (G-AIRMET); and • CWA. Users of FIS-B should familiarize themselves with the operational characteristics and limitations of the system, including: system architecture, service environment, product lifecycles, modes of operation, and indications of system failure. Up date intervals are defined as the rate at which the product data is available from the source for transmission. Transmission intervals are defined as the amount of time within which a new or updated product transmission must be completed and/or the rate or repetition interval at which the product is rebroadcast. Table 1-1, FIS-B Over UAT Product Update and Transmission Intervals, provides update and transmission intervals for each product. Where applicable, FIS-B products include a look-ahead range expressed in nautical miles for three service domains: Airport Surface, Terminal Airspace, and En route/Gulf of Mexico. Table 1-2, Product Parameters for Low/Medium/High Altitude Tier Radios, provides service domain availability and look-ahe ad ranging for each FIS-B product. Prior to using this capability, users should familiarize themselves with the operation of FIS-B avionics by referencing the applicable user’s guides. Users should obtain guidance concerning the interpretation of information displayed from the appropriate avionics manufacturer. Us ers should report FIS-B malfunctions not attributed to aircraft system failures or covered by active NOTAM via the ADS-B/Traffic Information Services-Broadcast (TIS-B)/FIS-B Problem Report on the following website: http://www.faa.gov/exit/?pageName=this%20form&pgLnk=http%3A%2F%2Fgoo%2Egl %2Fforms%2FisWDKYpYYv. Users may also report malfunctions by submitting FAA Form 8740-5, Safety Improvement Report, via mail, fax, or email to your local F light District Standards Office (FSDO) Safety Program Manager (SPM). ### Page 26 1-11 Table 1-1. FIS-B Over UAT Product Update and Transmission Intervals Product FIS-B Over UAT Service Update Interval1 FIS-B Service Transmission Interval2 AIRMET As available 5 minutes Convective SIGMET As available 5 minutes METARs/SPECIs 1 minute/as available 5 minutes NEXRAD Composite Reflectivity (CONUS) 15 minutes 15 minutes NEXRAD Composite Reflectivity (Regional) 5 minutes 2.5 minutes NOTAMs–D/FDC/TFR As available 10 minutes PIREP As available 10 minutes SIGMET As available 5 minutes Special Use Airspace Status As available 10 minutes TAF/AMEND 6 hours/as available 10 minutes Temperature Aloft 12 hours 10 minutes Winds Aloft 12 hours 10 minutes Lightning Strikes 5 minutes 5 minutes Turbulence 15 minutes 15 minutes Icing Forecasts 15 minutes 15 minutes Cloud Tops 15 minutes 15 minutes Graphical AIRMETS 00Z,03Z, 06Z, 09Z/12 hr forecasts 3 hour Center Weather Advisories As available As available 1. The Update Interval is the rate at which the product data is available from the source. 2. The Transmission Interval is the amount of time within which a new or updated product transmission must be completed and the rate or repetition interval at which the product is rebroadcast. 3. Notice to Airmen-Distant (NOTAM-D) and Notice to Airmen-Flight Data Center (NOTAM-FDC) products broadcast via FIS-B are limited to those issued or effective within the past 30 days. ### Page 27 1-12 Table 1-2. Product Parameters for Low/Medium/High Altitude Tier Radios Product Surface Radios Low Altitude Tier Medium Altitude Tier High Altitude Tier CONUS NEXRAD N/A CONUS NEXRAD not provided CONUS NEXRAD imagery CONUS NEXRAD imagery Winds & Temps Aloft 500 NM look-ahead range 500 NM look-ahead range 750 NM look-ahead range 1,000 NM look-ahead range METAR 100 NM look-ahead range 250 NM look-ahead range 375 NM look-ahead range CONUS: CONUS Class B & C airport METARs and 500 NM look-ahead range Outside of CONUS: 500 NM look-ahead range TAF 100 NM look-ahead range 250 NM look-ahead range 375 NM look-ahead range CONUS: CONUS Class B & C airport TAFs and 500 NM look-ahead range Outside of CONUS: 500 NM look-ahead range AIRMET, SIGMET, PIREP and Special Use Airspace/SAA 100 NM look-ahead range. PIREP/SUA is N/A 250 NM look-ahead Range 375 NM look-ahead range 500 NM look-ahead range Regional NEXRAD 150 NM look-ahead range 150 NM look-ahead range 200 NM look-ahead range 250 NM look-ahead range NOTAMs–D/FDC/TFR 100 NM look-ahead range 100 NM look-ahead range 100 NM look-ahead range 100 NM look-ahead range Lightning Strikes N/A 150 NM look-ahead range 200 NM look-ahead range 250 NM look-ahead range Turbulence N/A 150 NM look-ahead range 200 NM look-ahead range 250 NM look-ahead range Icing Forecasts N/A 150 NM look-ahead range 200 NM look-ahead range 250 NM look-ahead range Cloud Tops N/A 150 NM look-ahead range 200 NM look-ahead range 250 NM look-ahead range Graphical AIRMETS N/A 250 NM look-ahead range 375 NM look-ahead range 500 NM look-ahead range Center Weather Advisories N/A 250 NM look-ahead range 375 NM look-ahead range 500 NM look-ahead range Users should obtain guidance concerning the content, format, and symbology of individual FIS-B products from the manufacturer of the avionics equipment used to receive and display them. ### Page 28 1-13 1.3.5 Operational Use of FIS-B Products. FIS-B information may be used by the pilot for the safe conduct of flight and aircraft movement. However, FIS-B does not replace a thorough preflight briefing that may be from one or several Meteorological Information and AI sources. Pilots are encouraged, but not required, to use the dispatch/System Operations Control (SOC) (if applicable), or access an FSS via telephone. Title 14 CFR part 91, § 91.103 requires the pilot in command (PIC) to be familiar with all available information concerning that flight. A pil ot should be particularly alert and understand the limitations and quality assurance (QA) issues associated with individual products. This includes graphical representation of NEXRAD imagery and NOTAM/TFR. ### Page 29 2-1 CHAPTER 2. AVIATION WEATHER PRODUCT POLICY The demand for new and improved aviation weather products continues to grow, and with new products introduced to meet the demand, some confusion has resulted in the aviation community regarding the relationship between regulatory requirements and new weather products. This chapter will clarify that relationship by providing: • Policy guidance for using aviation weather products, • Descriptions of the types of aviation weather information, and • Categorization of the sources of aviation weather information. 2.1 Use of Aviation Weather Products. This AC describes the weather products distributed by the NWS. Pilots and operators using the Internet to access weather from a third-party vendor should request and/or review an appropriate description of services and provider disclosure. This should include, but is not limited to, the type of weather product (i.e., current weather or forecasted weather), the currency of the product (i.e., product issue and valid times) and the relevance of the product. Pilots and operators should be cautious when using unfamiliar weather products. When in doubt, consult with a Flight Service Specialist. Note that the FAA does not approve or qualify Internet providers of aviation weather service. The development of new weather products, coupled with the termination of some legacy textual and graphical products, may create confusion between regulatory requirements and the new products. All flight-related aviation weather decisions must be based on all available pertinent weather products. As every flight is unique and the weather conditions for that flight vary hour-by-hour, day-to-day, multiple weather products may be necessary to meet aviation weather regulatory requirements. Many new weather products have a precautionary use statement displayed that details the proper use or application of the specific product. 2.2 Types of Aviation Weather Information. The FAA has identified the following three distinct types of weather information that may be needed to conduct aircraft operations: observations, analyses, and forecasts. 2.2.1 Observations. Observations are raw weather data collected by sensor(s). The observations can either be in situ (i.e., surface or airborne) or remote (i.e., weather radar, satellite, profiler, and lightning). 2.2.2 Analyses. Analyses of weather information are an enhanced depiction and/or interpretation of observed weather data. Examples of these types of analyses can be seen in paragraph 4.1.1. Another type of analysis is the representation of an atmospheric variable (e.g., temperature, ceiling height, and visibility) derived from a finite set of irregularly ### Page 30 2-2 dis tributed observations onto a regular grid. See the figures in paragraph 4.2 for examples. 2.2.3 Forecasts. Forecasts are the predictions of the development and/or movement of weather phenomena based on meteorological observations and various mathematical models. In-flight weather advisories, including Terminal Aerodrome Forecast (TAF), significant meteorological information (SIGMET), Convective SIGMETs, Airman’s Meteorological Information (AIRMET), Center Weather Advisories (CWA), and Meteorological Impact Statements (MIS) are considered forecast weather information products. 2.3 Categorizing Aviation Weather Sources. The regulations pertaining to aviation weather reflect that, historically, the Federal Government was the only source of aviation weather information. That is, the FAA and NWS, or their predecessor organizations, were solely responsible for the collection and dissemination of weather data, including forecasts. Thus, the term “approved source(s)” referred exclusively to the Federal Government. The Federal Government is no longer the only source of weather information, due to the growing sophistication of aviation operations and scientific and technological advances. Si nce all three types of weather information defined in paragraph 2.2 are not available from all sources of aviation weather information, the FAA has categorized the sources as fol lows: Federal Government and commercial weather information providers. 2.3.1 Federal Government. The FAA and NWS collect weather observations. The NWS analyzes the observations and produces forecasts, including in-flight aviation weather advisories (e.g., SIGMETs). The FAA and NWS disseminate meteorological observations, analyses, and forecast products through a variety of systems. The Federal Government is the only approval authority for sources of weather observations (e.g., contract towers and airport operators). Commercial weather information providers contracted by the FAA to provide weather observations (e.g., contract towers and Lockheed Martin) are included in the Federal Government category of approved sources by virtue of maintaining required technical and quality assurance standards under FAA and NWS oversight. 2.4 Commercial Weather Information Providers. Commercial weather information providers are a major source of weather products for the aviation community. In general, they produce proprietary weather products based on NWS information with formatting and layout modifications, but no material changes to the weather information itself. This is also referred to as “repackaging.” In other cases, commercial providers produce forecasts, analyses, and other proprietary weather products which may substantially differ from the information contained in NWS-produced products. Operators who desire to use products prepared by a commercial weather provider, as opposed to using products that are simply repackaged, may require FAA approval. This approval is granted under the provisions in Operations Specification ### Page 31 2-3 (OpSpec) paragraph A010. Please provide which services and products you are contemplating using, to include the appropriate description of the service. This should include, but is not limited to: • The type of weather product (e.g., current weather or forecast weather); • The currency of the product (i.e., product issue and valid times); and • The relevance of the product. Pilots and operators should be cautious when using unfamiliar products, or products not supported by FAA/NWS technical specifications. ### Page 32 3-1 CHAPTER 3. OBSERVATIONS The first of three distinct types of weather (meteorological) information is observations. Observations are weather data collected by one or more sensors, and are the basic information upon which forecasts and advisories are made in support of a wide range of weather-sensitive activities within the public and private sectors, including aviation. Chapter 3 describes and discusses surface observations, aircraft observations, radar observations, and satellite observations (e.g., imagery). 3.1 Aviation Routine Weather Reports (METAR) and Special Weather Reports (SPECI). Surface weather observations are fundamental to all meteorological services. Aviators typically view surface observations through METARs and SPECIs. Although the METAR/SPECI code is used worldwide, there are some code differences among countries. Each country is allowed to make modifications to the code for use in their particular country, as long as they notify the International Civil Aviation Organization (ICAO). These paragraphs will focus on the METAR/SPECI code as used in the United States. 3.1.1 Aviation Routine Weather Report (METAR). The METAR report has been adopted by the United States to provide surface observations in support of aviation for the terminal. A METAR report includes the airport identifier, time of observation, wind, visibility, Runway Visual Range (RVR), present weather phenomena, sky conditions, temperature, dewpoint, and altimeter setting. Excluding the airport identifier and the time of observation, this information is collectively referred to as the “body” of the report. Coded and/or plain language information elaborating on data in the body may be appended to the end of the METAR as “remarks.” The contents of the remarks section varies with the type of reporting station. At some designated stations, the METAR may be abridged to include only a few of the mentioned elements. 3.1.2 Special Weather Report (SPECI). A SPECI is an unscheduled report taken when any of the criteria given in Table 3-1, SPECI Criteria, are observed during the period between hourly reports. SPECIs contain al l data elements found in a METAR. All SPECIs are issued as soon as possible when relevant criteria are observed. Whenever SPECI criteria are met at the time of the routine METAR, a METAR is issued. ### Page 33 3-2 Table 3-1. SPECI Criteria 1 Wind Shift Wind direction changes by 45 degrees or more, in less than 15 minutes and the wind speed is 10 knots or more throughout the wind shift. 2 Visibility Surface visibility, as reported in the body of the report, decreases to less than, or if below, increases to equal to or exceeding: • miles • mile s • 1 mile • The lowest standard instrument approach procedure minimum as published in the Nat ional Ocean Service (NOS) U.S. Instrument Procedures. If none published, use ½ mile 3 Runway Visual Range (RVR) The highest value from the designated RVR runway decreases to less than, or if below, increases to equal to or exceeding 2,400 feet during the preceding 10 minutes. U.S. military stations may not report a SPECI based on RVR. 4 Tornado, Funnel Cloud, or Waterspout • Is observed • Disappears from sight, or ends 5 Thunderstorm • Begins (a SPECI is not required to report the beginning of a new thunderstorm if one is currently reported) • Ends 6 Precipitation • Hail begins or ends • Freezing precipitation begins, ends or changes intensity • Ice pellets begin, end or change intensity 7 Squalls When a squall occurs. 8 Ceiling The ceiling (rounded to reportable values) forms or dissipates below, decreases to less than, or if below, increases to equal to or exceeding: • 3,000 feet • 1,5 00 feet • 1,000 feet • 500 feet • The lowest standard instrument approach procedure minimum as published in the Nat ional Ocean Service (NOS) U.S. Instrument Procedures. If none published, use 200 feet. 9 Sky Condition A layer of clouds or obscurations aloft is present below 1,000 feet and no layer aloft was reported below 1,000 feet in the preceding METAR or SPECI. 10 Volcanic Eruption When an eruption is first noted. 11 Aircraft Mishap Upon notification of an aircraft mishap, unless there has been an intervening observation. 12 Miscellaneous Any other meteorological situation designated by the responsible agency of which, in the opinion of the observer, is critical. The information in Table 3-1 can be found in the Federal Meteorological Handbook No. 1 (FMH-1), Section 2.5.2, Aviation Selected Special Weather Report (SPECI). ### Page 34 3-3 3.1.3 General Types of Observations. There are three general types of surface observations: • Manual Observation. Weather observations done by a human weather observer who is certified by the FAA. • A utomated Observation. Automated observations are derived from instruments and algorithms without human input or oversight. In the United States, there are two main kinds of automated observing systems: the automated surface observing system (ASOS) and the Automated Weather Observing System (AWOS). Detailed information on ASOS and AWOS can be found in the Aeronautical Information Manual (AIM), Chapter 7, Safety of Flight, Section 1, Meteorology. Automated METARs and SPE CIs contain AUTO in the report (see paragraph 3.1.5.4). • Augmented Observation. At select airports in the United States, the automated observing system will have input and oversight by human weather observers or tower controllers certified in weather observing. These are referred to as augmented stations. Human observers report weather elements that are beyond the capabilities of the automated system and/or are deemed operationally significant. The weather elements observed and reported by the human observer vary, depending on the selected airport. AUTO is not used in augmented reports. Whereas manual stations were most common prior to the mid-1990s in the United States, the vast majority of today’s METARs and SPECIs are from fully-automated stations. 3.1.4 Recency of Observed Elements at Automated Stations. For those elements that the human observer evaluates using spatial averaging techniques (e.g., sky cover and visibility), the automated station substitutes time averaging of sensor data. Therefore, in an automated observation, sky condition is an evaluation of sensor data gathered during the 30-minute period ending at the actual time of the observation. All other elements are based on sensor data that is within 10 minutes or less of the actual time of the observation. ### Page 35 3-4 3.1.5 Format. Figure 3-1. METAR/SPECI Coding Format A U.S. METAR/SPECI has two major sections: the body (consisting of a maximum of 11 groups) and the remarks (consisting of 2 categories). When an element does not occur, or cannot be observed, the corresponding group is omitted from that particular report. 3.1.5.1 Type of Report. METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 The type of report, METAR or SPECI, precedes the body of all reports, but may not be shown or displayed on all aviation weather websites. 3.1.5.2 Station Identifier. METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 The station identifier, in ICAO format, is included in all reports to identify the station to which the coded report applies. The ICAO airport code is a four-letter alphanumeric code designating each airport around the world. The ICAO codes are used for flight planning by pilots and airline operation departments. These codes are not the same as the International Air Transport Association (IATA) codes encountered by the general public used for reservations, baggage handling, and in airline timetables. ### Page 36 3-5 Unlike the IATA codes, the ICAO codes have a regional structure. The first letter identifies the region and country (see Figure 3-2, ICAO Continental Codes). In some regions, the second letter identifies the country. ICAO station identifiers in Alaska begin with PA, Hawaii begins with PH, Guam begins with PG, and Puerto Rico begins with TS. For example, the San Juan Puerto Rico IATA identifier “SJU” becomes the ICAO identifier “TSJU.” The remaining letters are used to identify each airport. Figure 3-2. ICAO Continental Codes In the continental United States (CONUS), ICAO station identifiers are coded K, followed by the three-letter IATA identifier. For example, the Seattle, WA IATA identifier SEA becomes the ICAO identifier KSEA. ICAO station identifiers in Alaska, Hawaii, and Guam begin with the continent code P. For a list of all U.S. identifiers, refer to FAA Order 7350.9, Location Identifiers. For a complete worldwide listing, refer to ICAO Document 7910, Location Indicators. Both are available online. 3.1.5.3 Date and Time of Report. METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 ### Page 37 3-6 The date and time are coded in all reports as follows: the day of the month is the first two digits (01), followed by the hour (19) and the minutes (55). The coded time of observations is the actual time of the report, or when the criteria for a SPECI is met or noted. If the report is a correction to a previously disseminated report, the time of the corrected report is the same time used in the report being corrected. The date and time group always ends with a Z, indicating Zulu time (or Coordinated Universal Time (UTC)). For example, METAR KOKC 011955Z would be disseminated as the 2000 hour routine report for station KOKC, taken on the 1st of the month at 1955 UTC. 3.1.5.4 Report Modifier (As Required). METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 The report modifier AUTO identifies the METAR/SPECI as a fully-automated report with no human intervention or oversight. In the event of a corrected METAR or SPECI, the report modifier COR is substituted for AUTO. 3.1.5.5 Wind Group. METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 Wind is the horizontal motion of air past a given point. It is measured in terms of velocity, which is a vector that includes direction and speed. It indicates the direction the wind is coming from. In the wind group, the wind direction is coded as the first three digits (220) and is determined by averaging the recorded wind direction over a 2-minute period. It is coded in tens of degrees relative to true north using three figures. Directions less than 100 degrees are preceded with a 0. For example, a wind direction of 90 degrees is coded as 090. A wind from the north is coded as 360. Immediately following the wind direction is the wind speed coded in two or three digits (15). Wind speed is determined by averaging the speed over a 2-minute period and is coded in whole knots (kts) using the units, tens digits, and, when required, the hundreds digit. When wind speeds are less than 10 kts, a leading 0 is used to maintain at least a two-digit wind code. For example, a wind speed of 8 kts will be coded 08KT. The wind group is ### Page 38 3-7 always coded with a KT to indicate wind speeds are reported in knots. Other countries may use kilometers per hour (KPH) or meters per second (MPS) instead of knots. Examples: • 05008KT Wind 50 degrees at 8 kts • 15014KT Wind 150 degrees at 14 kts • 340112KT Wind 340 degrees at 112 kts 3.1.5.5.1 Wind Gust. Wind speed data for the most recent 10 minutes is examined to evaluate the occurrence of gusts. Gusts are defined as rapid fluctuations in wind speed with a variation of 10 kts or more between peaks and lulls. The coded speed of the gust is the maximum instantaneous wind speed. Wind gusts are coded in two or three digits immediately following the wind speed. Wind gusts are coded in whole knots using the units, tens, and, if required, the hundreds digit. For example, a wind out of the west at 20 kts with gusts to 35 kts would be coded 27020G35KT. 3.1.5.5.2 Variable Wind Direction (speed 6 kts or less). Wind direction may be considered variable when, during the previous 2-minute evaluation period, the wind speed was 6 kts or less. In this case, the wind may be coded as VRB in place of the three-digit wind direction. For example, if the wind speed was recorded as 3 kts, it would be coded VRB03KT. 3.1.5.5.3 Variable Wind Direction (speed greater than 6 kts). Wind direction may also be considered variable when, during the 2-minute evaluation period, it varies by 60 degrees or more and the speed is greater than 6 kts. In this case, a variable wind direction group immediately follows the wind group. The directional variability is coded in a clockwise direction and consists of the extremes of the wind directions separated by a V. For example, if the wind is variable from 180 degrees to 240 degrees at 10 kts, it would be coded 21010KT 180V240. 3.1.5.5.4 Calm Wind. When no motion of air is detected, the wind is reported as calm. A calm wind is coded as 00000KT. 3.1.5.6 Visibility Group. METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 ### Page 39 3-8 Visibility is a measure of the opacity of the atmosphere. It is defined as the greatest horizontal distance at which selected objects can be seen and identified or its equivalent derived from instrumental measurements. Prevailing visibility is the reported visibility considered representative of recorded visibility conditions at the manual station during the time of observation. It is the greatest distance that can be seen throughout at least half of the horizon circle, not necessarily continuous. Surface visibility is the prevailing visibility from the surface at manual stations or the visibility derived from sensors at automated stations. The visibility group is coded as the surface visibility in statute miles (sm). A space is coded between whole numbers and fractions of reportable visibility values. The visibility group ends with SM to indicate that the visibility is in statute miles. For example, a visibility of 1½ sm is coded 1 1/2SM. Most other countries use meters (m). U.S. automated stations use an M to indicate “less than.” For example, M1/4SM means a visibility of less than ¼ sm. 3.1.5.7 Runway Visual Range (RVR) Group. METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 The RVR is an instrument-derived value representing the horizontal distance a pilot may see down the runway. RVR is reported whenever the station has RVR equipment and prevailing visibility is 1 sm or less and/or the RVR for the designated instrument runway is 6,000 feet (ft) or less. Otherwise the RVR group is omitted. RVR is coded in the following format: the initial R is code for runway and is followed by the runway number. When more than one runway is defined with the same runway number, a directional letter is coded on the end of the runway number. Next is a solidus (/) followed by the visual range in feet and then FT completes the RVR report. For example, an RVR value for Runway 01L of 800 ft would be coded R01L/0800FT. Most other countries use meters. In the United States, RVR values are coded in increments of 100 ft up to 1,000 ft, increments of 200 ft from 1,000 ft to 3,000 ft, and increments of 500 ft from 3,000 ft to 6,000 ft. Manual RVR is not reported below 600 ft. For U.S. airports only, the touchdown zone’s (TDZ) RVR is reported. For U.S. airports with multiple runways, the operating runway with the lowest ### Page 40 3-9 touchdown RVR is reported. RVR may be reported for up to four designated runways in other countries. When the RVR varies by more than one reportable value, the lowest and highest values will be shown with V between them, indicating variable conditions. For example, the 10-minute RVR for Runway 01L varying between 600 ft and 1,000 ft would be coded R01L/0600V1000FT. If RVR is less than its lowest reportable value, the visual range group is preceded by M. For example, an RVR for Runway 01L of less than 600 ft is coded R01L/M0600FT. If RVR is greater than its highest reportable value, the visual range group is preceded by a P. For example, an RVR for Runway 27 of greater than 6,000 ft will be coded R27/P6000FT. 3.1.5.8 Present Weather Group. METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 Pr esent weather includes precipitation, obscurations, and other weather phenomena. The appropriate notations found in Table 3-2, METAR/SPECI Notations for Reporting Present Weather, are used to code present weather. ### Page 41 3-10 Table 3-2. METAR/SPECI Notations for Reporting Present Weather1 **QUALIFIER WEATHER PHENOMENA** **INTENSITY OR** **PROXIMITY 1** **DESCRIPTOR 2** **PRECIPITATION 3** **OBSCURATION 4 OTHER 5** - Light MI Shallow DZ Drizzle BR Mist PO Dust/Sand Whirls Moderate2 PR Partial RA Rain FG Fog SQ Squalls + Heavy BC Patches SN Snow FU Smoke FC Funnel Cloud, Tornado or Waterspout4 VC In the Vicinity3 DR Low Drifting SG Snow Grains VA Volcanic Ash SS Sandstorm BL Blowing IC Ice Crystals (Diamond Dust) DU Widespread Dust DS Duststorm SH Shower(s) PL Ice Pellets SA Sand TS Thunderstorms GR Hail HZ Haze FZ Freezing GS Snow Pellets PY Spray UP Unknown Precipitation 1. The weather groups are constructed by considering columns 1 through 5 in the table above in sequence, i.e., intensity followed by description, followed by weather phenomena, e.g., heavy rain shower(s) is coded as +SHRA. 2. To denote moderate intensity no entry or symbol is used. 3. See text for vicinity definitions. 4. Tornadoes and waterspouts are coded as +FC. Separate groups are used for each type of present weather. Each group is separated from the other by a space. METAR/SPECI reports contain no more than three present weather groups. ### Page 42 3-11 When more than one type of present weather is reported at the same time, present weather is reported in the following order: • Tornadic activity (tornado, funnel cloud, or waterspout). • Thunderstorm(s) (with and without associated precipitation). • Present weather in order of decreasing dominance (i.e., the most dominant type reported first). • Left-to-right in T able 3-2 (columns 1 through 5). Qualifiers may be used in various combinations to describe weather phenomena. Present weather qualifiers fall into two categories: intensity (see paragraph 3.1.5.8.1) or proximity (see paragraph 3.1.5.8.2) and descriptors (see paragraph 3.1.5.8.3). 3.1.5.8.1 Intensity Qualifier. The intensity qualifiers are light, moderate, and heavy. They are coded with precipitation types, except ice crystals (IC) and hail (GR), including those associated with a thunderstorm (TS) and those of a showery nature (SH). Tornadoes and waterspouts are coded as heavy (+FC). No intensity is ascribed to the obscurations of blowing dust (BLDU), blowing sand (BLSA), and blowing snow (BLSN). Only moderate or heavy intensity is ascribed to sandstorm (SS) and duststorm (DS). When more than one form of precipitation is occurring at a time, or precipitation is occurring with an obscuration, the reported intensities are not cumulative. The reported intensity will not be greater than the intensity for each form of precipitation. For example, -FZRAPL is light freezing rain and light ice pellets, not light freezing rain and moderate ice pellets. 3.1.5.8.2 Proximity Qualifier. Weather phenomena occurring beyond the point of observation (between 5 and 10 sm) are coded as in the vicinity (VC). VC can be coded in combination with thunderstorm (TS), fog (FG), shower(s) (SH), well-developed dust/sand whirls (PO), blowing dust (BLDU), blowing sand (BLSA), blowing snow (BLSN), sandstorm (SS), and duststorm (DS). Intensity qualifiers are not coded in conjunction with VC. For example, VCFG can be decoded as meaning some form of fog is between 5 and 10 sm of the point of observation. If VCSH is coded, showers are occurring between 5 and 10 sm of the point of observation. Weat her phenomena occurring at the point of observation (at the station) or in the vicinity of the point of observation are coded in the body of the report. Weather phenomena observed beyond 10 sm from the point of observation (at the station) is not coded in the body, but may be coded in the remarks section (see paragraph 3.1.5.12). ### Page 43 3-12 3.1.5.8.3 Descriptor Qualifier. Descriptors are qualifiers that further amplify weather phenomena and are used in conjunction with some types of precipitation and obscurations. The descriptor qualifiers are: shallow (MI), partial (PR), patches (BC), low drifting (DR), blowing (BL), shower(s) (SH), thunderstorm (TS), and freezing (FZ). Only one descriptor is coded for each weather phenomena group (e.g., FZDZ). The descriptors shallow (MI), partial (PR), and patches (BC) are only coded with fog (FG) (e.g., MIFG). Mist (BR) is not coded with any descriptor. The descriptors low drifting (DR) and blowing (BL) will only be coded with dust (DU), sand (SA), and snow (SN) (e.g., BLSN or DRSN). DR is coded with DU, SA, or SN for raised particles drifting less than 6 ft above the ground. When blowing snow is observed with snow falling from clouds, both phenomena are reported (e.g., SN BLSN). If blowing snow is occurring and the observer cannot determine whether or not snow is also falling, then BLSN is reported. Spray (PY) is coded only with blowing (BL). The descriptor for showery-type precipitation (SH) is coded only with one or more of the precipitation qualifiers for rain (RA), snow (SN), ice pellets (PL), or hail (GR). When any type of precipitation is coded with VC, the intensity and type of precipitation is not coded. The descriptor for thunderstorm (TS) may be coded by itself when the thunderstorm is without associated precipitation. A thunderstorm may also be coded with the precipitation types of rain (RA), snow (SN), ice pellets (PL), snow pellets (GS), or hail (GR). For example, a thunderstorm with snow and/or GR hail and/or snow pellets would be coded as TSSNGRGS. TS is not coded with SH. The descriptor freezing (FZ) is only coded in combination with fog (FG), drizzle (DZ), or rain (RA) (e.g., FZRA). FZ is not coded with SH. 3.1.5.8.4 Precipitation. Precipitation is any form of water particle, whether liquid or solid, that falls from the atmosphere and reaches the ground. The precipitation types are: drizzle (DZ), rain (RA), snow (SN), snow grains (SG), ice crystals (IC), ice pellets (PL), hail (GR), snow pellets (GS), and unknown precipitation (UP). UP is reported if an automated station detects the occurrence of precipitation, but the precipitation sensor cannot recognize the type. ### Page 44 3-13 Up to three types of precipitation may be coded in a single present weather group. They are coded in order of decreasing dominance based on intensity. 3.1.5.8.5 Obscuration. Obscurations are any phenomenon in the atmosphere, other than precipitation, that reduces the horizontal visibility in the atmosphere. The obscuration types are: mist (BR), fog (FG), smoke (FU), volcanic ash (VC), widespread dust (DU), sand (SA), haze (HZ), and spray (PY). Spray (PY) is coded only as BLPY. With the exception of volcanic ash, low drifting dust, low drifting sand, low drifting snow, shallow fog, partial fog, and patches (of) fog, an obscuration is coded in the body of the report if the surface visibility is less than 7 miles (mi), or considered operationally significant. Volcanic ash is always reported when observed. 3.1.5.8.6 Other Weather Phenomena. Other weather phenomena types include: well-developed dust/sand whirls (PO), sandstorms (SS), duststorms (DS), squalls (SQ), funnel clouds (FC), and tornados and waterspouts (+FC). Examples: -DZ Light drizzle -RASN Light rain and (light) snow SN BR (Moderate) snow, mist -FZRA FG Light freezing rain, fog SHRA (Moderate) rain shower VCBLSA Blowing sand in the vicinity -RASN FG HZ Light rain and (light) snow, fog, haze TS Thunderstorm (without precipitation) +TSRA Thunderstorm, heavy rain +FC TSRAGR BR Tornado, thunderstorm, (moderate) rain, hail, mist 3.1.5.9 Sky Condition Group. METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 Sky condition is a description of the appearance of the sky. It includes either cloud cover, vertical visibility, or clear skies. ### Page 45 3-14 The sky condition group is based on the amount of cloud cover (the first three letters) followed by the height of the base of the cloud cover (final three digits). No space is between the amount of cloud cover and the height of the layer. The height of the layer is recorded in feet above ground level (AGL). Sky condition is coded in ascending order and ends at the first overcast layer. At mountain stations, if the layer is below station level, the height of the layer will be coded as ///. Vertical visibility is coded as VV, followed by the vertical visibility into the indefinite ceiling. An indefinite ceiling is a ceiling classification applied when the reported ceiling value represents the vertical visibility upward into surface-based obscuration. No space is between the group identifier and the vertical visibility. Figure 3-3, Obscuration Effects on Slant Range Visibility, illustrates the effect of an obscuration on the vision from a descending aircraft. Figure 3-3. Obscuration Effects on Slant Range Visibility The ceiling is 500 ft in both examples, but the indefinite ceiling example (bottom) produces a more adverse impact to landing aircraft. This is because an obscuration (e.g., fog, blowing dust, snow) limits runway acquisition due to reduced slant range visibility. This pilot would be able to see the ground, but not the runway. If the pilot was at approach minimums, the approach could not be continued and a missed approach must be executed. ### Page 46 3-15 Clear skies are coded in the format SKC or CLR. When SKC is used, an observer indicates no layers are present; CLR is used by automated stations to indicate no layers are detected at or below 12,000 ft. Each coded layer is separated from the others by a space. Each layer reported is coded by using the appropriate reportable contraction seen in Table 3-3, METAR/SPECI Contractions for Sky Cover. A report of clear skies (SKC or CLR) is a complete layer report within itself. The abbreviations FEW, SCT, BKN, and OVC will be followed (without a space) by the height of the layer. Table 3-3. METAR/SPECI Contractions for Sky Cover Reportable Contraction Meaning Summation Amount of Layer VV Vertical Visibility 8/8 SKC or CLR1 Clear 0 FEW2 Few 1/8 – 2/8 SCT Scattered 3/8 – 4/8 BKN Broken 5/8 – 7/8 OVC Overcast 8/8 1. The abbreviation CLR will be used at automated stations when no layers at or below 12,000 ft are reported; the abbreviation SKC will be used at manual stations when no layers are reported. 2. Any layer amount less than 1/8 is reported as FEW. The height is coded in hundreds of feet above the surface using three digits in accordance with Table 3-4, METAR/SPECI Increments of Reportable Values of Sky Cover Height. Table 3-4. METAR/SPECI Increments of Reportable Values of Sky Cover Height Range of Height Values (feet) Reportable Increment (feet) Less than or equal to 5,000 To nearest 100 5,001 to 10,000 To nearest 500 Greater than 10,000 To nearest 1,000 The ceiling is the lowest layer aloft reported as broken or overcast. If the sky is totally obscured with ground-based clouds, the vertical visibility is the ceiling. ### Page 47 3-16 Figure 3-4. METAR/SPECI Sky Condition Coding Clouds at 1,200 ft obscure 2/8ths of the sky (FEW). Higher clouds at 3,000 ft obscure an additional 1/8th of the sky, and because the observer cannot see above the 1,200-ft layer, he is to assume that the higher 3,000-ft layer also exists above the lower layer (SCT). The highest clouds at 5,000 ft obscure 2/8ths of the sky, and again since the observer cannot see past the 1,200 and 3,000-ft layers, he is to assume the higher 5,000-ft layer also exists above the lower layers (BKN). The sky condition group would be coded as: FEW012 SCT030 BKN050. At manual stations, cumulonimbus (CB) or towering cumulus (TCU) is appended to the associated layer. For example, a scattered layer of towering cumulus at 1,500 ft would be coded SCT015TCU, and would be followed by a space if there were additional higher layers to code. Examples: SKC No layers are present CLR No layers are detected at or below 12,000 ft AGL FEW004 Few at 400 ft AGL SCT023TCU Scattered layer of towering cumulus at 2,300 ft BKN100 Broken layer (ceiling) at 10,000 ft OVC250 Overcast layer (ceiling) at 25,000 ft VV001 Indefinite ceiling with a vertical visibility of 100 ft FEW012 SCT046 Few clouds at 1,200 ft, scattered layer at 4,600 ft ### Page 48 3-17 SCT033 BKN085 Scattered layer at 3,300 ft, broken layer (ceiling) at 8,500 ft SCT018 OVC032CB Scattered layer at 1,800 ft, overcast layer (ceiling) of cumulonimbus at 3,200 ft SCT009 SCT024 BKN048 Scattered layer at 900 ft, scattered layer at 2,400 ft, broken layer (ceiling) at 4,800 ft 3.1.5.10 Temperature/Dewpoint Group. METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 Temperature is the degree of hotness or coldness of the ambient air, as measured by a suitable instrument. Dewpoint is the temperature to which a given parcel of air must be cooled at constant pressure and constant water vapor content for the air to become fully saturated. Temperature and dewpoint are coded as two digits rounded to the nearest whole degree Celsius. For example, a temperature of 0.3 ºC would be coded at 00. Sub-zero temperatures and dewpoints are prefixed with an M. For example, a temperature of 4 ºC with a dewpoint of -2 ºC would be coded as 04/M02; a temperature of -2 ºC would be coded as M02. If temperature is not available, the entire temperature/dewpoint group is not coded. If dewpoint is not available, temperature is coded followed by a solidus (/) and no entry is made for dewpoint. For example, a temperature of 1.5 ºC and a missing dewpoint would be coded as 02/. 3.1.5.11 Altimeter. METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 The altimeter setting group codes the current pressure at elevation. This setting is then used by aircraft altimeters to determine the true altitude above a fixed plane of mean sea level (MSL). The altimeter group always starts with an A and is followed by the four-digit group representing the pressure in tens, units, tenths, and hundredths of inches (in) of mercury. The decimal point is not coded. For example, an altimeter setting of 29.92 in of Mercury would be coded as A2992. 3.1.5.12 Remarks (RMK). METAR KOKC 011955Z AUTO 22015G25KT 180V250 3/4SM R17L/2600FT +TSRA BR OVC010CB 18/16 A2992 RMK AO2 TSB25 TS OHD MOV E SLP132 ### Page 49 3-18 Remarks are included in METAR and SPECI, when appropriate. Remarks are separated from the body of the report by the contraction RMK. When no remarks are necessary, the contraction RMK is not required. METAR/SPECI remarks fall into two categories: (1) Automated, Manual, and Plain Language, and (2) Additive and Automated Maintenance Data. Table 3-5. METAR/SPECI Order of Remarks Automated, Manual, and Plain Language 1. Volcanic Eruptions 2. Funnel Cloud 3. Type of Automated Station 4. Peak Wind 5. Wind Shift 6. Tower or Surface Visibility 7. Variable Prevailing Visibility 8. Sector Visibility 9. Visibility at Second Location 10. Lightning 11. Beginning and Ending of Precipitation 12. Beginning and Ending of Thunderstorms 13. Thunderstorm Location 14. Hails tone Size 15. Virga 16. Variable Ceiling Height 17. Obscurations 18. Variable Sky Condition 19. Significant Cloud Types 20. Ceiling Height at Second Location 21. Pressure Rising or Falling Rapidly 22. Sea-Level Pressure 23. Aircraft Mishap 24. No SPECI Reports Taken 25. Snow Increasing Rapidly 26. Other Significant Information Additive and Automated Maintenance Data 27. Precipitation amount within a specified time period* 28. Cloud Types* 29. Duration of Sunshine* 30. Hourly Temperature and Dewpoint 31. 6-Hourly Maximum Temperature* 32. 6-Hourly Minimum Temperature* 33. 24-Hour Maximum and Minimum Temperature* 34. 3-Hourly Pressure Tendency* 35. Sens or Status Indicators 36. Maintenance Indicator Note: Additive data is primarily used by the National Weather Service for climatological purposes. *These groups should have no direct impact on the aviation community and will not be discussed in this document. Remarks are made in accordance with the following: • Time entries are made in minutes past the hour if the time reported occurs during the same hour the observation is taken. Hours and minutes are used if the hour is different. • Present weather coded in the body of the report as VC may be further described (e.g., direction from the station, if known). Weather phenomena beyond 10 sm of the point(s) of observation are coded as distant (DSNT) followed by the direction from the station. For example, precipitation of unknown intensity within 10 sm east of the station would be coded as VCSH E; lightning 25 sm west of the station would be coded as LTG DSNT W. ### Page 50 3-19 • Distance remarks are in statute miles except for automated lightning remarks, which are in nautical miles (NM). • Movement of clouds or weather, when known, is coded with respect to the direction toward which the phenomena are moving. For example, a thunderstorm moving toward the northeast would be coded as TS MOV NE. • Directions use the eight points of the compass coded in a clockwise order. • Inso far as possible, remarks are entered in the order they are presented in the following paragraphs (and Table 3-5, METAR/SPECI Order of Remarks). 3.1.5.13 Automated, Manual, and Plain Language Remarks. These remarks generally elaborate on parameters reported in the body of the report. An automated station or observer may generate automated and manual remarks. Only an observer can provide plain language remarks. 3.1.5.13.1 Volcanic Eruptions (Plain Language). Volcanic eruptions are coded in plain language and contain the following, when known: • Name of volcano; • Latitude and longitude or the direction and approximate distance from the station; • Date/Time (UTC) of the eruption; • Size description, approximate height, and direction of movement of the ash cloud; and • Any other pertinent data about the eruption. For example, a remark on a volcanic eruption would look like the following: RMK MT. AUGUSTINE VOLCANO 70 MILES SW ERUPTED AT 231505 LARGE ASH CLOUD EXTENDING TO APRX 30000 FEET MOVING NE. Pre-eruption volcanic activity is not coded. Pre-eruption refers to unusual and/or increasing volcanic activity which could presage a volcanic eruption. 3.1.5.13.2 Funnel Cloud. At manual stations, tornadoes, funnel clouds, and waterspouts are coded in the following format: tornadic activity, TORNADO, FUNNEL CLOUD, or WATERSPOUT, followed by the beginning and/or ending time, followed by the location and/or direction of the phenomena from the station, and/or movement, when known. For example, TORNADO B13 6 NE would indicate ### Page 51 3-20 that a tornado began at 13 minutes past the hour and was 6 sm northeast of the station. 3.1.5.13.3 Type of Automated Station. AO1 or AO2 is coded in all METAR/SPECI from automated stations. Automated stations without a precipitation discriminator are identified as AO1; automated stations with a precipitation discriminator are identified as AO2. 3.1.5.13.4 Peak Wind. Peak wind is coded in the following format: the remark identifier PK WND, followed by the direction of the wind (first three digits), peak wind speed (next two or three digits) since the last METAR, and the time of occurrence. A space is between the two elements of the remark identifier and the wind direction/speed group; a solidus (/), without spaces, separates the wind direction/speed group and the time. For example, a peak wind of 45 kts from 280 degrees that occurred at 15 minutes past the hour is coded PK WND 28045/15. 3.1.5.13.5 Wind Shift. Wind shift is coded in the following format: the remark identifier WSHFT, followed by the time the wind shift began. The contraction FROPA is entered following the time if there is reasonable data to consider the wind shift was the result of a frontal passage. A space is between the remark identifier and the time and, if applicable, between the time and the frontal passage contraction. For example, a remark reporting a wind shift accompanied by a frontal passage that began at 30 minutes after the hour would be coded WSHFT 30 FROPA. 3.1.5.13.6 Tower or Surface Visibility. Tower or surface visibility is coded in the following format: tower (TWR VIS) or surface (SFC), followed by the observed tower/surface visibility value. A space is coded between each of the remark elements. For example, the control tower visibility of 1½ sm would be coded TWR VIS 1 1/2. 3.1.5.13.7 Variable Prevailing Visibility. Variable prevailing visibility is coded in the following format: the remark identifier VIS, followed by the lowest and highest visibilities evaluated, separated by the letter V. A space follows the remark identifier and no spaces are between the letter V and the lowest/highest values. For example, a visibility that was varying between ½ and 2 sm would be coded VIS 1/2V2. 3.1.5.13.8 Sector Visibility (Plain Language). Sector visibility is coded at manual stations in the following format: the remark identifier VIS, followed by the sector referenced to eight points of the ### Page 52 3-21 compass, and the sector visibility in statute miles. For example, a visibility of 2½ sm in the northeastern octant is coded VIS NE 2 1/2. 3.1.5.13.9 Visibility at Second Location. At designated automated stations, the visibility at a second location is coded in the following format: the remark identifier VIS, followed by the measured visibility value and the specific location of the visibility sensor(s) at the station. This remark will only be generated when the condition is lower than that contained in the body of the report. For example, a visibility of 2½ sm measured by a second sensor located at Runway 11 is coded VIS 2 1/2 RWY11. 3.1.5.13.10 Lightning. When lightning is observed at a manual station, the frequency, type of lightning, and location are reported. The contractions for the type and frequency of lightning are based on Table 3-6, METAR/SPECI Type and Frequency of Lightning, for example, OCNL LTGICCG NW, FRQ LTG VC, or LTG DSNT W. When lightning is detected by an automated system: • Within 5 NM of the Airport Location Point (ALP), it is reported as TS in the body of the report with no remark. • Between 5 and 10 NM of the ALP, it is reported as VCTS in the body of the report with no remark. • Beyond 10 but less than 30 NM of the ALP, it is reported in remarks only as LTG DSNT followed by the direction from the ALP. Table 3-6. METAR/SPECI Type and Frequency of Lightning Type of Lightning Type Contraction Definition Cloud-ground CG Lightning occurring between cloud and ground In-cloud IC Lightning which takes place within the cloud Cloud-cloud CC Streaks of lightning reaching from one cloud to another Cloud-air CA Streaks of lightning which pass from a cloud to the air, but do not strike the ground Frequency of Lightning Frequency Contraction Definition Occasional OCNL Less than 1 flash/minute Frequent FRQ About 1 to 6 flashes/minute Continuous CONS More than 6 flashes/minute ### Page 53 3-22 3.1.5.13.11 Beginning and Ending of Precipitation. At designated stations, the beginning and ending times of precipitation are coded in the following format: the type of precipitation, followed by either a B for beginning or an E for ending, and the time of occurrence. No spaces are coded between the elements. The coded times of the precipitation start and stop times are found in the remarks section of the next METAR. The times are not required to be in the SPECI. The intensity qualifiers are coded. For example, if rain began at 0005 and ended at 0030, and then snow began at 0020 and ended at 0055, the remarks would be coded RAB05E30SNB20E55. If the precipitation was showery, the remark is coded SHRAB05E30SHSNB20E55. If rain ended and snow began at 0042, the remark would be coded as RAESNB42. 3.1.5.13.12 Beginning and Ending of Thunderstorms. The beginning and ending times of thunderstorms are coded in the following format: The thunderstorm identifier TS, followed by either a B for beginning or an E for ending, and the time of occurrence. No spaces are between the elements. For example, if a thunderstorm began at 0159 and ended at 0230, the remark is coded TSB0159E30. 3.1.5.13.13 Thunderstorm Location. Thunderstorm locations are coded in the following format: the thunderstorm identifier TS, followed by the location of the thunderstorm(s) from the station, and the direction of movement, when known. For example, a thunderstorm southeast of the station and moving toward the northeast is coded TS SE MOV NE. 3.1.5.13.14 Hailstone Size. At designated stations, the hailstone size is coded in the following format: the hail identifier GR, followed by the size of the largest hailstone. The hailstone size is coded in ¼ in increments. For example, GR 1 3/4 would indicate that the largest hailstones were 1¾ in. in diameter. If small hail or snow pellets, GS, are coded in the body of the report, no hailstone size remark is required. 3.1.5.13.15 Virga. At designated stations, Virga is coded in the following format: the identifier VIRGA, followed by the direction from the station. The direction of the phenomena from the station is optional, e.g., VIRGA or VIRGA SW. 3.1.5.13.16 Variable Ceiling Height. The variable ceiling height is coded in the following format: the identifier CIG, followed by the lowest ceiling height recorded, V denoting variability between two values, and ending with the highest ceiling height. A single space follows the identifier with no other spaces between the letter V and the lowest/highest ceiling values. For example, CIG 005V010 would indicate a ceiling is variable between 500 ft and 1,000 ft. ### Page 54 3-23 3.1.5.13.17 Obscurations. Obscurations, surface-based or aloft, are coded in the following format: the weather identifier causing the obscuration at the surface or aloft, followed by the sky cover of the obscuration aloft (FEW, SCT, BKN, OVC) or at the surface (FEW, SCT, BKN), and the height. Surface-based obscurations have a height of 000. A space separates the weather causing the obscuration and the sky cover; no space is between the sky cover and the height. For example, fog hiding 3/8 to 4/8 of the sky is coded FG SCT000; a broken layer at 2,000 ft composed of smoke is coded FU BKN020. 3.1.5.13.18 Variable Sky Condition. Variable sky condition remarks are coded in the following format: the two operationally significant sky conditions (FEW, SCT, BKN, and OVC), separated by spaces, and V denoting the variability between the two ranges. If several layers have the same condition amount, the layer height of the variable layer is coded. For example, a cloud layer at 1,400 ft varying between broken and overcast is coded BKN014 V OVC. 3.1.5.13.19 Significant Cloud Types. At manual stations, significant cloud type remarks are coded in all reports. 3.1.5.13.20 Cumulonimbus. Cumulonimbus not associated with thunderstorms is coded as CB, followed by the direction from the station, and the direction of movement, when known. The location, direction, and direction of movement entries are separated from each other by a space. For example, a CB up to 10 sm west of the station moving toward the east would be coded CB W MOV E. If the CB was more than 10 sm to the west, the remark is coded CB DSNT W. Cumulonimbus (CB) always evolves from the further development of towering cumulus (TCU). The unusual occurrence of lightning and thunder within or from a CB leads to its popular title, thunderstorm. A thunderstorm usually contains severe or greater turbulence, severe icing, and Low-Level Wind Shear (LLWS). 3.1.5.13.21 Towering Cumulus. Towering cumulus clouds are coded in the following format: the identifier TCU, followed by the direction from the station. The cloud type and direction entries are separated by a space. For example, a towering cumulus cloud up to 10 sm west of the station is coded TCU W. 3.1.5.13.22 Standing Lenticular or Rotor Clouds. Stratocumulus (SCSL), altocumulus (ACSL), cirrocumulus (CCSL), or rotor clouds are coded in the following format: the cloud type followed by the direction from the station. The cloud type and direction entries are separated by a space. For example, altocumulus standing lenticular clouds observed southwest through west of the station are coded ACSL SW-W; an apparent ### Page 55 3-24 rotor cloud 5 to 10 sm northeast of the station is coded APRNT ROTOR CLD NE; and cirrocumulus clouds south of the station are coded CCSL S. 3.1.5.13.23 Ceiling Height at Second Location. At designated stations, the ceiling height at a second location is coded in the following format: the identifier CIG, followed by the measured height of the ceiling and the specific location of the ceilometer(s) at the station. This remark is only generated when the ceiling is lower than that contained in the body of the report. For example, if the ceiling measured by a second sensor located at Runway 11 is broken at 200 ft, the remark would be coded CIG 002 RWY11. 3.1.5.13.24 Pressure Rising or Falling Rapidly. At designated stations, the reported pressure is evaluated to determine if a pressure change is occurring. If the pressure is rising or falling at a rate of at least 0.06 in per hour and the pressure change totals 0.02 in or more at the time of the observation, a pressure change remark is reported. When the pressure is rising or falling rapidly at the time of observation, the remark pressure rising rapidly (PRESRR) or pressure falling rapidly (PRESFR) is included in the remarks. 3.1.5.13.25 Sea-Level Pressure. At designated stations, the sea-level pressure is coded in the following format: the identifier SLP, immediately followed by the sea-level pressure in millibars (mb)1. The hundreds and thousands units are not coded and must be inferred. For example, a sea-level pressure of 998.2 mb is coded SLP982. A sea-level pressure of 1,013.2 mb would be coded SLP132. For a METAR, if sea-level pressure is not available, it is coded SLPNO. 3.1.5.13.26 Aircraft Mishap. If a SPECI report is taken to document weather conditions when notified of an aircraft mishap, the remark ACFT MSHP is coded in the report, but the SPECI is not transmitted. 3.1.5.13.27 No SPECI Reports Taken (Plain Language). At manual stations where SPECIs are not taken, the remark NOSPECI is coded to indicate no changes in weather conditions will be reported until the next METAR. 3.1.5.13.28 Snow Increasing Rapidly. At designated stations, the snow increasing rapidly remark is reported in the next METAR whenever the snow depth increases by 1 in or more in the past hour. The remark is coded in the following format: the remark indicator 1 Although the international unit of pressure measurement is now the Pascal, we will use the term millibar in this document version because, at this time, it is felt the reader is probably more familiar with millibars. Increasingly in meteorology literature, however, the term hectopascal (hPa) has replaced the term millibar to conform to international standards and because 1 hPa = 1 mb. ### Page 56 3-25 SNINCR, the depth increase in the past hour, and the total depth of snow on the ground at the time of the report. The depth of snow increase in the past hour and the total depth on the ground are separated from each other by a solidus (/). For example, a snow depth increase of 2 in. in the past hour with a total depth on the ground of 10 in is coded SNINCR 2/10. 3.1.5.13.29 Other Significant Information (Plain Language). Agencies may add to a report other information significant to their operations, such as information on fog dispersal operations, runway conditions, FIRST or LAST reports from station, etc. 3.1.5.13.30 Additive and Automated Maintenance Data. Additive data groups (see Table 3-5) are only reported at designated stations and are primarily used by the NWS for climatological purposes. Most have no dir ect impact on the aviation community, but a few are discussed below. 3.1.5.13.31 Hourly Temperature and Dewpoint. At designated stations, the hourly temperature and dewpoint group are further coded to the tenth of a degree Celsius. For example, a recorded temperature of +2.6 ºC and dewpoint of -1.5 ºC would be coded T00261015. The format for the coding is as follows: T Group indicator 0 Indicates the following number is positive; a 1 would be used if the temperature was reported as negative at the time of observation 026 Temperature disseminated to the nearest tenth and read as 02.6 1 Indicates the following number is negative; a 0 would be used if the number was reported as positive at the time of observation 015 Dewpoint disseminated to the nearest tenth and read as 01.5 No spaces are between the entries. For example, a temperature of 2.6 ºC and dewpoint of -1.5 ºC is reported in the body of the report as 03/M01 and the hourly temperature and dewpoint group as T00261015. If the dewpoint is missing, only the temperature is reported; if the temperature is missing, the hourly temperature and dewpoint group are not reported. 3.1.5.13.32 Maintenance Data Groups. The following maintenance data groups, Sensor Status Indicators and the Maintenance Indicator, are only reported from automated stations. 3.1.5.14 Sensor Status Indicators. Sensor status indicators are reported as indicated below: ### Page 57 3-26 • If the RVR is missing and would normally be reported, RVRNO is coded. • When automated stations are equipped with a present weather identifier and the sensor is not operating, the remark PWINO is coded. • When automated stations are equipped with a tipping bucket rain gauge and the sensor is not operating, PNO is coded. • When automated stations are equipped with a freezing rain sensor and the sensor is not operating, the remark FZRANO is coded. • When automated stations are equipped with a lightning detection system and the sensor is not operating, the remark TSNO is coded. • When automated stations are equipped with a secondary visibility sensor and the sensor is not operating, the remark VISNO LOC is coded. • When automated stations are equipped with a secondary ceiling height indicator and the sensor is not operating, the remark CHINO LOC is coded. 3.1.5.14.1 Maintenance Indicator. A maintenance indicator ($) is coded when an automated system detects that maintenance is needed on the system. 3.2 Aircraft Observations and Reports. There are two kinds of aircraft observations: Pilot Weather Reports (PIREP) and Aircraft Reports, or air-reports (AIREP). Each kind has two types: • Routine PIREPs and Urgent PIREPs. • Routine AIREPs and Special AIREPs. PIREPs are reported by the pilot (or aircrew), while AIREPs can either be reported by the pilot or generated from sensors onboard the aircraft (automated AIREPs). PIREPs and AIREPs are coded differently. The PIREP format is a U.S.-only format. The AIREP format is used worldwide. Automated AIREPs are common over the United States. 3.2.1 Pilot Weather Report (PIREP). Pilots should report any observation, good or bad, to assist other pilots with flight planning and preparation. If conditions were forecasted to occur but not encountered, a pilot should also report the observed condition. This will help the NWS verify forecast products and create more accurate products for the aviation community. A PIREP is prepared using a prescribed format (see Figure 3-5, Pilot Weather Report (PIREP) Coding Format). Required elements for all PIREPs are: message type, loca tion, time, altitude/flight level, type aircraft, and at least one other element to describe the reported phenomena. The other elements are omitted when no data is reported. All altitude references are MSL unless otherwise noted. Distance for visibility is in statute miles and all other distances are in nautical miles. Time is reported in UTC. ### Page 58 3-27 Figure 3-5. Pilot Weather Report (PIREP) Coding Format 3.2.1.1 Mes sage Type (UUA/UA). The two types of PIREPs are Urgent (UUA) and Routine (UA). 3.2.1.1.1 Urgent PIREPs. Urgent (UUA) PIREPs contain information about: • Tornadoes, funnel clouds, or waterspouts; • Severe or extreme turbulence (including Clear Air Turbulence (CAT)); • Severe icing; • Hail; • LLWS within 2,000 ft of the surface. LLWS PIREPS are classified as UUA if the pilot reports air speed fluctuations of 10 kts or more, or if air speed fluctuations are not reported but LLWS is reported, the PIREP is classified as UUA; • Volcanic ash clouds; and/or • Any other weather phenomena reported that are considered by the briefer as being hazardous, or potentially hazardous, to flight operations. 3.2.1.1.2 Routine PIREPs. Routine PIREPs are issued after receiving a report from a pilot that does not contain any urgent information as listed in paragraph 3.2.1.1.1. 3.2.1.2 Location (/OV). The Location (/OV) is the position reference where the phenomenon occurred. It is not the location of the aircraft when the report is submitted. Location can be referenced either by geographical position or by route segment. A position ### Page 59 3-28 reference is preferred by meteorologists to aid forecast precision, monitoring, and verification. 3.2.1.2.1 Location. Location can be referenced to a very high frequency (VHF) Navigational Aid (NAVAID) or an airport, using either the three-letter IATA or four-letter ICAO identifier. If appropriate, the PIREP is encoded using the identifier, then three digits to define a radial and three digits to define the distance in nautical miles. Examples: /OV APE Over Appleton Very High Frequency Omni-Directional Range Station (VOR) /OV KJFK Over John F. Kennedy International Airport, New York, NY /OV APE230010 230 degrees at 10 NM from the Appleton Very High Frequency Omni-Directional Range Station (VOR) /OV KJFK107080 107 degrees at 80 NM from John F. Kennedy International Airport, New York, NY 3.2.1.2.2 Route Segment. A PIREP can also be referenced using two or more fixes to describe a route. Examples: /OV KSTL-KMKC From Lambert-Saint Louis International Airport, MO to Charles B. Wheeler Downtown Airport, Kansas City, MO /OV KSTL090030-KMKC045015 From 90 degrees at 30 NM from Lambert-Saint Louis International Airport, MO to 45 degrees at 15 NM from Charles B. Wheeler Downtown Airport, Kansas City, MO 3.2.1.3 Time (/TM). Time (/TM) is the time that the reported phenomenon occurred or was encountered. It is coded in four digits UTC. Example: /TM 1315 1315 UTC ### Page 60 3-29 3.2.1.4 Altitude/Flight Level (/FL). The altitude/flight level (/FL) is the altitude in hundreds of feet MSL where the phenomenon was first encountered. If not known, UNKN is entered. If the aircraft was climbing or descending, the appropriate contraction (DURC or DURD) is entered in the remarks (/RM). If the condition was encountered within a layer, the altitude range is entered within the appropriate element that describes the condition. Examples: /FL085 8,500 ft MSL /FL310 Flight level 310 /FLUNKN /RM DURC Flight level unknown, remarks, during climb 3.2.1.5 Aircraft Type (/TP). Aircraft type (/TP) is entered. If not known, UNKN is entered. Icing and turbulence reports always include aircraft type. Examples: /TP BE20 Super King Air 200 /TP SR22 Cirrus 22 /TP P28R Piper Arrow /TP UNKN Type unknown 3.2.1.6 Sky Condition (/SK). Sky condition (/SK) group is used to report height of cloud bases, tops, and cloud cover. The height of the base of a layer of clouds is coded in hundreds of feet MSL. The top of a layer is entered in hundreds of feet MSL preceded by the word TOP. If reported as clear above the highest cloud layer, SKC is coded following the reported level. Examples: /BKN040-TOP065 Base of broken layer 4,000 ft MSL, top 6,500 ft MSL /SK OVC100-TOP110/ SKC Base of an overcast layer 10,000 ft MSL, top 11,000 ft MSL, clear above /SK OVC015-TOP035/OVC230 Base of an overcast layer 1,500 ft MSL, top 3,500 ft MSL, base of an overcast layer 23,000 ft MSL /SK OVC-TOP085 Overcast layer, top 8,500 ft MSL ### Page 61 3-30 Cloud cover amount ranges are entered with a hyphen separating the amounts, e.g., BKN-OVC. Examples: /SK SCT-BKN050-TOP100 Base of a scattered to broken layer 5,000 ft MSL, top 10,000 ft MSL /SK BKN-OVCUNKN-TOP060/BKN120-TOP150/ SKC Base of a broken to overcast layer unknown, top 6,000 ft MSL, base of a broken layer 12,000 ft MSL, top 15,000 ft MSL, clear above Unknown heights are indicated by the contraction UNKN. Examples: /SK OVC065-TOPUNKN Base of an overcast layer 6,500 ft MSL, top unknown. If a pilot indicates he/she is in the clouds, IMC is entered. /SK OVC065-TOPUNKN /RM IMC Base of an overcast layer 6,500 ft MSL, top unknown, remark, in the clouds. When more than one layer is reported, layers are separated by a solidus (/). 3.2.1.7 Flight Visibility and Weather (/WX). The pilot reports the weather conditions he or she encounters as follows: Flight visibility, when reported, is entered first in the /WX field. It is coded FV followed by a two-digit visibility value rounded down, if necessary, to the nearest whole statute mile and appended with SM (FV03SM). If visibility is reported as unrestricted, FV99SM is entered. Flig ht weather types are entered using one or more of the surface weather reporting phenomena contained in Table 3-2. Example: /WX FV01SM +DS000-TOP083/SKC /RM DURC Flight visibility 1 sm, base heavy duststorm layer at the surface, top 8,300 ft MSL, clear above, remarks, during climb ### Page 62 3-31 When more than one form of precipitation is combined in the report, the dominant type is reported first. Examples: /WX FV00SM +TSRAGR Flight visibility 0 sm, thunderstorm, heavy rain, hail /WX FV02SM BRHZ000-TOP083 Flight visibility 2 sm, base of a haze and mist layer at the surface, top 8,300 ft MSL If a funnel cloud is reported, it is coded FC following /WX group and is spelled out as Funnel Cloud after /RM group. If a tornado or waterspout is reported, it is coded +FC following /WX group and TORNADO or WATERSPOUT is spelled out after the /RM group. Examples: /WX FC /RM FUNNEL CLOUD Funnel cloud, remarks, funnel cloud /WX +FC /RM TORNADO Tornado, remarks, tornado When more than one type of weather is reported, they are reported in the following order: • TORNADO, WATERSPOUT, or FUNNEL CLOUD. • Thunderstorm with or without associated precipitation. • Weather phenomena in order of decreasing predominance. No more than three groups are used in a single PIREP. Weather layers are entered with the base and/or top of the layer when reported. The same format as in the sky condition (/SK) group is used. Example: /WX FU002-TOP030 Base of a smoke layer, 200 ft MSL, top 3,000 ft MSL 3.2.1.8 Air Temperature (/TA). Outside air temperature (/TA) is reported using two digits in degrees Celsius. Negative temperatures is prefixed with an M (e.g., /TA 08 or /TA M08). 3.2.1.9 Wind Direction and Speed (/WV). Wind direction and speed are encoded using three digits to indicate wind direction, relative to true north, and two or three digits to indicate reported wind speed. When the reported speed is less than 10 kts, a leading 0 is used. The wind group will always have KT appended to represent the units in knots. ### Page 63 3-32 Examples: /WV 02009KT Wind 20 degrees at 9 kts /WV 28057KT Wind 280 degrees at 57 kts /WV 350102KT Wind 350 degrees at 102 kts 3.2.1.10 Turbulence (/TB). Turbulence intensity, type, and altitude are reported after wind direction and speed. Duration (intermittent (INTMT), occasional (OCNL), or continuous (CONS)) is coded first (if reported by the pilot), followed by the intensity (light (LGT), moderate (MOD), severe (SEV), or extreme (EXTRM)). Range or variation of intensity is separated with a hyphen (e.g., MOD-SEV). If turbulence was forecast, but not encountered, negative (NEG) is entered. Type is coded second. Clear Air Turbulence (CAT) or CHOP is entered if reported by the pilot. High-level turbulence not associated with clouds (including thunderstorms) is reported as CAT. Altitude is reported (last) only if it differs from value reported in the altitude/flight level (/FL) group. When a layer of turbulence is reported, height values are separated with a hyphen. If lower or upper limits are not defined, BLO or ABV is used. Examples: /TB LGT Light turbulence /TB LGT 040 Light turbulence at 4,000 ft MSL /TB OCNL MOD-SEV BLO 080 Occasional moderate to severe turbulence below 8,000 ft MSL /TB MOD-SEV CAT 350 Moderate to severe CAT at 35,000 ft MSL /TB NEG 120-180 Negative turbulence between 12,000 ft and 18,000 ft MSL /TB CONS MOD CHOP 220/NEG 230-280 Continuous moderate chop at 22,000 feet MSL, negative turbulence between 23,000 ft and 28,000 ft MSL /TB MOD CAT ABV 290 Moderate CAT above 29,000 ft MSL ### Page 64 3-33 Turbulence reports should include location, altitude (or range of altitudes), and aircraft type, as well as, when reported, whether in clouds or clear air. The pilot determines the degree of turbulence, intensity, and duration (occasional, intermittent, and continuous). The report should be obtained and disseminated, when possible, in conformance with the Turbulence Reporting Criteria Table in the AIM, Chapter 7, Section 1. 3.2.1.11 Icing (/IC). Icing intensity, type, and altitude are reported after turbulence. Intensity is coded first using contractions TRACE, light (LGT), moderate (MOD), or severe (SEV). Reports of a range or variation of intensity is separated with a hyphen. If icing was forecast but not encountered, negative (NEG) is coded. Icing type is reported second. Reportable types are RIME, clear (CLR), or mixed (MX). The AIM, Chapter 7, Section 1 provides classification of icing intensity according to its operational effects on aircraft, as well as tables of icing types and icing conditions. The reported icing/altitude is coded (last) only if different from the value reported in the altitude/flight level (/FL) group. A hyphen is used to separate reported layers of icing. Above (ABV) or below (BLO) is coded when a layer is not defined. Pilot reports of icing should also include air temperature (/TA). Examples: /IC LGT-MOD MX 085 Light to moderate mixed icing, 8,500 ft MSL /IC LGT RIME Light rime icing /IC MOD RIME BLO 095 Moderate rime icing below 9,500 ft MSL /IC SEV CLR 035-062 Severe clear icing 3,500 ft to 6,200 ft MSL 3.2.1.12 Remarks (/RM). The remarks (/RM) group is used to report a phenomenon that is considered important but does not fit in any of the other groups. This includes, but is not limited to, LLWS reports, thunderstorm lines, coverage and movement, lightning, sulfur dioxide (SO2) gas smell, clouds observed but not encountered, and geographical or local descriptions of where the phenomenon occurred. Hazardous weather is reported first. LLWS is described to the extent possible. ### Page 65 3-34 3.2.1.12.1 Windshear. Ten kts or more fluctuations in wind speed (+/-10KTS), within 2,000 ft of the surface, require an Urgent (UUA) pilot report. When LLWS is entered in a pilot report, LLWS is entered as the first remark in the remarks (/RM) group. Example: /RM LLWS +/-15 KT SFC-008 DURC RY22 JFK Remarks, LLWS, air speed fluctuations of plus or minus 15 kts, surface to 800 ft during climb, Runway 22, John F. Kennedy International Airport, NY. 3.2.1.12.2 FUNNEL CLOUD, TORNADO, and WATERSPOUT. Funnel cloud, tornado, and waterspout are entered with the direction of movement, when reported. Example: /RM TORNADO W MOV E Remarks, tornado west moving east 3.2.1.12.3 Thunderstorm. Thunderstorm coverage is coded as isolated (ISOL), few (FEW), scattered (SCT), numerous (NMRS) followed by description as line (LN), broken line (BKN LN), solid line (SLD LN), when reported. This is followed with TS, the location and movement, and the type of lightning, when reported. Example: /RM NMRS TS S MOV E Remarks, numerous thunderstorms south moving east 3.2.1.12.4 Lightning. Lightning frequency is coded as occasional (OCNL) or frequent (FRQ), followed by type as lightning in cloud (LTGIC), lightning cloud to cloud (LTGCC), lightning cloud to ground (LTGCG), lightning cloud to air (LTGCA), or combinations, when reported. Example: /RM OCNL LTGICCG Remarks, occasional lighting in cloud, cloud to ground 3.2.1.12.5 Electrical Discharge. For an electrical discharge, DISCHARGE is coded followed by the altitude. ### Page 66 3-35 Example: /RM DISCHARGE 120 Remarks, discharge, 12,000 ft MSL 3.2.1.12.6 Clouds. Remarks are used when clouds can be seen but were not encountered and reported in the sky condition group (/SK). Examples: /RM CB E MOV N Remarks, cumulonimbus east moving north /RM OVC BLO Remarks, overcast below 3.2.1.12.7 Plain Language. If specific phraseology is not adequate, plain language is used to describe the phenomena or local geographic locations. Remarks that do not fit in other groups like during climb (DURC), during descent (DURD), reach cruising altitude (RCA), top of climb (TOP or TOC), are included. Examples: /RM DONNER SUMMIT PASS 3.2.1.12.8 Volcanic Eruptions. Volcanic ash alone is an Urgent PIREP. A report of volcanic activity includes as much information as possible including the name of the mountain, ash cloud and movement, height of the top and bottom of the ash, etc., is included. Example: /UUA/OV ANC240075/TM 2110/FL370/TP DC10/WX VA/RM VOLCANIC ERUPTION 2008Z MT AUGUSTINE ASH 40S MOV SSE Urgent Pilot Weather Report, 240 degrees at 75 NM from Anchorage International Airport, AK, 2110 UTC, flight level 370, a DC10 reported volcanic ash, remarks, volcanic eruption occurred at 2008 UTC Mount Augustine, ash 40 NM south moving south-southeast. **3.2.1.12.9 SKYSPOTTER.** The SKYSPOTTER program is a result of a recommendation from the Safer Skies FAA/Industry Joint Safety Analysis and Implementation Teams. The term SKYSPOTTER indicates a pilot has received specialized training in observing and reporting in-flight weather phenomenon or PIREPs. When a PIREP is received from a pilot identifying themselves as a SKYSPOTTER aircraft, the additional comment “/AWC” is added at the end of the remarks section of the PIREP. ### Page 67 3-36 An AWC-WEB/xxxx in the remarks indicates the pilot report was submitted by an airline dispatcher or Center Weather Service Unit (CWSU) meteorologist directly to the Aviation Weather Center (AWC). The xxxx represents the airline abbreviation or Air Route Traffic Control Center (ARTCC) of the CWSU that submitted the PIREP. Example: **PIREP TEXT/RM REMARKS/AWC** **PIREP TEXT/RM REMARKS/AWC-WEB/KZFW** 3.2.2 Air-Reports (AIREP). AIREPs are messages from an aircraft to a ground station. AIREPs are normally comprised of the aircraft’s position, time, flight level (FL), estimated time of arrival (ETA) over its next reporting point, destination ETA, fuel remaining, and meteorological information. It is beyond the scope of this AC to describe the details of all the elements in the AIREP, but this paragraph will focus on the meteorological information. The AWC’s website provides AIREPs over the CONUS and portions of the Atlantic and Pacific Oceans. 3.2.2.1 AIREP Types and Content. There are two types of AIREPs, routine or position report (ARP) and special (ARS). AIREPs can be reported by the pilot but the majority of routine AIREPs are automated and downlinked from the aircraft to a service provider, e.g., a flight planning company, for processing and forwarding to an airline and the NWS. The majority of AIREPs report wind and temperature at selected intervals along the flight route, derived from onboard sensors and probes. Some aircraft are equipped with sensors and probes to measure humidity/water vapor, turbulence, and icing data. The format for the AIREP is governed by ICAO. The AWC’s website includes AIREPs on their PIREP Web page that is formatted for Web display with some weather elements decoded. 3.2.2.2 AIREP Examples. The following examples are from https://www.aviationweather.gov. The actual airline’s call sign was replaced with a fictitiou s call sign and the Special AIREP was created from the routine report. Routine AIREP Example: **ARP XXX836 2443N 15516W 2229 F350 M43 315/128 TB LGT** ### Page 68 3-37 ARP Routine report XXX836 Aircraft call sign 2423N 15516W Location in latitude and longitude, 24 degrees 23 minutes north, 155 degrees 16 minutes west F350 Flight level or altitude, FL350 M43 Temperature in Celsius, minus 43 ºC 315/128 Wind direction (true) and speed, 315 degrees (true) and 128 kts TB LGT Light turbulence Special AIREP Example: ARS XXX836 2443N 15516W 2229 F350 M43 315/128 TB SEV ARS Special AIREP Same as the routine example except: TB SEV Severe turbulence 3.3 Radar Observations. 3.3.1 Description. Weather radar observations and their resultant images are graphical displays of precipitation and non-precipitation targets detected by weather radars. Weather Surveillance Radar–1988 Doppler (WSR-88D), also known as next generation weather radar (NEXRAD), displays these targets on a variety of products, which can be found on the websites of all NWS Weather Forecast Offices (WFO), the AWC, Storm Predication Center (SPC), and various flight planning and weather service providers. 3.3.2 Issuance. WSR-88D radars are continuously generating radar observations. Each radar observation, called a volume scan, consists of 5 to 14 separate elevation “tilts,” and takes between 4 and 11 minutes to generate, depending on the radar’s mode of operation. Once one observation is complete, the next one begins. Radar observation times are not standard, nor are they synchronized with other radars. The valid time of the observation is the time assigned to the product, which is the end of the last radar scan. 3.3.2.1 WSR-88D Radar (NEXRAD) Network. The WSR-88D radar network consists of 160 radars operated by the NWS, FAA, and Department of Defense (DOD). Figure 3-6, Location of WSR-88D Weather Radar in the CONUS and Their Respective Coverage at 4,000 Feet AGL, 6,000 Feet A GL, and 10,000 Feet AGL, and Figure 3-7, Location of ### Page 69 3-38 WSR-88D Weather Radar Outside of the CONUS, show the location of the radars. Figure 3-6. Location of WSR-88D Weather Radar in the CONUS and Their Respective Coverage at 4,000 Feet AGL, 6,000 Feet AGL, and 10,000 Feet AGL ### Page 70 3-39 Figure 3-7. Location of WSR-88D Weather Radar Outside of the CONUS 3.3.2.2 Coverage. Figu re 3-6 depicts the radar coverage over the CONUS at 4,000 ft AGL, 6,000 ft AGL, and 10,000 ft AGL (i.e., above the height of the radar). Several WSR-88D radars are located in mountainous areas, such as the western United States. For example, the radar in southern Utah (near Cedar City) is on top of a 10,000-foot mountain. This means that the coverage begins at 10,000 ft AGL in that area. Any precipitation from low-topped clouds would not be detected by this radar due to overshooting of the radar beam (see paragraph 3.3.7.1). 3.3.3 Modes of Operation. The WSR-88D employs scanning strategies in which the antenna automatically raises to higher and higher preset angles, or elevation scans, as it rotates. These elevation scans comprise a volume coverage pattern (VCP). Once the radar sweeps through all elevation slices, a volume scan is complete. The WSR-88D radar can use several VCPs. There are two main classes of VCPs, which are commonly referred to as Clear Air and Precipitation Modes. ### Page 71 3-40 3.3.3.1 Clear Air Mode. In Clear Air Mode, the radar is in its most sensitive operation. This mode has the slowest antenna rotation rate, which permits the radar to sample the atmosphere longer. This slower sampling increases the radar’s sensitivity and ability to detect smaller objects in the atmosphere. The term “clear air” does not imply “no precipitation” mode. Even in Clear Air Mode, the WSR-88D can detect light, stratiform precipitation (e.g., snow) due to the increased sensitivity. Many of the radar returns in Clear Air Mode are airborne dust and particulate matter. The WSR-88D images are updated approximately every 10 minutes when operating in this mode. 3.3.3.2 Precipitation Mode. Precipitation targets typically provide stronger return signals to the radar than non-precipitation targets. Therefore, the WSR-88D is operated in Precipitation Mode when precipitation is present, although some non-precipitation echoes can still be detected in this operating mode. The faster rotation of the WSR-88D in Precipitation Mode allows images to update at a faster rate approximately every 4 to 6 minutes. 3.3.4 Echo Intensities. Figure 3-8. WSR-88D (NEXRAD) Weather Radar Echo Intensity Legend The colors on radar images represent the reflective power of the precipitation target. In general, the amount of radar power received is proportional to the intensity of the precipitation. This reflective power, commonly referred to by meteorologists as ### Page 72 3-41 “ref lectivity,” is measured in terms of decibels (dBZ). A decibel is a unit that describes the change of power emitted versus the power received. Since the power emitted is constant, the power received is related to the intensity of the precipitation target. Each reflectivity image includes a color scale that describes the relationship among reflectivity value, color on the radar image, and precipitation intensity. Figure 3-8, WSR-88D (NEXRAD) Weather Radar Echo Intensity Legend, depicts the correlations for most NW S radar images. The scale ranges from -30 dBZ to greater than 75 dBZ. The scale also includes ND correlated to black, which indicates no data was measured. The colors and decibel scale can vary depending on the service provider and website. Reflectivity is correlated to intensity of precipitation. For example, in Precipitation Mode, when the decibel value reaches 15, light precipitation is present. The higher the indicated reflectivity value, the higher the rainfall rate. The interpretation of reflectivity values is the same for both Clear Air and Precipitation Modes. Reflectivity is also correlated with intensity terminology (phraseology) for air traffic control (ATC) purposes. Table 3-7, WSR-88D Weather Radar Precipitation Intensity Terminology, defines this correlation. Table 3-7. WSR-88D Weather Radar Precipitation Intensity Terminology Reflectivity (dBZ) Ranges Weather Radar Echo Intensity Terminology <30 dBZ Light 30-40 dBZ Moderate >40-50 dBZ Heavy 50+ dBZ Extreme Note: En route ATC radar’s weather and radar processor (WARP) does not display light precipitation. Values below 15 dBZ are typically associated with clouds. However, they may also be caused by atmospheric particulate matter such as dust, insects, pollen, or other phenomena. The scale cannot reliably be used to determine the intensity of snowfall. However, snowfall rates generally increase with increasing reflectivity. 3.3.5 Radar Products. The NWS produces many radar products that serve a variety of users. Some of these products are of interest to the aviation community. This paragraph will discuss radar mosaics, Composite Reflectivity, Base Reflectivity, and the radar coded message products. 3.3.5.1 Radar Mosaic. A radar mosaic consists of multiple single-site radar images combined to produce a radar image on a regional or national scale. Regional and national mosaics can be found on the websites of NWS, AWC, all NWS WFOs, as well as commercial aviation weather providers. ### Page 73 3-42 Rad ar mosaics can be assembled from either Composite Reflectivity (see paragraph 3.3.5.2) o r Base Reflectivity (see paragraph 3.3.5.3), depending on the website or data provider. At this time, NWS national (see Figur e 3-9, NWS National Radar Mosaic Example, Which Utilizes NEXRAD Base Reflectivity) and regional (see Figure 3-10, NWS Regional Radar Mosaic Sector Example, Which Utilizes NEXRAD Base Reflectivity), and Al aska (see Figure 3-11, Alaska Radar Mosiac Example) radar mosaic sectors are assembled using only B ase Reflectivity data (0.5º radar beam angle with a 124 NM range) and are set up to display all echoes (precipitation and non-precipitation). Most commercial aviation weather providers use Composite Reflectivity for their mosaics and configure the display to eliminate most non-precipitation echoes. NEXRAD radar data data-linked to aircraft cockpit displays via FAA Flight Information Service-Broadcast (FIS-B) use the Composite Reflectivity data for their radar mosaics. Figure 3-9. NWS National Radar Mosaic Example, Which Utilizes NEXRAD Base Reflectivity ### Page 74 3-43 Figure 3-10. NWS Regional Radar Mosaic Sector Example, Which Utilizes NEXRAD Base Reflectivity F igure 3-11. Alaska Radar Mosaic Example Many areas of Alaska do not have radar coverage. These areas are shaded gray. ### Page 75 3-44 3.3.5.2 C omposite Reflectivity. Because the highest precipitation intensity can be at any altitude, the Composite Reflectivity product (see Figure 3-12, WSR-88D Weather Radar Composite Reflectivity, Single-Site Product Example) is needed. Composite Reflectivity is the maximum echo intensity (reflectivity) detected within a column of the atmosphere above a location. During its tilt sequence, the radar scans through all of the elevation slices to determine the highest decibel value in the vertical column (see Figure 3-13, Creation of a Composite Reflectivity, Single-Site Product), then displays that value on the product. When compared w ith Base Reflectivity, the Composite Reflectivity can reveal important storm structure features and intensity trends of storms (see Figure 3-14, Weather Radar 0.5º Base Reflectivity (left) versus Composite Reflectivity (rig ht) Comparison). NEXRAD radar displays on airplane avionics use the Composite Reflectivity data for their radar mosaics. Figure 3-12. WSR-88D Weather Radar Composite Reflectivity, Single-Site Product Example ### Page 76 3-45 Figure 3-13. Creation of a Composite Reflectivity, Single-Site Product The Composite Reflectivity product displays the highest reflectivity of all elevation scans. Figure 3-14. Weather Radar 0.5º Base Reflectivity (left) versus Composite Reflectivity (right) Comparison This Composite Reflectivity shows that in many locations the highest precipitation intensity occurs at an altitude higher than precipitation detected at the elevation of the base elevation angle. 3.3.5.3 Base Reflectivity. Base Reflectivity product is a display of both the location and intensity of reflectivity data from the lowest elevation angle, or 0.5º above the horizon. ### Page 77 3-46 The Base Reflectivity product is one elevation scan, whereas Composite Reflectivity looks at all elevation scans. Base Reflectivity products are available several minutes sooner than Composite Reflectivity products. Prec ipitation at any location may be heavier than depicted on the Base Reflectivity image because it is occurring above the lowest elevation angle. Both a short-range (see Figure 3-15, WSR-88D Weather Radar Short-Range (124 NM) Base Reflectivity, Single Site Product Example) and long-range (see Figure 3-16, WSR-88D Weather Radar Long-Range (248 NM) Base Reflectivity, Single Site Product Example) image are available from the 0.5º Bas e Reflectivity product. The maximum range of the short-range, single-site radar Base Reflectivity product is 124 NM from the radar location. Long-range, single-site, Base Reflectivity product’s range is 248 NM from the radar location. When using a single-site radar, i.e., not using a radar mosaic, echoes farther than 124 NM (short-range) or 248 NM (long-range) from the radar site will not be displayed, even if precipitation may be occurring at these greater distances. Figure 3-15. WSR-88D Weather Radar Short-Range (124 NM) Base Reflectivity, Single-Site Product Example ### Page 78 3-47 Figure 3-16. WSR-88D Weather Radar Long-Range (248 NM) Base Reflectivity, Single-Site Product Example 3.3.5.4 Radar Coded Message (RCM). With the deployment of the WSR-88D (NEXRAD) radar network in the 1990s, the manual radar observations, known by the acronyms ROB, SD, and RAREP, was replaced by the RCM. The RCM is the encoded and transmitted report of radar features observed by a WSR-88D radar. The RCM is generated automatically and has no human input or oversight. The actual RCM is highly detailed, complicated, and not intended for use by the pilot or operator in raw format. However, the RCM is used to generate certain radar charts or displays, such as “The Current RCM Radar Plot,” available on https://www.aviationweather.gov (see Figur e 3-17, Radar Coded Message (RCM) Display), as well as other weather service provider’s websites . The RCM display is updated every 30 minutes. The display is an image representation of the NEXRAD radar Composite Reflectivity overlaid with cloud and echo tops and centroid movement. The RCM display includes the maximum echo top for each radar’s area of coverage. The other tops shown on the display are derived from the satellite images. ### Page 79 3-48 The echo tops shown on the display can be erroneous in some cases. The algorithm computes echo tops (the altitude of echoes greater than or equal to 18 dBZ) within the range of the radar. This can lead to spurious values when precipitation is far from the site. Values greater than 50,000 ft (500 ft on the plot) can be disregarded, especially if their locations do not correspond to any precipitation. Figure 3-17. Radar Coded Message (RCM) Display The display is an image representation of the NEXRAD radar Composite Reflectivity overlaid with cloud and echo tops and centroid movement. 3.3.6 Precipitation. 3.3.6.1 Convective Precipitation. Convective precipitation (see Figure 3-18, WSR-88D Weather Radar Convective Precipitation on the 0.5º Base Reflectivity Product Example) is disting uished by the following radar characteristics: • Echoes tend to form as lines or cells, • Reflectivity gradients are strong, ### Page 80 3-49 • Precipitation intensities generally vary from moderate to extreme, • Occasionally precipitation intensities can be light, and • Echo patterns change rapidly when animating the image. Numerous hazards are associated with Convective precipitation. They include: turbulence, LLWS, strong and gusty surface winds, icing above the freezing level, hail, lightning, tornadoes, and localized instrument flight rules (IFR) conditions below the cloud base due to heavy precipitation. Figure 3-18. WSR-88D Weather Radar Convective Precipitation on the 0.5º Base Reflectivity Product Example 3.3.6.2 Stra tiform Precipitation. Stratiform precipitation (see Figure 3-19, WSR-88D Weather Radar Stratiform Precipitation on the 0.5º Base Reflectivity Product Example) has the fo llowing radar characteristics: • Widespread in aerial coverage; • Weak reflectivity gradients; • Precipitation intensities are generally light or moderate (39 dBZ or less); • Occasionally, precipitation intensities can be stronger; and • Echo patterns change slowly when animating the image. ### Page 81 3-50 Figure 3-19. WSR-88D Weather Radar Stratiform Precipitation on the 0.5º Base Reflectivity Product Example Hazards associated with stratiform precipitation include possible widespread icing above the freezing level, low ceilings, and reduced visibilities. 3.3.7 Limitations. Limitations associated with Composite Reflectivity and Base Reflectivity include: • The radar beam may overshoot or undershoot targets, and • The image may be contaminated by: o Beam blockage, o Ground clutter, o Anomalous Propagation (AP), o Ghosts, o Angels, and/or o Other non-meteorological phenomena. 3.3.7.1 Radar Beam Overshooting and Undershooting. Radar beam overshooting may occur because the lowest radar beam (typically the 0.5 degree elevation) can be higher than the top of precipitation. This will most likely occur with stratiform precipitation and low-topped convection. For ### Page 82 3-51 example, at a distance of 124 NM from the radar, the 0.5º radar beam is at an altitude of approximately 18,000 ft; at 248 NM the beam height is approximately 54,000 ft. Any precipitation with tops below these altitudes and distances will not be displayed on a single-site radar image. Therefore, it is quite possible that precipitation may be occurring where none appears on the single-site radar image. Radar overshooting occurs more often in the mountainous western United States where some radars are located on a mountain top (e.g., the WSR-88D near Cedar City, UT). Radar undershooting occurs when precipitation occurs above Base Reflectivity beam, usually with high-cloud based precipitation near the radar site. This often occurs in the western United States during the summer months. Undershooting may occur at and near the radar site even in Composite Reflectivity products when the precipitation is above the highest elevation angle. This region above the radar is known as the “cone of silence” (see Figure 3-20, Cone of Silence). Figure 3-20. Cone of Silence 3.3.7.2 Beam Blockage. Beam blockage (see Figure 3-21, WSR-88D Weather Radar Beam Blockage on Base Reflectivity Product E xample) occurs when the radar beam is blocked by terrain and is particularly predominant in mountainous terrain. Beam blockage is most easily seen on the 0.5º Base Reflectivity images where it appears as a pie-shaped area (or areas) perpetually void of echoes. When animating the imagery, the beam blockage area will remain clear of echoes even as precipitation and other targets pass through. In many cases, the beam blockage effect seen on a single-site radar can be minimized by viewing mosaic images. ### Page 83 3-52 Figure 3-21. WSR-88D Weather Radar Beam Blockage on Base Reflectivity Product Example 3.3.7.3 Ground Clutter. Ground clutter (see Figure 3-22, WSR-88D Weather Radar Ground Clutter Example) is radar echoes’ returns from trees, buildings, or other objects on the ground. It appears as a roughly circular region of high reflectivity at ranges close to the radar. Ground clutter appears stationary when animating images and can mask precipitation located near the radar. Most ground clutter is automatically removed from WSR-88D imagery, so typically it is does not interfere with image interpretation. Figure 3-22. WSR-88D Weather Radar Ground Clutter Example ### Page 84 3-53 3.3.7.4 Ghost. A ghost (see Figure 3-23, WSR-88D Weather Radar Ghost Example) is a diffused echo in apparently clear air caused by a “cloud” of point targets, such as insects or by refraction returns of the radar beam in truly clear air. The latter case commonly develops at sunset due to super-refraction during the warm season. The ghost develops as an area of low reflectivity echoes (typically less than 15 dBZ) near the radar site and quickly expands. When animating the imagery, the ghost echo shows little movement. Figure 3-23. WSR-88D Weather Radar Ghost Example 3.3.7.5 Angels . Angels are echoes caused by a physical phenomenon not discernible by the eye at the radar site. They are usually caused by bats, birds, or insects. Angels typically appear as a donut-shaped echo with low reflectivity values (see Figure 3-24, WSR-88D Weather Radar Angel Example). When animated, the echo expands and becomes more diffuse with time. Angels typically only appear when the radar is in Clear Air Mode because of their weak reflectivity. Echoes caused by birds are typically detected in the morning, when they take flight for the day. Echoes caused by bats are typically detected in the evening, when they are departing from caves. ### Page 85 3-54 Figure 3-24. WSR-88D Weather Radar Angel Example This angel was caused by bats departing Selman Bat Cave at Alabaster Caverns State Park, OK around sunset. 3.3.7.6 Anomalous Propagation (AP). AP (see Figure 3-25, WSR-88D Weather Radar Anomalous Propagation (AP) Example) is an extended pattern of ground echoes caused by superrefraction of th e radar beam. Superrefraction causes the radar beam to bend downward and strike the ground. It differs from ground clutter because it can occur anywhere within the radar’s range, not just at ranges close to the radar. AP typically appears as speckled or blotchy, high-reflectivity echoes. When animating images, AP tends to “bloom up” and dissipate, and has no continuity of motion. AP can sometimes be misinterpreted as thunderstorms; differentiating between the two is determined by animating the images. Thunderstorms move with a smooth, continuous motion while AP appears to “bloom up” and dissipate randomly. ### Page 86 3-55 Figure 3-25. WSR-88D Weather Radar Anomalous Propagation (AP) Example 3.3.7.7 O ther Non-Meteorological Phenomena. 3.3.7.7.1 Wind Farms. Wind farms can affect the return signal of the radar beam. Depending on the proximity of the wind farm to the radar site (generally within 10 NM), wind farm turbines can result in beam blockage, false echoes, or high reflectivity values (see Figure 3-26, Wind Farms Have Been Known to Make Benign Echoes Appear as Small Storms. This Example Is from the Buffalo Radar). ### Page 87 3-56 Figure 3-26. Wind Farms Have Been Known to Make Benign Echoes Appear as Small Storms. This Example Is from the Buffalo Radar 3.3.8 Use. Radar mosaic products based on Composite Reflectivity radar data provide a more complete depiction of precipitation than mosaics based on Base Reflectivity data. 3.3.9 Future Products. The NWS is working on two new types of radar displays that are not available for description in time for publication. 3.3.9.1 Product Update Rate. Today, composite radar images are updated after volume scans are complete (about every 5 minutes). In the future, composite radar products will be updated with every tilt sweep. 3.3.9.2 Mosaicked Composite Reflectivity. The NWS will add mosaics of Composite Reflectivity. This is especially important for aviation as the heaviest precipitation at any altitude in the ### Page 88 3-57 vertical will be depicted in mosaics. Today, only the base elevation angle (i.e., closest to the ground) contributes to the mosaic. 3.3.9.3 Hydr ometeorological classification. The implementation of dual polarization radar across the NEXRAD system means more information about detected targets can be observed. For more information on what dual polarization is, refer to Dual-Polarization Radar Training for NWS Partners provided by the NWS Warning Decision Training Division at https://training.weather.gov/wdtd/courses/dualpol/Outreach/. Dual polarization allows the NWS to make a best guess at the precipitation type detected from a pre-defined list. The precipitation types of interest to aviation include: • Graupel (also known as snow pellets). Graupel is ice crystals encased in a layer of ice. When graupel hits the ground, you can sometimes see it bounce. Graupel is formed when small cores of ice crystals fall through a region of supercooled (below freezing) liquid water droplets. These conditions are also favorable for airframe icing, thus detection of graupel implies potential icing conditions above the freezing level. • Hail (or hail mixed with rain). Research is continuing to identify hail size from dual-polarization radar information. 3.4 Terminal Doppler Weather Radar (TDWR). The TDWR network is a Doppler weather radar system operated by the FAA, which is used primarily for the detection of hazardous windshear conditions, precipitation, and winds aloft on and near major airports situated in climates with great exposure to thunderstorms in the United States. The National Centers for Environmental Information (NCEI) archives the products produced by the NWS Supplemental Product Generator (SPG) (called Level III) which are in the same data format as NEXRAD Level III. NCEI does not archive the base data (called Level II). 3.4.1 History. TDWR was developed in the early 1990s at Lincoln Laboratory, which is part of the Massachusetts Institute of Technology (MIT). The TDWR system was funded by the FAA to assist air traffic controllers by providing real-time windshear detection and high-resolution precipitation data. In 2004, the NWS established the policy with the FAA to allow the operational use of TDWR data and began to develop the SPG. Initial deployment of the SPG began in 2005 and finished in 2008. As of 2016, there were 47 operational TDWR-SPG radar systems in major metropolitan locations across the United States and Puerto Rico. ### Page 89 3-58 3.4.2 Data Access. 3.4.2.1 Radar Data Access. Weather radar data at NCEI are stored on a tape archive system and are accessed by placing orders through the NCEI website. There are several ordering systems available, optimized for different situations, including small orders, multisite and multiday orders, and large bulk orders. Currently, TDWR-SPG data are only available from the “Select by Site” and “Select by File” ordering systems. Small orders are typically completed in less than 15 minutes. There are occasional gaps and missing data for each site in the archive. These gaps are caused by a number of reasons, including scheduled maintenance at the radar sites, unplanned downtime due to severe weather, communications problems, or archival problems. The data access web pages include lists or visualizations of file availability. 3.4.3 Products. 3.4.3.1 TDWR-SPG Products. This complete list of all available data products (called Level-III products) includes descriptions and possible uses. There are 26 TDWR-SPG Level-III products routinely available from NCEI, including precipitation estimates, storm relative velocity, and echo tops. 3.4.3.2 Comparison of the TDWR to the WSR-88D. The range resolution of the TDWR is finer than what is available in the WSR-88D or any other FAA radar that has weather channel capability. The TDWR utilizes a range gate resolution of 150 m for Doppler data. It has a resolution of 150 m for reflectivity data within 135 kilometers (km) and 300 m from beyond 135 km to 460 km. By contrast, the WSR-88D employed by the NWS, FAA, and DOD has a maximum range gate resolution of 250 m for Doppler and 1 km for surveillance data. The angular (azimuth) resolution of the TDWR is nearly twice what is available in the WSR-88D. Each radial in the TDWR has a beam width of 0.55 degrees. The average beam width for the WSR-88D is 0.95 degrees. The following table shows a comparison of technical specifications between the TDWR and the WSR-88D. ### Page 90 3-59 Table 3-8. Technical Specifications: WSR-88D vs. TDWR WSR-88D TDWR Wavelength 10 cm 5 cm Volume Scan Time 4 minutes in Volume Coverage Pattern (VCP) 12 1 min 0.2 degree, Hazard (HAZ) Bean Width 1.25 degrees 0.5 degrees Range Gate 0.13 NM in velocity 0.54 NM in reflectivity 0.067 NM Max Unambiguous Velocity Up to 62 kts 20–30 kts Max Doppler Range 230 km 90 km The National Oceanic and Atmospheric Administration’s (NOAA) Radar Operations Center (ROC) has detailed information (http://www.roc.noaa.gov/). 3.4.4 How Does the Radar Collect Data? The FAA designed the TDWR to look for low-altitude phenomena, such as wind shifts over the runways, windshear along the immediate approach and departure corridors, and downbursts. These radars are located close to major airports with the scanning strategy optimized to sample the atmosphere over its associated airport. The TDWR employs a scanning strategy called monitor mode, which is similar to the WSR-88D clear-air mode. The TDWR remains in monitor mode until one of the two following conditions are recognized: 1. A region of 20 dBZ echoes located within 24.3 NM from the associated airport with a nominal areal extent of 1.3 NM and an altitude of at least 1.3 NM AGL, or 2. The radar detects windshear or a microburst. This is a departure from the WSR-88D operations that can switch from clear-air mode to precipitation mode if the areal coverage of precipitation exceeds an adaptable parameter anywhere on the scope. The TDWR ties its decision on the area of influence directly to its associated airport. 3.4.5 Volume Coverage Pattern (VCP) 90. VCP 90 is the TDWR monitor mode (clear air) and consists of 17 scans in about 6 minutes. The first cut is always a low Pulse Repetition Frequency (PRF), long-range scan (276 km). All remaining scans are short range (90 km). There is a small variation in the elevation angles of the lower scans, but all scans above 5 degrees are the same in all systems. Cuts two and three employ a split cut strategy. Cut two contains no dealiased Doppler data. Cut three (at the same elevation as cut two) is the first cut to contain dealiased Doppler velocity data. The first three cuts are used to initialize wind field models and velocity unfolding algorithms. ### Page 91 3-60 3.4.6 Volume Coverage Pattern (VCP) 80. VCP 80 is the TDWR hazardous mode (precipitation mode). The first cut is always a low PRF, long-range scan (276 km). Cuts two and three are the short-range, split cut scans used to collect information for unfolding and clutter migration algorithms. The first cut with dealiased Doppler data is cut three. There are two sub-volumes or aloft scans contained in each full volume, as denoted by the use of red and orange. Every fourth scan, the TDWR provides a short-range elevation scan at the same elevation as cuts two and three. 3.4.7 Where Can I Get Details on TDWR-SPG? 1. NOAA’s ROC maintains papers, presentations, and detailed information on the TDWR-SPG system (https://www.roc.noaa.gov/spg/default.aspx). 2. Details on a subset of the TDWR-SPG products are available in the Federal Meteorological Handbook No. 11 (FMH-11), Doppler Radar Meteorological Observations (WSR-88D). The handbooks are availabl e for download from the Office of the Federal Coordinator for Meteorology (OFCM) (http://www.ofcm.gov/). 3. NCEI provides descriptions and possible uses of radar products generated from the TDWR network operated by the FAA. All data are available digitally at NCEI for free (visit http://www.ncdc.noaa.gov/nexradinv/) and paper copies are available for purchase. Certification of the paper data is al so available at https://www.ncdc.noaa.gov/customer-support/certifica tion-data. 3.4.8 TDWR Level-II (Base) Data. The TDWR Level-II Base Data is not available from NCEI. 3.4.9 TDWR Level-III Products. There are 26 TDWR Level-III products routinely available from NCEI. Most Level-III products are available as digital images, color hard copy, grayscale hard copy, or acetate overlay copies. Each copy will include state, county, and city background maps. The Certification of Data page (https://www.ncdc.noaa.gov/customersupport/certification-data) has further information on hard copy radar products, pricing, and certification information. A detailed list of Lev el-III product codes is also available at http://www1.ncdc.noaa.gov/pub/data/radar/RadarProductsDetailedTable.pdf. Brief descriptions and possible uses of these products are included below. 3.4.9.1 General Products. General products include Base Reflectivity and Base Velocity, as well as graphical products derived from algorithms, including Spectrum Width, Vertically Integrated Liquid, and the Velocity Azimuth Display (VAD) Wind Profile. 3.4.9.1.1 Digital Base Reflectivity (TR0, TR1, TR2/181). A display of echo intensity measured in decibels. These products are used to detect precipitation, evaluate storm structure, locate boundaries, and ### Page 92 3-61 determine hail potential. The three lowest elevation angles are available, with a maximum range of 48 NM. 3.4.9.1.2 Digital Base Velocity (TV0, TV1, TV2/182). A measure of the radial component of the wind either toward the radar (negative values) or away from the radar (positive values). Cool colors (green) represent negative values and warm colors (red) represent positive values. These products are used to estimate wind speed and direction, locate boundaries, locate severe weather signatures, and identify suspected areas of turbulence. The three lowest elevation angles are available, with a maximum range of 48 NM. 3.4.9.1.3 Long Range Digital Base Reflectivity (TZL/186). Digital Reflectivity data are measured at an elevation angle of 0.6 degrees with a maximum range of 225 NM. 3.4.9.1.4 Composite Reflectivity (NCR/37). A display of maximum reflectivity for the total volume within the range of the radar. This product is used to reveal the highest reflectivity in all echoes, examine storm structure features, and determine intensity of storms. 3.4.9.1.5 Vertically Integrated Liquid (NVL/57). The water content of a 2.2 x 2.2 NM column of air, which is color-coded and plotted on a 124-NM map. This product is used as an effective hail indicator, to locate most significant storms, and to identify areas of heavy rainfall. 3.4.9.1.6 Echo Tops (NET/41). An image of the echo top heights color-coded in user-defined increments. This product is used for a quick estimation of the most intense Convection and higher echo tops, as an aid in identification of storm structure features, and for pilot briefing purposes. 3.4.9.1.7 VAD Wind Profile (NVW/48). A graphic display of windbarbs plotted on a height staff in 500-ft or 1,000-ft increments. The current (far right) and up to 10 previous plots may be displayed simultaneously. This product is an excellent tool for meteorologists in weather forecasting, severe weather, and aviation. 3.4.9.2 Precipitation Products. Precipitation products are estimated ground accumulated rainfall. Estimates are based on a reflectivity and rainfall rate relationship called Z-R (https://www.weather.gov/tae/research-zrpaper). 3.4.9.2.1 One-Hour Precipitation (N1P/78). A display of estimated 1 hour precipitation accumulation on a 1.1-NM x 1-degree grid using the Precipitation Processing System (PPS) algorithm. This product is used to assess rainfall intensities for flash flood warnings, urban flood statements, and special weather statements. ### Page 93 3-62 3.4.9.2.2 Storm Total Precipitation (NTP/80). The estimated storm total precipitation accumulation on a 1.1-NM x 1-degree grid continuously updated since the precipitation event began. This product uses the PPS algorithm. This product is used to locate flood potential over urban or rural areas, estimate total basin runoff, and provide rainfall data 24 hours a day. 3.4.9.2.3 Digital Precipitation Array (DPA/81). An array format of estimated 1-hour precipitation accumulations on the 1/4 Lyon Federal Maybery (LFM) (4.7625 km Hydrologic Rainfall Analysis Project (HRAP)) grid. This is an 8-bit product with 255 possible precipitation values. This product is used to assess rainfall intensities for flash flood warnings, urban flood statements, and special weather statements. 3.4.9.3 Overlay Products. Overlay products provide detailed information for identified storm cells. 3.4.9.3.1 Storm Structure (NSS/62). A table displaying information on storm attributes, which include maximum reflectivity, maximum velocity at lowest elevation angle, storm overhang, mass weighted storm volume, storm area base and top, storm position, and storm tilt. 3.4.9.3.2 Hail Index (NHI/59). A product designed to locate storms that have the potential to produce hail. Hail potential is labeled as either probable (hollow green triangle) or positive (filled green triangle). Probable means the storm is probably producing hail and positive means the storm is producing hail. 3.4.9.3.3 Digital Mesocyclone Detection Algorithm (NMD/141). This product is designed to display information regarding the existence and nature of rotations associated with thunderstorms. Numerical output includes azimuth, range, and height of the mesocyclone. 3.4.9.3.4 Tornadic Vortex Signature (NTV/61). A product that shows an intense gate-to-gate azimuthal shear associated with tornadic-scale rotation. It is depicted by a red triangle with numerical output of location and height. 3.4.9.3.5 Storm Tracking Information (NST/58). A product that shows a plot of the past hour’s movement, current location, and forecast movement for the next hour or less for each identified thunderstorm cell. This product is used to determine reliable storm movement. 3.4.9.4 Radar Messages. Radar messages provide information about the radar status and special product data. ### Page 94 3-63 3.4.9.4.1 General Status Message (GSM/2). A text message describing the status of the radar site. A reason may be provided if the radar is not transmitting. 3.4.9.4.2 Free Text Message (FTM/5). A text message describing an upcoming, ongoing, or completed event at the radar site, including downtime, maintenance, and more. 3.4.9.4.3 Radar Status Log (RSL/152). A running daily log of status, errors, and messages from the Radar Product Generator (RPG) and Radar Data Acquisition (RDA) processing systems. Discussion Credit: NOAA/NCEI/ROC. Figure 3-27. WSR-88D vs. TDWR (Image Credit: NWS) ### Page 95 3-64 Figure 3-28. TDWR Site Locations (Image Credit: MIT Lincoln Labs) The image below is a classic thunderstorm producing a microburst. The TDWR radar picture on the left is a vertical cross section of a storm’s core. The image on the right is a visual photograph of a similar storm that produced a microburst. Figure 3-29. Thunderstorm Producing a Microburst (Image Credit: MIT Lincoln Labs) ### Page 96 3-65 3.5 Satellite. 3.5.1 Description. Satellite is perhaps the single most important source of weather data worldwide, particularly over data-sparse regions, such as countries without organized weather data collection and the oceans. The NOAA’s Geostationary Operational Environmental Satellite (GOES) imagery can be found on the AWC’s website, as well as all NWS WFO websites. Additional satellite imagery for Alaska can be found on the NWS Alaska Aviation Weather Unit (AAWU) website. 3.5.2 Imagery Types. Three types of satellite imagery are commonly used: visible, infrared (IR), and water vapor. Visible imagery is only available during daylight hours. IR and water vapor imagery are available day or night. 3.5.2.1 Visible Imagery. Visible imagery (see Figure 3-30, Visible Satellite Image—U.S. Example) displays reflected sunlight from the Earth’s surface, clouds, and particulate matter in the atmosphere. Visible satellite images, which look like black and white photographs, are derived from the satellite signals. Clouds usually appear white, while land and water surfaces appear in shades of gray or black. The visible channel senses reflected solar radiation. Clouds, the Earth’s atmosphere, and the Earth’s surface all absorb and reflect incoming solar radiation. Since visible imagery is produced by reflected sunlight (radiation), it is only available during daylight. ### Page 97 3-66 Figure 3-30. Visible Satellite Image—U.S. Example 3.5.2.1.1 Visible Image Data Legend. The data legend (see Figure 3-31, Visible Satellite Data Legend) on a visible image displays albedo, or reflectance, expressed as a percentage. For example, an albedo of 72 means 72 percent of the sunlight that struck a feature was reflected back to space. Figure 3-31. Visible Satellite Data Legend The gray shades (values) represent albedo, or reflectance, expressed as a percentage. ### Page 98 3-67 3.5.2.2 Inf rared (IR) Imagery. IR images (see Figure 3-32, Infrared (Color) Satellite Image—U.S. Example, and Figure 3-33, Unenhanced Infrared (Black and White) Satellite Image—U.S. Example) display temperatures of the Earth’s surface, clouds, and p articulate matter. Generally speaking, the warmer an object, the more IR energy it emits. The satellite sensor measures this energy and calibrates it to temperature using a very simple physical relationship. Clouds that are very high in the atmosphere are generally quite cold (e.g., -50 ºC), whereas clouds very near the Earth’s surface are generally quite warm (e.g., +5 ºC). Likewise, land may be even warmer than the lower clouds (e.g., +20 ºC). Those colder clouds emit much less IR energy than the warmer clouds and the land emits more than those warm clouds. The data measured by satellite is calibrated and colorized according to the temperature. If the temperature of the atmosphere decreases with height (which is typical), cloud-top temperature can be used to roughly determine which clouds are high-level and which are low-level. When clouds are present, the temperature displayed on the IR images is that of the tops of clouds. When clouds are not present, the temperature is that of the ground or the ocean. A major advantage of the IR channel is that it can sense energy at night, so this imagery is available 24 hours a day. Figure 3-32. Infrared (Color) Satellite Image—U.S. Example The scale is in degrees Celsius. Blue/purple colors indicate colder temperatures, while orange/red colors indicate warmer temperatures. ### Page 99 3-68 Figure 3-33. Unenhanced Infrared (Black and White) Satellite Image—U.S. Example The scale is in degrees Celsius. Lighter gray shades indicate colder temperatures, while darker gray shades indicate warmer temperatures. 3.5.2.2.1 Infrared Image Data Legends. The data legend (see Figure 3-34, Infrared (Color) Satellite Image Data Legend, and Figure 3-35, Unenhanced Infrared (Black and White) Satellite Image Data Legend) on an IR image is calibrated to temperature expressed in degrees Celsius. The legend may vary based on the satellite image provider. Figure 3-34. Infrared (Color) Satellite Image Data Legend The colors (values) represent temperature in degrees Celsius. Figure 3-35. Unenhanced Infrared (Black and White) Satellite Image Data Legend The gray shades (values) represent temperature in degrees Celsius. 3.5.2.3 Water Vapor Imagery. The water vapor imagery (see Figure 3-36, Water Vapor Satellite Image— U.S. Example) displays the quantity of water vapor generally located in the middle and upper troposphere within the layer between 700 mb (approximately 10,000 ft MSL) and 200 mb (approximately FL390). The actual numbers displayed on the water vapor images correspond to temperature in degrees Celsius. No direct relationship exists between these values and the temperatures of clouds, unlike IR imagery. Water vapor ### Page 100 3-69 imagery does not really “see” clouds, but “sees” high-level water vapor instead. The most useful information to be gained from the water vapor images is the locations and movements of weather systems, jet streams, and thunderstorms. Another useful tidbit is aided by the color scale used on the images. In general, regions displayed in shades of red are very dry in the upper atmosphere and may correlate to crisp, blue skies from a ground perspective. On the contrary, regions displayed in shades of blue or green are indicative of a lot of high-level moisture and may also indicate cloudiness. This cloudiness could simply be high-level cirrus types or thunderstorms. That determination cannot be ascertained from this image by itself, but could easily be determined when used in conjunction with corresponding visible and IR satellite images. A major advantage of the water vapor channel is that it can sense energy at night, so this imagery is available 24 hours a day. Figure 3-36. Water Vapor Satellite Image—U.S. Example The scale is in degrees Celsius. Blue/green colors indicate moisture and/or clouds in the mid/upper troposphere, while dark gray/orange/red colors indicate dry air in the mid/upper troposphere. 3.5.2.3.1 Water Vapor Image Data Legend. The data legend (see Figure 3-37, Water Vapor Satellite Image Data Legend) on water vapor images is calibrated to temperature expressed in degrees Cels ius. The actual data values on the water vapor images are not particularly ### Page 101 3-70 useful. Interpretation of the patterns and how they change over time is more important. The legend may vary depending on the satellite image provider. Figure 3-37. Water Vapor Satellite Image Data Legend The colors (values) represent temperature in degrees Celsius 3.5.3 Polar Orbiting Environment Satellites (POES). POES stands for the Polar Orbiting Environment Satellites, although more recently the U.S. polar satellite program has been rechristened the Joint Polar Satellite System (JPSS). Polar satellites are not stationary. They track along various orbits around the poles. Typically, they are somewhere between 124 and 1,240 mi above the Earth’s surface. The satellites scan the Earth in swaths as they pass by on their tracks. 3.5.3.1 Benefits. Because they are so much closer to Earth, you can get very high resolution, i.e., better than 0.5 km (or ~5/8 mi). This allows for weather and surface features to be seen in much greater detail. This is particularly useful over the poles and arctic areas. Quality of geostationary satellite data degrades the further you get from the poles, while polar data provides high resolution in those areas. 3.5.3.2 Shortfalls. By far the most significant shortfall is the latency, or the time between the satellite scanning the area and the time that the data is available to a user. Because polar satellites are moving, they cannot continuously transmit to a single station. Instead, there are a series of stations around the globe through which the data are collected. Data is then transmitted from those stations to other locations. At times, it can be several hours (or more) old by the time it reaches operational users in the United States (in polar areas it is much quicker). There are some direct ground stations closer to the United States which can cut the latency to ~45 minutes when utilizing the newer polar satellites. For the purpose of the AWC, that is at the limit of usefulness for real-time operations. Discussion Credit: NOAA/NWS/AWC. 3.6 Upper-Air Observations. 3.6.1 Radiosonde Observations. Since the late 1930s, the NWS has taken upper air observations with radiosondes attached to weather balloons. Weather data from the radiosonde are foundational to all computer model forecasts produced by the NWS. ### Page 102 3-71 The radiosonde is a small, expendable instrument package (weighing 100 grams (g) to 500 g) that is suspended below a large balloon inflated with hydrogen or helium gas. As the radiosonde rises at about 300 m per minute (about 1,000 ft per minute), sensors on the radiosonde measure profiles of pressure, temperature, and moisture. These sensors are linked to a battery-powered radio transmitter that sends the sensor measurements to a ground tracking antenna. Wind speed and direction aloft are also obtained by tracking the position of the radiosonde in flight using Global Positioning Satellites (GPS). 3.6.1.1 Issuance. Weather balloons with radiosondes are launched twice a day worldwide from designated locations (see Figure 3-38, U.S. Radiosonde Network, for U.S. locations) at around 1100 UTC and 2300 UTC. It takes approximately 90 minutes for the balloon to reach an altitude of 100,000 ft. The weather data collected is assigned the observation times of 1200 UTC and 0000 UTC. Special radiosondes may be launched at select times for various reasons, including when severe weather is expected in a region. Figure 3-38. U.S. Radiosonde Network ### Page 103 3-72 3.7 Graphical Depiction of Surface Observations. 3.7.1 AAWU Surface Observations. A display of color-coded station plots (see Figure 3-39, AAWU Surface Observations Example) from Alaska is produced by the NWS’ AAWU. This product shows: surface temperature, dewpoint, ceiling, visibility, and wind direction/speed which have been taken from the METAR report. A key to the station plots is found on each map. Figure 3-39. AAWU Surface Observations Example 3.7.1.1 Issuance. The charts are issued hourly and can be found on the AAWU website. 3.7.1.2 Legends. AAWU Surface Observations depict numerous parameters, including the flying category, sky cover, and wind. 3.7.1.2.1 Flying Category. Each station plot is color-coded according to the weather flying category reported (see Table 3-9, AAWU Surface Observations Flying Categories and Criteria). Red indicates IFR, blue indicates Marginal Visual Flight Rules (MVFR ), and black is plotted for stations reporting Visual Flight Rules (VFR) ceiling and/or visibility conditions. ### Page 104 3-73 Table 3-9. AAWU Surface Observations Flying Categories and Criteria FLYING CATEGORY CEILING (feet) VISIBILITY (miles) VFR (black) Greater than 3,000 feet Greater than 5 miles MVFR (blue) 1,000 to 3,000 feet 3 to 5 miles IFR (red) Less than 1,000 feet Less than 3 miles 3.7.1.2.2 Station Plot. METAR elements are plotted for each station in the form of a station plot (see Figure 3-44, Surface Observations—Aviationweather.gov METAR Static Display Example). Figure 3-40. AAWU Surface Observation Plot Legend 3.7.1.2.3 Sky Cover. The sky cover symbol is plotted at the station location and is filled according to the summation total of the sky condition element from the METAR report. For example, if the METAR sky condition element was SCT030 BKN060 OVC090, the sky cover would be overcast and have a completely filled circle. 3.7.1.2.4 Station Identifier (STNID). The four-letter ICAO station identifier is entered to the upper right of the station. 3.7.1.2.5 Wind. Wind is plotted the same as the NWS surface analysis charts. 3.7.1.2.6 Temperature (TMP deg F). Temperature in degrees Fahrenheit is plotted to the upper left of the sky cover symbol. 3.7.1.2.7 Visibility (VIS). Visibility in statute miles is plotted to the left of the sky cover symbol. Decimals are used to represent tenths of miles, when necessary. 3.7.1.2.8 Dewpoint Temperature (DEWPT deg F). Dewpoint temperature, in degrees Fahrenheit, is plotted to the lower left of the sky cover symbol. ### Page 105 3-74 3.7.1.2.9 Ceiling (CIG). Ceiling is the height from the base of the lowest layer aloft covering more than one-half of the sky. Additionally, vertical visibility into a total surface-based obscuration is defined as a ceiling. For a METAR report, the first broken (BKN) or overcast (OVC) layer is the ceiling. For example, if the METAR sky condition element is SCT030 BKN060 OVC090, the ceiling is 6,000 ft. For a total surface-based obscuration, no ceiling is plotted and the METAR must be consulted. If the sky cover is clear, few, or scattered, no ceiling is plotted. The ceiling is plotted to the lower right of the station circle. Ceilings are reported as hundreds of feet AGL. If no ceiling is present, the code 999 will be plotted. 3.7.1.2.10 Present Weather (WX). Present weather symbols are entered to the left of the station. Note that the older Surface Aviation Observation (SAO) code is used instead of surface analysis chart symbols or the modern METAR code. ### Page 106 3-75 Table 3-10. AAWU Surface Observation Precipitation Symbols Symbol Meaning T Thunderstorm R Rain RW Rain Shower L Drizzle ZR Freezing Rain ZL Freezing Drizzle A Hail IP Ice Pellets IPW Ice Pellet Showers S Snow SW Snow Showers SP Snow Pellets SG Snow Grains IC Ice Crystals ### Page 107 3-76 Table 3-11. AAWU Surface Observation Obstruction to Visibility Symbols Symbol Meaning BD Blowing Dust BN Blowing Sand BS Blowing Snow BY Blowing Spray D Dust F Fog GF Ground Fog H Haze IF Ice Fog K Smoke Table 3-12. AAWU Surface Observation Precipitation Intensity Symbols Symbol Meaning - Light (No symbol) Moderate + Heavy 3.7.1.3 Use. AAWU Surface Observations provide an overview of weather flying categories and other adverse weather conditions for the valid time shown on the graphic display. This product often does not completely represent the en route conditions because of terrain variations and the possibility of weather occurring between reporting stations. This product should be used in addition to the latest METAR/SPECIs, PIREPs, and radar and satellite imagery for a complete look at the latest flying conditions. ### Page 108 3-77 3.7.2 METARs on Aviationweather.gov. 3.7.2.1 Issuance. Surface observations (METAR and SPECI) are provided on https://www.aviationweather.gov in several formats. The formats range from traditional text (referred to as the raw format) to different ty pes of map displays. The two types of map displays are interactive (see Figure 3-41, Surface Observations—Aviationweather.gov METAR Interactive Display Example) and static (see Figure 3-44). 3.7.2.1.1 Interactive Display. The interactive display includes zoom and pan. Increasing the zoom allows more METARs to be displayed. Figure 3-41. Surface Observations—Aviationweather.gov METAR Interactive Display Example ### Page 109 3-78 “Plot Options” allows users to choose the weather elements displayed (e.g., view only ceilings and visibilities). The following parameters can be displayed using a station plot model (see Figure 3-42, Interactive Display Station Plot Example): Figure 3-42. Interactive Display Station Plot Example • Temp: Temperature in Fahrenheit. Celsius is also available, if metric is selected. • Alt: Atmospheric pressure reduced to MSL. o Where expressed in inches of mercury (Hg) or altimeter setting, the value is expressed in hundredths of an inch where the leading 2 or 3 is omitted (e.g., 982 would be 29.82 in, 025 would be 30.25 in). When trying to determine whether to add a 2 or 3, use the number that will give you a value closest to 30.00 in. o Where expressed in millibars, the value is expressed in tenths of a millibar with the leading 9 or 10 omitted (e.g., 998 would be 999.8 mb, 164 would be 1,016.4 mb). When trying to determine whether to add a 9 or 10, use the number that will give you a value closest to 1,000 mb. • Ceil: Cloud ceiling in hundreds of feet (BKN, OVC, and OVX only). • Id: The four-letter ICAO identifier for the airport. • Windbarb: The stick points in the direction the winds are coming from. Wind speed is in knots. Red represents wind gusts. • Dew: Dewpoint in Fahrenheit. Celsius is also available, if metric is selected. • Vis: Visibility in miles. Kilometers (km), if metric is selected. Visibility value on the chart may not represent actual value reported in the METAR or SPECI, and may be fractionally lower than shown. (e.g., 2½ in the METAR is shown as 3 on the surface plot) (see Table 3-12, Rounding of Actual METAR Visibility Values for Depiction on Aviat ionweather.gov/AWC METAR Display). • Wx: Weather symbol representing current weather. ### Page 110 3-79 • Sky: Cloud cover s ymbol (see Figure 3-43, Station Plot Sky Symbols) is plotted in the center. The amount the circle is filled represents the amount of cloud cover. The color of the circle represents one of the following weather categories (see Table 3-14, Station Plot Weather Categories): o Green: VFR, o Blue: MVFR, o Red: IFR, or o Purple: Low instrument flight rules (LIFR). A click on the station circle displays the raw METAR in a separate text window. When the Hover option is selected, the raw METAR text window appears by merely placing the cursor over the station circle. If the Decoded option is set, the METAR appears in plain language instead of METAR code. If the TAF option is set, the latest Terminal Aerodrome Forecast (TAF) will be appended to the information in the text window. Table 3-13. Rounding of Actual METAR Visibility Values for Depiction on Aviationweather.gov/AWC METAR Display Visibility Reported in METAR Visibility, as depicted on surface plot 0, 1/16, 1/8, 3/16, 1/4, 3/8 0.3 ½ 0.5 ¾ 0.8 1½, 1¾, 2 2½, 3 3 ### Page 111 3-80 Table 3-14. Station Plot Weather Categories Category* Color Ceiling Visibility LIFR (Low IFR) Magenta Below 500 feet AGL and/or Less than 1 mile IFR Red 500 to below 1,000 feet AGL and/or 1 mile to less than 3 miles MVFR (Marginal VFR) Blue 1,000 to 3,000 feet AGL and/or 3 to 5 miles VFR Green Greater than 3,000 feet AGL and Greater than 5 miles *These categories are not flight rules and should not be confused with the flight rules provided in Part 91, including those for Basic VFR Weather Minimums. Rather, these categories were created for weather charts as a means to visually enhance the products. Figure 3-43. Station Plot Sky Symbols Automated stations report “CLR” when clouds may exist above 12,000 ft, so a square is used to represent this uncertainty, whereas an unfilled circle is used for “SKC,” when a human reports the sky is completely clear overhead. The abbreviation “OVX” is unofficial, but AWC uses it to indicate the sky is obscured, which is the case when a METAR reports vertical visibility and no cloud information. “Miss” means the sky condition is missing. 3.7.2.1.2 Static Display. Static display products are provided on the AWC’s website for 19 select regions plus the CONUS. Each product is updated every 5 minutes using the latest METAR or SPECI information. The Web page automatically refresh every 15 minutes. The static display has fewer features than the interactive display. Primarily, the static display does not provide ceiling height. Also, the static display does not have the functionality to display separately the raw METAR or TAF, or change units to metric. ### Page 112 3-81 Figure 3-44. Surface Observations—Aviationweather.gov METAR Static Display Example Fi gure 3-45. Static Display Station Plot Example St ation plot descriptions are: • Temp: Temperature in Fahrenheit (F). • Windbarb: The windbarb. The stick points in the direction the winds are coming from. ### Page 113 3-82 • Alt: Last three digits of the altimeter setting in inches of mercury. For example, 032 represents 30.32 in Hg. • Id: The last three letters of the ICAO identifier for the airport. • Dew: The dewpoint temperature in Fahrenheit. • Vis: The visibility in statute miles. • Wx: Weather symbol representing current weather. • Sky: The cloud cover symbol is plotted in the center. The amount the circle is filled represents the amount of cloud cover. The color of the circle represents one of the following weather categories: o Green: VFR, o Blue: MVFR, o Red: IFR, or o Purple: LIFR. 3.7.2.2 Use. Graphical depictions of surface observations, both interactive and static display, provide an overview of weather flying categories and other adverse weather conditions for the valid time shown on the graphic display. 3.8 Aviation Weather Cameras. 3.8.1 FAA Aviation Weather Cameras. The FAA’s Aviation Weather Camera network (see Figure 3-46, Map of FAA’s Aviation Weather Camera Network in Alaska) consists of more t han 220 camera facilities in Alaska. Each site has up to four cameras. The direction of each camera (see Figure 3-47, Sectional Chart Depicting Shungnak (SHG), Alaska Camera Orientations) is provided with reference to a Secti onal Chart. A clear day image (see Figure 3-48, “Clear Day” Image from Aviation Weather Camera Image at Shungnak (SHG), Alaska) is provided for reference and co mparison with the latest image (see Figure 3-49, Aviation Weather Camera Image at Shungnak (SHG), Alaska). 3.8.2 Issuance. Camera images are available on the FAA’s Aviation Weather Camera website. Images are generally updated every 10 minutes. The time of the last update is indicated on each image. Actual site conditions may differ from displayed images due to a variety of reasons (e.g., rapidly changing weather conditions, image update frequency, optical distortion). ### Page 114 3-83 Figure 3-46. Map of FAA’s Aviation Weather Camera Network in Alaska F igure 3-47. Sectional Chart Depicting Shungnak (SHG), Alaska Camera Orientations ### Page 115 3-84 Figure 3-48. “Clear Day” Image from Aviation Weather Camera Image at Shungnak (SHG), Alaska F igure 3-49. Aviation Weather Camera Image at Shungnak (SHG), Alaska 3.8.3 Use. The images should only be used for flight planning and to improve situational awareness (SA). They are not to be used to comply with regulatory requirements (e.g., to determine weather minimums for IFR flight). ### Page 116 4-1 CHAPTER 4. ANALYSIS The second of three distinct types of weather (meteorological) information are analyses. Analyses of weather information are an enhanced depiction and/or interpretation of observed weather data. Prior to the 1990s, most analysis charts were hand drawn by forecasters. Today’s analyses are automated, and depending on the weather information provider (i.e., NWS, commercial weather services, and flight planning services), the appearance and content of these analyses will vary. Chapter 4 will only focus on those analyses produced by the NWS and made available on various websites, including the Aviation Weather Center (AWC), the Weather Prediction Center (WPC), the Ocean Prediction Center (OPC), and the Alaska Aviation Weather Unit (AAWU). 4.1 Surface Analysis Charts. Surface analysis charts are analyzed charts of surface weather observations. The chart depicts the distribution of several items, including sea-level pressure; the positions of highs, lows, ridges, and troughs; the location and type of fronts; and the various boundaries such as drylines. Pressure is referred to in mean sea level (MSL) on the surface analysis chart while all other elements are presented as they occur at the surface point of observation. Figure 4-1. NWS Surface Analysis Chart Example ### Page 117 4-2 4.1.1 WPC Surface Analysis Charts. The WPC in College Park, MD, produces a variety of surface analysis charts for North America that are available on their website. Figure 4-1, NWS Surface Analysis Chart Example, is one example of several surface analysis products available on their website. WPC’ s surface analysis is also available at https://www.aviationweather.gov. Some of WPC’s surface analysis charts are combined with radar or satellite imagery (Figure 4-2, NWS Surface Analysis and Radar Composite Example) as well as have different background features (e.g., terrain). Figure 4-2. NWS Surface Analysis and Radar Composite Example 4.1.1.1 Issuance. The WPC issues surface analysis charts for North America eight times daily, valid at 00, 03, 06, 09, 12, 15, 18, and 21 Coordinated Universal Time (UTC). 4.1.1.2 Anal ysis Symbols. Figure 4-3, NWS Surface Analysis Chart Symbols, shows analysis symbols used on NWS surface analysis charts. ### Page 118 4-3 Figure 4-3. NWS Surface Analysis Chart Symbols 4.1.1.3 Station Plot Models. Land, ship, buoy, and Coastal-Marine Automated Network (C-MAN) stations are plotted on the chart to aid in analyzing and interpreting the surface weather features. These plotted observations are referred to as station models. Some stations may not be plotted due to space limitations. However, all reporting stations are used in the analysis. Figure 4-4, NWS Surfa ce Analysis Chart Station Plot Model, and Figure 4-5, NWS Surface Analysis Chart Ship/Buoy Plot Model, contain the most commonly us ed station plot models used in surface analysis charts. ### Page 119 4-4 Figure 4-4. NWS Surface Analysis Chart Station Plot Model F igure 4-5. NWS Surface Analysis Chart Ship/Buoy Plot Model WPC al so produces surface analysis charts specifically for the aviation community. Figure 4-6, NWS Surface Analysis Chart for Aviation Interests Station Plot Model, contains the station plot model for these charts. ### Page 120 4-5 Figure 4-6. NWS Surface Analysis Chart for Aviation Interests Station Plot Model 4.1.1.3.1 Station Identifier. The format of the station identifier depends on the observing platform. • Ship: Typically four or five characters. If five characters, then the fifth will usually be a digit. • Buoy: Whether drifting or stationary, a buoy will have a five-digit identifier. The first digit will always be a 4. • C-MAN: Usually located close to coastal areas. Their identifier will appear like a five-character ship identifier; however, the fourth character will identify off which state the platform is located. • Land: Land stations will always be three characters, making them easily distinguishable from ship, buoy, and C-MAN observations. 4.1.1.3.2 Temperature. The air temperature is plotted in whole degrees Fahrenheit. 4.1.1.3.3 Dewpoint. The dewpoint temperature is plotted in whole degrees Fahrenheit. 4.1.1.3.4 Weather. A weather symbol is plotted if, at the time of observation, precipitation is either occurring or a condition exists causing reduced visibility. Figure 4-7, NWS Surface Analysis Chart Common Weather Symbols, contains a list of the most common weather symbols. ### Page 121 4-6 Figure 4-7. NWS Surface Analysis Chart Common Weather Symbols 4.1.1.3.5 Wind. Wind is plotted in increments of 5 knots (kts). The wind direction is referenced to true north and is depicted by a stem (line) pointed in the direction from which the wind is blowing. Wind speed is determined by adding the values of the flags (50 kts), barbs (10 kts) and half-barbs (5 kts) found on the stem. If the wind is calm at the time of observation, only a single circle over the station is depicted. Figur e 4-8, NWS Surface Analysis Chart Wind Plotting Model, includes some sample wind symbols. ### Page 122 4-7 Figure 4-8. NWS Surface Analysis Chart Wind Plotting Model 4.1.1.3.6 Ceiling. Ceiling is plotted in hundreds of feet above ground level (AGL). 4.1.1.3.7 Visibility. Surface visibility is plotted in whole statute miles (sm). 4.1.1.3.8 Pressure. Sea-level pressure is plotted in tenths of millibars (mb), with the first two digits (generally 10 or 9) omitted. For reference, 1013 mb is equivalent to 29.92 inches (in) of mercury. Below are some sample conversions between plotted and complete sea-level pressure values. 410 1041.0 mb 103 1010.3 mb 987 998.7 mb 872 987.2 mb 4.1.1.3.9 Pressure Trend. The pressure trend has two components, a number and a symbol, to indicate how the sea-level pressure has changed during the past 3 hours. The number provides the 3-hour change in tenths of millibars, while the symbol provides a graphic illustration of how this change occurred. ### Page 123 4-8 Figure 4-9, NWS Surface Analysis Chart Pressure Trends, contains the meanings of the pressure trend symbols. Figure 4-9. NWS Surface Analysis Chart Pressure Trends 4.1.1.3.10 Sky Cover. The approximate amount of sky cover can be determined by the circle at the center of the station plot. The amount that the circle is filled reflects the amount of sky covered by clouds. Figure 4-10, NWS Surface Analysis Chart Sky Cover Symbols, contains the common cloud cover depictions. ### Page 124 4-9 Figure 4-10. NWS Surface Analysis Chart Sky Cover Symbols 4.1.1.3.11 Water Temperature. Water temperature is plotted in whole degrees Fahrenheit. 4.1.1.3.12 Swell Information. Swell direction, period, and height are represented in the surface observations by a six-digit code. The first two digits represent the swell direction, the middle digits describe the swell period (in seconds), and the last two digits are the swell’s height (in half meters). 090703: 09: The swell direction is from 90 degrees (i.e., it is coming from due east) 07: The period of the swell is 7 seconds 03: The height of the swell is 3 half meters (m) 271006: 27: The swell direction is from 270 degrees (due west) 10: The period is 10 seconds 06: The height of the swell is 6 half meters 4.1.1.3.13 Wave Information. Period and height of waves are represented by a five-digit code. The first digit ### Page 125 4-10 is always 1. The second and third digits describe the wave period (in seconds), and the final two digits give the wave height (in half meters). 10603: 1: A group identifier. The first digit will always be 1 06: The wave period is 6 seconds 03: The wave height is 3 half meters 10515: 1: A group identifier 05: The wave period is 5 seconds 15: The wave height is 15 half meters In some charts by the OPC, only the wave height (in feet) is plotted. 4.1.1.4 Analyses. Isobars, pressure systems, and fronts are the most common analyses depicted on surface analysis charts. 4.1.1.4.1 Isobars. An isobar is a line of equal or constant pressure commonly used in the analysis of pressure patterns. On a surface analysis chart, isobars are solid lines usually spaced at intervals of 4 mb. Each isobar is labeled. For example, 1032 signifies 1,032.0 mb and 992 signifies 992.0 mb. 4.1.1.4.2 Pressure Systems. On a surface analysis chart, a High (H) is a maximum of atmospheric pressure, while a Low (L) is a minimum of atmospheric pressure. Central pressure is the atmospheric pressure located at the center of a High or Low. In general, the central pressure is the highest pressure in the center of a High and the lowest pressure at the center of a Low. The central pressure is denoted near each pressure center. A trough or an elongated area of low atmospheric pressure is denoted by dashed lines and identified with the word TROF. A ridge or an elongated area of high atmospheric pressure is denoted by saw-toothed symbols. Ridges are rarely denoted on charts produced by the WPC. Tropical storms, hurricanes, and typhoons are low-pressure systems with their names and central pressures denoted. ### Page 126 4-11 4.1.1.4.3 Fronts. NWS surface analysis charts positions and types of fronts are shown by symbols in Figure 4-3. The symbols on the front indicate the type of front and point in the direction toward which the front is moving. Two short lines across a front indicate a change in front type. 4.1.2 Unified Surface Analysis Chart. The NWS Unified Surface Analysis Chart is a surface analysis product produced collectively and collaboratively by NWS’s WPC, the OPC, the National Hurricane Center (NHC), and Weather Forecast Office (WFO) Honolulu. The chart contains an analysis of isobars, pressure systems, and fronts. This chart is available on the OPC website as well as the AWC. Users can zoom in by clicking an area on the map (see Figure 4-11, Unified Surface Analysis Chart Example) to enlarge (see Figure 4-12, Unified Surface Analysis Chart Example) and show station plot models (see paragraph 4.1.1.3). 4.1.2.1 Issuance. The Unified Surface Analysis Chart is issued four times daily for valid times 00, 06, 12, and 18 UTC. Figure 4-11. Unified Surface Analysis Chart Example ### Page 127 4-12 Figure 4-12. Unified Surface Analysis Chart Example 4.1.2.2 Analysis Symbols. Unified Surface Analysis Charts use the symbols shown in Figure 4-3. 4.1.3 AAWU Surface Chart. The NWS’s Unified Surface Analysis chart covers the Alaskan area. The AAWU also provides a fixed area image of the Unified Surface Analysis Chart centered over Alaska (see Figure 4-13, Unified Surface Analysis Chart Example With Fixed Area Coverage Over Alaska). This chart is available fro m AAWU’s website. ### Page 128 4-13 Figure 4-13. Unified Surface Analysis Chart Example With Fixed Area Coverage Over Alaska 4.2 C eiling and Visibility. 4.2.1 Ceiling and Visibility Analysis (CVA). The CVA product provides a real-time analysis of current observed and estimated ceiling and visibility conditions across the continental United States (CONUS). The product is primarily intended to help the general aviation pilot (particularly the Visual Flight Rules (VFR)-only pilot) avoid instrument flight rules (IFR) conditions. However, CVA’s overview of ceiling and visibility conditions can be useful to others involved in flight planning or weather briefing. 4.2.1.1 Issuance. CVA is issued every 5 minutes and is available through https://www.aviationweather.gov. 4.2.1.2 Content. CVA presents information via full-CONUS graphic and 18 regional graphics. Each graphic is rendered on a horizontal grid of 5 km resolution and shows viewer-selectable representations of ceiling height (AGL), surface visibility in statute miles (sm) and flight category designation. Each regional display includes an overlay of station plots showing the current ceiling and visibility observations reported at selected Aviation Routine ### Page 129 4-14 Weather Report (METAR) stations. To avoid overcrowding, the overlay shows only a subset of the total number of stations available at the CONUS scale view. Additional stations are displayed when zooming in. 4.2.1.3 CVA Ceiling Analysis. The CVA Ceiling Analysis uses observed and estimated ceiling heights as follows: • At display points corresponding to METAR locations, CVA uses the ceiling values observed by the nearest METAR. • At display points between METAR locations (where there are no direct observations), CVA uses estimated ceiling values derived by adjusting the ceiling observation from the nearest METAR to take into account intervening changes in terrain height. When corresponding Geostationary Operational Environmental Satellite (GOES) observations yield an unambiguous indication that no cloud is present, adjusted ceiling observations are reset to indicate clear conditions. • At display points where observed or estimated ceiling height is less than 200 feet (ft) AGL, CVA reports “Possible Terrain Obstruction.” The CVA Ceiling Analysis graphic is rendered as follows: • Ceiling less than 1,000 ft AGL: pale yellow. • Ceiling less than 200 ft. AGL and Possible Terrain Obstruction: orange. Fi gure 4-14, CONUS Display of CVA Ceiling Analysis, shows a CONUS-scale CVA Ceiling Analysis. Figure 4-15, Regional Display of CVA Ceiling Analysis, shows a regional-scal e Ceiling Analysis. ### Page 130 4-15 Figure 4-14. CONUS Display of CVA Ceiling Analysis ### Page 131 4-16 Figure 4-15. Regional Display of CVA Ceiling Analysis 4.2.1.4 CVA V isibility Analysis. The CVA Visibility Analysis (shown as a regional view in Figure 4-16, Regional Display of CVA Visibility Analysis) reports the observed visibility at the near est METAR. The graphic is rendered as follows: • Surface visibility less than 3 sm: pale yellow. ### Page 132 4-17 Figure 4-16. Regional Display of CVA Visibility Analysis 4.2.1.5 CVA Flight Category Analysis. The CVA Flight Category Analysis (shown as a regional view in Figure 4-17, Regional Display of CVA Flight Category Analysis) uses observed and estimate d ceiling and visibility values to report the flight category at each display point. The graphic is rendered as follows: • IFR—pale yellow: Ceiling less than 1,000 ft AGL and/or visibility less than 3 sm. • LIFR—orange: Ceiling less than 200 ft AGL and Possible Terrain Obstruction. ### Page 133 4-18 Figure 4-17. Regional Display of CVA Flight Category Analysis Areas of possible terrain obscuration (indicated by estimated ceiling less than 200 ft AGL) are also shown. 4.2.1.6 Strengths and Limitations. 4.2.1.6.1 CVA Strengths. • The CVA’s area-wide graphics highlight observed and estimated IFR, VFR, and possible ground obscuration conditions, enabling a user’s quick recognition of ceiling and visibility hazards, as represented by existing observational tools. • The CVA is issued every 5 minutes using the most current METAR and GOES observations available. These rapid updates enhance recognition of trends when conditions are changing. The CVA incorporates “special” METAR observations as they are issued. ### Page 134 4-19 4.2.1.6.2 CVA Limitations. • The CVA is an analysis of estimated real-time conditions only. It is not a forecast and cannot be used in place of a forecast. • The CVA must only be used with products such as METARs, Terminal Aerodrome Forecasts (TAF), and Airmen’s Meteorological Information (AIRMET). • The CVA’s representation of ceiling and visibility conditions in regions between METARS (where direct observations are unavailable) can be significantly in error. As distance from the nearest METAR increases, the uncertainty in represented conditions increases. • Impacted ceiling and visibility conditions can be highly localized and smaller in scale than the 5 km grid used to convey CVA information. Thus, small-scale variations in ceiling and visibility conditions may not be represented by CVA, even in regions close to observing stations. 4.2.1.7 Use. General aviation pilots, weather briefers, and others involved in flight planning can use CVA as a companion to METARS, TAFs, and AIRMETs to help them recognize and avoid IFR conditions. Along with the foregoing weather information sources, the CVA can aid IFR avoidance and escape for the en route pilot who inadvertently encounters IFR conditions. Pilots and flight planners must exercise judgment when using CVA information, as it is based in recent weather information and calculated estimations. Degrading conditions are more likely as the distance and time increase from the station’s reported weather. The CVA is one of the data fields included in the part of the helicopter emergency medical services (HEMS) Tool (see paragraph 6.1). 4.2.2 Weather Depiction Chart. The Weather Depiction Chart (see Figure 4-18, Weather Depiction Chart—Example) is being phased out by the NWS, in favor of newer ceiling and visibility products, like the CVA pr oduct. ### Page 135 4-20 Figure 4-18. Weather Depiction Chart—Example 4.3 Upper-Air Analyses. 4.3.1 Issuance. The NWS National Centers for Environmental Prediction (NCEP) Central Operations (NCO) produces and provides upper-air analyses and forecast products. Their website, Model Analyses and Guidance (see Figure 4-19, NWS NCEP Central Operations Model Analyses and Guidance Website), contains a user’s guide that provides descriptions, details, a nd examples of the various products. A select subset of these products is available on https://www.aviationweather.gov. ### Page 136 4-21 Figure 4-19. NWS NCEP Central Operations Model Analyses and Guidance Website A user’s guide, located within Website Information (upper right corner), provides descriptions, details, and examples of the various products. 4.3.1.1 Constant Pressure Analyses. Constant Pressure Charts (see Figure 4-20, 500 MB Analysis Chart—Example) are being phased out by the NWS in favor of other products (see Figure 4-21 ( i.e., Model’s Analyses from the NWS’s North American Model [NAM] Example)) a vailable on NCO’s Model Analyses and Guidance website. See paragraph , 500 MB Height, Temperature, and Wind, 00-hour Forecast 5.17.1 for additional information. ### Page 137 4-22 Figure 4-20. 500 MB Analysis Chart—Example Fi gure 4-21. 500 MB Height, Temperature, and Wind, 00-hour Forecast (i.e., Model’s Analyses from the NWS’s North American Model (NAM) Example) Specific details regarding the content of the product is provided in the Product Description tab on the NCO’s Model Analyses and Guidance Web page. ### Page 138 4-23 4.3.1.2 Use. See paragraph 5.17.1.2. 4.3.2 Radiosonde Observation Analysis. A common means of analyzing radiosonde observations (see paragraph 3.6.1) is the skew-T diagram (see Figure 4-22, Skew-T Diagram—Example). Skew-T diagrams are primarily intended for, and used by, meteorologists as part of their analyses of the atmosphere. For example, the skew-T diagram can be used to: • Determine the freezing level (or levels); • Determine the stability of the atmosphere; • Determine the potential for severe weather; • Determine the height and depth of inversions; • Infer cloud bases, tops, and cloud layers; and • Determine soaring conditions. 4.3.2.1 Issuan ce. Skew-T diagrams are available from the NWS NCO. Their website, Model Analyses and Guidance (see Figure 4-19), contains a user’s guide that provides descriptions, details, and examples of the various products, including the sk ew-T. 4.3.2.2 Format. The skew-T diagram provided on the NWS NCO Model Analyses and Guidance website uses the following format (other weather providers and websites may have different formats, especially colors): • Horizontal axis is temperature in degrees Celsius, skewed to the right, labeled (minus) -20, 0, and 20 (Celsius). • Vertical axis is pressure levels in millibars, labeled 1,000 (near sea level) to 100 (approximately 53,000 ft MSL). • Bold solid red line represents the temperature profile over the station taken from the radiosonde observation (weather balloon). • Bold solid green line represents the dewpoint profile. • Wind aloft is shown on the far right side. ### Page 139 4-24 Figure 4-22. Skew-T Diagram—Example 4.3.2.3 Example. Tw o examples are provided: a multiple freezing level example (see Figure 4-23, Skew-T Diagram—Multiple Freezing Level Example) and a cloud top example (see Figure 4-24, Skew-T Diagram—Cloud Top Example). ### Page 140 4-25 4.3.2.3.1 Multiple Freezing Level Example. Figure 4-23. Skew-T Diagram—Multiple Freezing Level Example Horizontal axis is temperature in degrees Celsius, skewed to the right, labeled -20, 0, and 20 (Celsius). Vertical axis is pressure levels in millibars, labeled 1000 (near sea level) to 500 (18,000 ft). Bold solid red line represents the temperature profile over the station taken from the radiosonde observation (weather balloon). Bold solid green line represents the dewpoint profile. Note how the temperature profile (bold red line) crosses the 0 degree temperature line (also known as an isotherm) 5 times (near 900 MB, 860 MB, 775 MB, 725 MB, and 675 MB). 4.3.2.3.2 Cloud Top Example. Figure 4-24 is the radiosonde observation from Vandenberg Air Force Base (VAFB), California, for 1200 UTC for a typical coastal stratus cloud. Figure 4-24. Skew-T Diagram—Cloud Top Example At about 950 MB, the temperature profile (bold red line) and dewpoint profile (bold green line) almost touch each other. This is the profile of a cloud top. The temperature and dewpoint quickly diverge, representing a change from the cool, moist air (and associated stratus cloud) to the dry and warmer air (cloud free) above. 4.3.2.4 Use. Skew-T diagrams are primarily intended for, and used by, meteorologists as part of their analyses of the atmosphere and formulation of various forecasts. ### Page 141 5-1 CHAPTER 5. FORECASTS The third distinct type of weather (meteorological) information is forecasts. Chapter 5 will discuss many forecast products, including in-flight advisories, produced by the NWS that are either specific to aviation or are public products of interest to aviation users. 5.1 Significant Meteorological Information (SIGMET). A SIGMET is a concise description of the occurrence or expected occurrence of specified en route weather phenomena that may affect the safety of aircraft operations. SIGMETs are issued in text format and intended for dissemination to all pilots in flight to enhance safety. SIGMETs are issued as soon as practical to give notice to operators and aircrews of potentially hazardous en route conditions. • All SIGMETs are a vailable on https://www.aviationweather.gov. • SIGMETs for Alaska are also available on the Alaska Aviation Weather Unit (AAWU) website. • SIGMETs for Hawaii are also available on the NWS Weather Forecast Office (WFO) Honolulu website. Although the areas where the SIGMETs apply may be shown graphically, a graphical depiction of the SIGMET area is not the entire SIGMET. Additional information regarding the SIGMET may be contained in the text version. 5.1.1 SIGMET Issuance. SIGMETs are issued from Meteorological Watch Offices (MWO). The United States has three MWOs: the Aviation Weather Center (AWC), the AAWU, and the WFO Honolulu. Their Areas of Responsibility (AOR) are as follows: The AWC: • Twenty dome stic Air Route Traffic Control Center (ARTCC) Flight Information Regions (FIRs) covering the continental United States (CONUS) and adjacent coastal waters. CONUS SIGMETs except for Convective SIGMETs are grouped into six areas (see Figure 5-1, AWC SIGMET Areas of Responsibility—Continental U.S.). • The New York, Houston, Miami, and San Juan Oceanic FIRs (see Figure 5-2, AWC SIGMET Areas of Responsibility—Atlantic Basin). • The Oakland Oceanic FIR north of 30 north latitude and the portion east of 140 west longitude, which is between the equator and 30 north latitude (see Figure 5-3, SIGMET Areas of Responsibility—Pacific Basin). The AAWU is responsible for the Anchorage Continental FIR and Anchorage Oceanic FIR (see Figure 5-3). WFO Honolulu is responsible for the Oakland Oceanic FIR south of 30 north latitude and between 140 west and 130 east longitude (see Figure 5-3). ### Page 142 5-2 Figure 5-1. AWC SIGMET Areas of Responsibility—Continental U.S. F igure 5-2. AWC SIGMET Areas of Responsibility—Atlantic Basin ### Page 143 5-3 Figure 5-3. SIGMET Areas of Responsibility—Pacific Basin Note that KKCI refers to the AWC that is in Kansas City, MO. 5.1.1.1 SIGMET Identification. U.S. SIGMETs (other than Convective SIGMETs) are assigned a series identifier: • AWC for CONUS: o NOVEMBER, OSCAR, PAPA, QUEBEC, ROMEO, UNIFORM, VICTOR, WHISKEY, XRAY and YANKEE. Note that SIERRA and TANGO are excluded. • AWC for Oakland Oceanic FIR: o ALFA, BRAVO, CHARLIE, DELTA, ECHO, FOXTROT, GOLF and HOTEL. • Honolulu MWO for Oakland Oceanic FIR: o NOVEMBER, OSCAR, PAPA, QUEBEC, ROMEO, SIERRA, TANGO, UNIFORM, VICTOR, WHISKEY, XRAY, YANKEE and ZULU. ### Page 144 5-4 • AAWU for Anchorage FIR: o INDIA, JULIET, KILO, LIMA and MIKE. A number is assigned sequentially with each issuance until the phenomenon ends. At 0000 Coordinated Universal Time (UTC) each day, all continuing SIGMETs are renumbered to 1 regardless of a continuation of the phenomena (e.g., YANKEE 1, YANKEE 2, YANKEE 3). 5.1.2 SIGMET Standardization. SIGMETs follow these standards: • All heights or altitudes are referenced to above Mean Sea Level (MSL), unless otherwise noted, and annotated using the height in hundreds of feet, consisting of three digits (e.g., 040). For heights at or above 18,000 feet (ft), the level is preceded by flight levels (FL) (e.g., FL180). • References to latitude and longitude are in whole degrees and minutes following the model: Nddmm or Sddmm, Wdddmm, or Edddmm with a space between latitude and longitude and a hyphen between successive points. Example: N3106 W07118— N3011 W7209. • Messages a re prepared in abbreviated plain language using contractions from FAA Order JO 7340.2, Contractions, for domestic products and International Civil Aviation Organization (ICAO) document 8400 for international products issued for Oceanic FIRs. A limited number of non-abbreviated words, geographical names, and numerical values of a self-explanatory nature may also be used. 5.1.3 SIGMET (Non-Convective)—CONUS. 5.1.3.1 SIGMET (Non-Convective) Issuance Criteria—CONUS. A SIGMET may be issued in the CONUS when any of the following conditions are affecting or, in the judgment of the forecaster, are expected to affect an area of at least 3,000 square miles (m2) or an area judged to have a significant impact on the safety of aircraft operations: • Severe or greater turbulence (SEV TURB). • Severe icing (SEV ICE). • Widespread duststorm (WDSPR DS). • Widespread sandstorm (WDSPR SS). • Volcanic ash (VA). 5.1.3.2 SIGMET (Non-Convective) Issuance Time and Valid Period—CONUS. A SIGMET is an unscheduled product issued any time conditions reaching SIGMET criteria are occurring or expected to occur within a 4-hour period. A SIGMET can have a valid period up to, but not exceeding, 4 hours. SIGMETs ### Page 145 5-5 for continuing phenomena will be reissued at least every 4 hours as long as SIGMET conditions continue to occur in the AOR. 5.1.3.3 SIGMET (Non-Convective) Format and Example—CONUS. The content and order of elements in the SIGMET are as follows: • Series name and number. • Valid beginning and ending time (UTC). • List of states affected by the phenomena. • Location of phenomena delineated by high-altitude Very High Frequency Omni-Directional Range (VOR) coordinates covering the affected area during the SIGMET valid time. • Phenomena description (e.g., SEV ICE). • Vertical extent (base and top), if appropriate. • Movement, if appropriate. • Intensity change (INTSF—intensifying, WKN—weakening, NC—no change). • Indication that the weather condition will continue during the 4 hours beyond the valid time of the SIGMET. Figure 5-4. SIGMET for the Continental U.S.—Example SFOR UWS 100130 SIGMET ROMEO 1 VALID UNTIL 100530 OR WA **FROM SEA TO PDT TO EUG TO SEA SEV TURB BTN FL280 AND FL350. CONDS CONTG BYD 0530Z.** ### Page 146 5-6 Table 5-1. Decoding a SIGMET (Non-Convective) for the Continental U.S. Line Content Description 1 SFO SIGMET area identifier R SIGMET series WS Product identifier 100130 Issuance date/time UTC 2 SIGMET Product type ROMEO SIGMET series name 1 Series issuance number VALID UNTIL 100530 Ending valid date/time UTC 3 OR WA Phenomenon location (states) 4 FROM SEA TO PDT TO EUG TO SEA Phenomenon location (high-altitude VOR coordinates) 5 SEV TURB BTN FL280 AND FL350. CONDS CONTG BYD 1000Z Phenomenon description The SIGMET in Figure 5-4, SIGMET for the Continental U.S.—Example, is decoded as the following: (Line 1) SIGMET ROMEO series issued for the San Francisco Area at 0130 UTC on the 10th day of the month. (Line 2) This is the first issuance of the SIGMET ROMEO series and is valid until the 10th day of the month at 0530 UTC. (Line 3) The affected states within the SFO area are Oregon and Washington. (Line 4) From Seattle, WA; to Pendleton, OR; to Eugene, OR; to Seattle, WA. (Line 5) Severe turbulence between Flight Level 280 and Flight Level 350. Conditions continuing beyond 0530Z. ### Page 147 5-7 5.1.3.4 SIGMET (Non-Convective) Cancellations—CONUS. A CONUS Non-Convective SIGMET is canceled when the phenomena is no longer occurring or no longer expected to occur or has moved out of the AOR. 5.1.3.5 SIGMET (Non-Convective) Amendments—CONUS. Amendments to CONUS Non-Convective SIGMETs are not issued. Instead, a new SIGMET is issued using the next series number. 5.1.3.6 SIGMET (Non-Convective) Corrections—CONUS. Corrections to CONUS Non-Convective SIGMETs are issued as necessary. The corrected SIGMET is identified by a “COR” located at the end of the first line after the issuance UTC date/time. 5.1.4 Convective SIGMET. Convective SIGMETs are issued for the CONUS instead of SIGMETs for thunderstorms. Any Convective SIGMET implies severe or greater turbulence, severe icing, and low-level windshear. Although the areas where the Convective SIGMETs apply may be shown graphically, such a graphical depiction of the Convective SIGMET polygon is a “snapshot” that outlines the area (or line) of thunderstorms at the issuance time of 55 past each hour. During the valid time of the SIGMET, the area/line will move according to the movement vector given in the SIGMET. For fast moving areas or lines, they will very likely end up outside the SIGMET polygon by the end of the hour. Slow moving or stationary areas or lines will likely remain in or very close to the original polygon. For additional clarification, the movement “MOV FROM...” within the Convective SIGMET describes the current movement of the SIGMET area or line. In cases when cell movements are different within the area, the SIGMET will include an additional line that states “CELL MOV FROM.” Detailed information regarding the Convective SIGMET depiction should be compared to the textual version for storm movement, velocity, cloud tops, and several other important elements. Users should exercise caution as areas of convection and their associated polygons can change and should only be used for strategic planning. 5.1.4.1 Convective SIGMET—Routine Issuance Criteria. A Convective SIGMET will be issued when any of the following conditions are occurring or, in the judgment of the forecaster, are expected to occur: • A line of thunderstorms at least 60 miles (mi) long with thunderstorms affecting at least 40 percent of its length. • An area of active thunderstorms affecting at least 3,000 mi2 covering at least 40 percent of the area concerned and exhibiting a very strong radar reflectivity intensity or a significant satellite or lightning signature. • Embedded or severe thunderstorm(s) expected to occur for more than 30 minutes during the valid period regardless of the size of the area. ### Page 148 5-8 5.1.4.2 Convective SIGMET—Special Issuance Criteria. A special Convective SIGMET may be issued when any of the following criteria are occurring or, in the judgment of the forecaster, are expected to occur for more than 30 minutes of the valid period. • Tornado, hail greater than or equal to ¾ inch (in) (at the surface), or wind gusts greater than or equal to 50 knots (kts) (at the surface) are reported. • Indications of rapidly changing conditions if, in the forecaster’s judgment, they are not sufficiently described in existing Convective SIGMETs. • Special issuance is not required for a valid Convective SIGMET. 5.1.4.3 Convective SIGMET Issuance Time and Valid Period. Convective SIGMET bulletins for the eastern, central, and western regions of the CONUS (see Figure 5-5, AWC Convective SIGMET Areas of Responsibility) are issued on a scheduled basis, hourly at 55 minutes past the hour. Each bulletin contains all valid Convective SIGMETs within the region. Convective SIGMETs are valid for 2 hours or until superseded by the next hourly issuance. A Convective SIGMET bulletin must be transmitted each hour for each region. When conditions do not meet or are not expected to meet Convective SIGMET criteria within a region at the scheduled time of issuance, a “CONVECTIVE SIGMET...NONE” message is transmitted. Figure 5-5. AWC Convective SIGMET Areas of Responsibility 5.1.4.4 Convective SIGMET Format and Example. Each Convective SIGMET bulletin includes one or more individually numbered Convective SIGMETs for the region. The content and order of each bulletin is as follows: ### Page 149 5-9 • Convective SIGMET series number and region letter (E, W or C). • Valid ending time (UTC). • List of states affected by the phenomena. • Location of phenomena delineated by high-altitude VOR coordinates covering the affected area during the SIGMET valid time. • Phenomena description (e.g., AREA SEV EMBD TS). • Movement (e.g., MOV FROM 26030KT). • Cloud top (e.g., TOPS ABV FL450). • Remarks (e.g., TORNADOES...HAIL TO 2.5 IN...WIND GUSTS TO 70KT POSS). Note 1: Tropical cyclone information will be added to remarks section of the CONUS Convective SIGMETs when appropriate. Note 2: Se e Appendix A, Definition of Common Terms Used in En Route Forecasts and Advisories, for definitions of common terms used in C onvective SIGMETs. Figure 5-6. Convective SIGMET Decoding—Example ### Page 150 5-10 Table 5-2. Decoding a Convective SIGMET Line Content Description 1 MKC Issuing Office (AWC) C Region (East, Central or West) WST Product Identifier 221855 Issuance date/time (DDHHMM) 2 CONVECTIVE SIGMET Product type 20 Issuance number C Region (East, Central or West) 3 VALID UNTIL 2055Z Valid ending time (UTC) 4 ND SD States/areas affected 5 FROM 90W MOT-GFK-ABR-90W MOT Phenomenon location (high altitude VOR coordinates) 6 INTSFYG AREA SEV TS MOVG FROM 24045KT. TOPS ABV FL450. WIND GUSTS TO 60KTS RPRTD. TORNADOES…HAIL TO 2 IN… WIND GUSTS TO 65KTS POSS ND PTN Phenomenon description, movement, cloud top, remarks The Convective SIGMET in Figure 5-6, Convective SIGMET Decoding— Example, is decoded as the following: (Line 1) Convective SIGMET issued for the central portion of the United States on the 22nd at 1855Z. (Line 2) This is the 20th Convective SIGMET issued on the 22nd for the central United States as indicated by “20C.” (Line 3) Valid until 2055Z. (Line 4) The affected states are North and South Dakota. (Line 5) From 90 nautical miles west of Minot, ND; to Grand Forks, ND; to Aberdeen, SD; to 90 nautical miles west of Minot, ND. (Line 6) An intensifying area of severe thunderstorms moving from 240 degrees at 45 knots (to the northeast). Thunderstorm tops above Flight Level 450. Wind gusts to 60 knots reported. Tornadoes, hail to 2 inches in diameter and wind gusts to 65 knots possible in the North Dakota portion. 5.1.4.5 Convective SIGMET Outlook. Each Convective SIGMET bulletin includes a 2- to 6-hour outlook at the end of the bulletin. The content and order of each bulletin is as follows: ### Page 151 5-11 • Beginning and ending valid times. • Location of expected Convective SIGMET issuances delineated by high-altitude VOR coordinates for the outlook valid time. 5.1.4.6 Convective SIGMET Cancellations. Convective SIGMETs are not canceled but are superseded by the next Convective SIGMET in the series. 5.1.4.7 Convective SIGMET Amendments. Amended Convective SIGMETs are not issued. Instead, a new Convective SIGMET is issued for that region. 5.1.4.8 Convective SIGMET Corrections. Corrections to Convective SIGMETs are issued as necessary. The corrected Convective SIGMET is identified by a “COR” located at the end of the first line after the issuance UTC date/time. 5.1.5 SIGMET—Outside the CONUS. 5.1.5.1 SIGMET Issuance Criteria—Outside the CONUS. U.S. SIGMETs outside the CONUS are issued when any of the following is occurring or expected to occur affecting an area greater than 3,000 mi2 or, in the judgment of the forecaster, an area having the potential to have a significant effect on the safety of aircraft operations. • Thunderstorm—of type below* o Obscured (OBSC TS) o Embedded (EMBD TS) o Widespread (WDSPR TS) o Squall line (SQL TS) o Isolated severe (ISOL SEV TS) • Severe turbulence (SEV TURB) • Severe icing (SEV ICE) o With freezing rain (SEV ICE (FZRA)) • Widespread duststorm (WDSPR DS) • Widespread sandstorm (WDSPR SS) • Volcanic ash (VA) • Tropical cyclone (TC) • Radioactive cloud (RDOACT CLD) ### Page 152 5-12 Note: Obscured, embedded, or squall line thunderstorms do not have to reach 3,000 mi2 criteria. *Tornado (TDO), funnel cloud (FC), waterspout (WTSPT) and heavy hail (HVY GR) may be used as a further description of the thunderstorm, as necessary. 5.1.5.2 SIGMET Issuance Time and Valid Period—Outside the CONUS. A SIGMET is an unscheduled product issued any time conditions reaching SIGMET criteria are occurring or expected to occur within a 4-hour period. A SIGMET outside the CONUS can have a valid period up to, but not exceeding, 4 hours, except for volcanic ash (VA) and tropical cyclone (TC), which can be valid up to 6 hours. SIGMETs for continuing phenomena will be reissued at least every 4 (or 6) hours as long as SIGMET conditions continue to occur in the area for responsibility. 5.1.5.3 SIGMET Format and Example—Outside the CONUS. SIGMETs outside the CONUS contain the following information, related to the specific phenomena and in the order indicated: • Phenomenon and its description (e.g., SEV TURB). • An indication whether the information is observed, using OBS and/or FCST. The time of observation will be given in UTC. • Location of the phenomenon referring, where possible the latitude and longitude, and FLs (altitude) covering the affected area during the SIGMET valid time. SIGMETs for volcanic ash cloud and tropical cyclones contain the positions of the ash cloud, tropical cyclone center, and radius of convection at the start of the validity time of the SIGMET. • Movement toward or expected movement using sixteen points of the compass, with speed in knots, or stationary, if appropriate. • Thunderstorm maximum height as FL. • Changes in intensity, using, as appropriate, the abbreviations intensifying (INTSF), weakening (WKN), or no change (NC). • Forecast position of volcanic ash cloud or the center of the tropical cyclone at the end of the validity period of the SIGMET message. ### Page 153 5-13 Figure 5-7. SIGMET Outside the Continental U.S. Decoding Example Tab le 5-3. Decoding a SIGMET Outside of the Continental U.S. Line Content Description 1 WSPA07 ICAO communication header PHFO Issuance MWO 010410 Issuance UTC date/time 2 SIGPAT NWS AWIPS communication header 3 KZOA Area Control Center SIGMET Product type TANGO SIGMET series 2 Issuance number VALID 010410/010800 Valid period UTC date/time PHFO Issuance office 4 OAKLAND OCEANIC FIR Flight Information Region (FIR) FRQ TS OBS AND FCST WI 200NM N3006 W14012 - N2012 W15016 CB TOP FL400 MOV W 10KT WKN. Phenomenon description The SIGMET in Figure 5-7, SIGMET Outside the Continental U.S. Decoding Example, is decoded as the following: (Line 1) The WMO product header is WSPA07. Issued by the PHFO on the 1st day of the month at 0410 UTC. (Line 2) The NWS AWIPS communication header is SIGPAT. (Line 3) For the Oakland (KZOA) Area Control Center. This is the 2nd issuance of SIGMET Tango series, valid from the 1st day of the month at 0410 UTC until the 1st day of the month at 0800 UTC, issued by the Honolulu Meteorological Watch Office. ### Page 154 5-14 (Line 4) Concerning the Oakland Oceanic Flight Information Region (FIR), frequent thunderstorms observed and forecast within 200 nautical miles of 30 degrees and 6 minutes north; 140 degrees and 12 minutes west; to 20 degrees and 12 minutes north, 150 degrees and 16 minutes west, cumulonimbus tops to flight level 400 moving west at 10 knots, weakening. 5.1.5.4 SIGMETs for Volcanic Ash—Outside the CONUS. A SIGMET for volcanic ash cloud is issued for volcanic eruptions. A volcanic eruption is any volcanic activity, including the emission of volcanic ash, regardless of the eruption’s magnitude. Initial volcanic ash SIGMETs may be issued based on credible pilot reports in the absence of a volcanic ash advisory (VAA), but are updated once a VAA is issued. Volcanic ash SIGMETs will continue to be issued until the ash cloud is no longer occurring or expected to occur over the AOR. SIGMETs for volcanic ash cloud are valid up to 6 hours and provide an observed or forecast location of the ash cloud at the beginning of the SIGMET. A 6-hour forecast position for the ash cloud, valid at the end of the validity period of the SIGMET message, is also included. SIGMETs are reissued at least every 6 hours while the volcanic ash cloud hazard exists or is expected to exist. 5.1.5.5 SIGMETs for Tropical Cyclone—Outside the CONUS. SIGMETs for a tropical cyclone may be issued for non-frontal synoptic-scale cyclones meeting the following criteria: • Originates over tropical or sub-tropical waters with organized convection and definite cyclonic surface wind circulation. • Wind speeds reach 35 kts independent of the wind averaging time used by the Tropical Cyclone Advisory Center (TCAC). SIGMETs for tropical cyclones will be valid up to 6 hours. SIGMETs for tropical cyclones will include two positions. The first position included will be the TCAC advisory position. The second position will be the forecast position valid at the end of the SIGMET period. In addition to the two storm positions, SIGMETs will include associated convection when applicable. SIGMETs will be reissued at least every 6 hours while the tropical cyclone wind remains or are expected to remain above 34 kts. 5.1.5.6 SIGMET Cancellation—Outside the CONUS. SIGMETs are canceled when the phenomena is no longer occurring or expected to occur in the AOR. 5.1.5.7 SIGMET Amendments—Outside the CONUS. SIGMET amendments will not be issued. Instead, the next SIGMET in the ### Page 155 5-15 series is issued to accomplish the update. The valid time of the new SIGMET is reset to reflect the new 4-hour valid period (6-hour for VA and TC SIGMETs). 5.1.5.8 SIGMET Corrections—Outside the CONUS. Corrections to SIGMETs are issued as necessary. This is done by issuing a new SIGMET in the series, which advances the SIGMET number and cancels the previous SIGMET. 5.1.5.9 SIGMET for Volcanic Ash Example—Outside the CONUS. WVNT06 KKCI 082030 TJZS SIGMET FOXTROT 2 VALID 082030/090230 KKCI- SAN JUAN FIR VA FROM SOUFRIERE HILLS LOC 1642N06210W VA CLD OBS AT 2030Z WI N1730 W06400 - N1700 W06300 - N1650 W06300 - N1710 W06400 - N1730 W06400. SFC/060. MOV W 15KT. FCST 0230Z VA CLD APRX N1730 W06500 - N1700 W06300 - N1650 W06300 - N1710 W06500 - N1730 W06500. The ICAO communication header for this product is WVNT06. It is a SIGMET issued by the Aviation Weather Center (KKCI) in Kansas City, Missouri, on the 8th day of the month at 2030 UTC. This is the second (2) issuance of SIGMET series Foxtrot valid from the 8th day of the month at 2030 UTC until the 9th day of the month at 0230 UTC. Within the San Juan Oceanic FIR, volcanic ash from Soufriere Hills volcano located at 16 degrees/42 minutes north, 62 degrees/10 minutes west. Volcanic ash cloud observed at 2030 UTC within an area bounded by 17 degrees/30 minutes north, 64 degrees/00 minutes west to 17 degrees/00 minutes north, 63 degrees/00 minutes west to 16 degrees/50 minutes north, 63 degrees/00 minutes west to 17 degrees/ 10 minutes north, 64 degrees/00 minutes west to 17 degrees/30 minutes north, 64 degrees/00 minutes west. From the surface to 6,000 feet MSL. Moving to the west at 15 knots. Forecast at 0230 UTC, volcanic ash cloud located approximately at 17 degrees/30 minutes north, 65 degrees/00 minutes west to 17 degrees/00 minutes north, 63 degrees/00 minutes west to 16 degrees/50 minutes north, 63 degrees/00 minutes west to 17 degrees/10minutes north, 65 degrees/00 minutes west to 17 degrees/30 minutes north, 65 degrees/ 00 minutes west. ### Page 156 5-16 5.1.5.10 SIGMET for Tropical Cyclone Example—Outside the CONUS. WSNT03 KKCI 081451 SIGA0C KZNY SIGMET CHARLIE 11 VALID 081500/082100 KKCI- NEW YORK OCEANIC FIR TC KYLE OBS N3106 W07118 AT 1500Z CB TOP FL500 WI 120NM OF CENTER MOV WSW 5 KT NC FCST 2100Z TC CENTER N3142 W07012 The ICAO communication header for this product is WSNT03. It is a SIGMET issued by the Aviation Weather Center (KKCI) in Kansas City, Missouri, on the 8th day of the month at 1451 UTC. The National Weather Service AWIPS communication header for this product is SIGPAT. This is the eleventh (11) issuance of SIGMET series Charlie valid from the 8th day of the month at 1500 UTC until the 8th day of the month at 2100 UTC. Within the New York Oceanic FIR, Tropical Cyclone Kyle observed at 31 degrees/6 minutes north, 71 degrees/18 minutes west at 1500 UTC, cumulonimbus tops to flight level 500 (approximately 50,000 feet MSL), within 120 nautical miles of the center, moving from west-southwest at 5 knots, no change in intensity is forecast, at 2100 UTC the tropical cyclone center will be at 31 degrees/42 minutes north, 70 degrees/12 minutes west. 5.2 Airmen’s Meteorological Information (AIRMET). An AIRMET is a concise description of the occurrence or expected occurrence of specified en route weather phenomena that may affect the safety of aircraft operations, but at intensities lower than those that require the issuance of a SIGMET. AIRMETs are intended for dissemination to all pilots in flight to enhance safety and are of particular concern to operators and pilots of aircraft sensitive to the phenomena described and to pilots without instrument ratings. AIRMETs give notice to operators and aircrews of potentially hazardous en route conditions. Although the areas where the AIRMETs apply may be shown graphically, a graphical depiction of the AIRMET area is not the entire AIRMET. Additional information regarding the AIRMET may be contained in the text version. • AIR METs are available for the CONUS on https://www.aviationweather.gov. • AIRMETs are available for Alaska on the AAWU website. • AIRMETs are available for Hawaii on the NWS WFO Honolulu website. ### Page 157 5-17 5.2.1 AIRMET Issuance. AIRMETs are issued from the three MWOs located at the AWC, AAWU, and WFO Honolulu. Their AORs are: • AWC: The CONUS and adjacent coastal waters (see Figure 5-8, AWC AIRMET Areas of Responsibility—Continental U.S.). • AAWU: Alaska and adjacent coastal waters (see Figure 5-9, AAWU Flight Advisory and Area Forecast (FA) Zones—Alaska). • WFO Honolulu: Hawaii and adjacent waters (see Figure 5-10, WFO Honolulu AIRMET Areas of Responsibility—Hawai i). Figure 5-8. AWC AIRMET Areas of Responsibility—Continental U.S. F igure 5-9. AAWU Flight Advisory and Area Forecast (FA) Zones—Alaska ### Page 158 5-18 Table 5-4. AAWU Flight Advisory and Area Forecast (FA) Zones—Alaska 1 Arctic Coast Coastal 2 North Slopes of the Brooks Range 3 Upper Yukon Valley 4 Koyukuk and Upper Kobuk Valley 5 Northern Seward Peninsula-Lower Kobuk Valley 6 Southern Seward Peninsula-Eastern Norton Sound 7 Tanana Valley 8 Lower Yukon Valley 9 Kuskowim Valley 10 Yukon-Kuskowim Delta 11 Bristol Bay 12 Lynn Canal and Glacier Bay 13 Central Southeast Alaska 14 Southern Southeast Alaska 15 Coastal Southeast Alaska 16 Eastern Gulf Coast 17 Copper River Basin 18 Cook Inlet-Susitna Valley 19 Central Gulf Coast 20 Kodiak Island 21 Alaska Peninsula-Port Heiden to Unimak Pass 22 Unimak Pass to Adak 23 St. Lawrence Island-Bering Sea Coast 24 Adak to Attu 25 Pribilof Islands and Southeast Bering Sea Figure 5-10. WFO Honolulu AIRMET Areas of Responsibility—Hawaii ### Page 159 5-19 5.2.2 AIRMET Issuance Criteria. An AIRMET may be issued when any of the following weather phenomena are occurring or are expected to occur over an area of at least 3,000 mi2: • Ceiling less than 1,000 ft and/or visibility less than 3 statute miles (sm) (IFR). o Weather phenomena restricting the visibility including, but not limited to, precipitation (PCPN), smoke (FU), haze (HZ), mist (BR), fog (FG), and blowing snow (BS). • Widespread mountain obscuration (MTN OBSCN). o Weather phenomena causing the obscuration are included, but not limited to, clouds (CLDS), precipitation (PCPN), smoke (FU), haze (HZ), mist (BR), and fog (FG). • Moderate turbulence (MOD TURB). o Top and bottom of MOD TURB layer are specified. • Sustained surface wind greater than 30 kts (STG SFC WND). • Moderate icing (MOD ICE). o Top and bottom of MOD ICE are specified. o The range of freezing level altitudes is given when the bottom altitude of MOD ICE is the freezing level (FRZLVL). o Areas with multiple freezing levels are specified. o Range of freezing levels over the area is specified. o Lowest freezing levels above ground level (AGL) at intervals of 4,000 ft MSL (or SFC as appropriate) are specified. • Non-Convective low-level windshear potential below 2,000 ft AGL (LLWS POTENTIAL). 5.2.3 AIRMET Standardization. All AIRMETs follow these standards: • All heights or altitudes are referenced to above MSL, unless otherwise noted, and annotated using the height in hundreds of feet, consisting of three digits (e.g., 040). For heights at or above 18,000 ft, the level is preceded by FL to represent flight levels (e.g., FL180). • Messages are prepared in abbreviated plain language using contractions from Order 7340.2. A limited number of non-abbreviated words, geographical names, and numerical values of a self-explanatory nature may also be used. • We ather and obstructions to visibility are described using the weather abbreviations for surface weather observations (Aviation Routine Weather Report (METAR)/Special Weather Report (SPECI)). See the Federal Meteorological ### Page 160 5-20 Handbook No. 1 (FMH-1), Surface Weather Observations and Reports, or paragraph 3.1 of this document. Note: See Appendix A for definitions of common terms used in AIRMETs. 5.2.4 AIRMET Bulletins, Issuance Times, and Valid Period. AIRMETs are issued as bulletins containing one or more AIRMET messages following the schedule listed in Table 5-5, AIRMET Issuance Schedule. AIRMETs are issued 4 times a day except for those in Alaska, which are issued three times a day. Unscheduled AIRMETs are issued when conditions are occurring or expected to occur but were not forecast. Table 5-5. AIRMET Issuance Schedule 1st Scheduled Issuance (UTC) 2nd Scheduled Issuance (UTC) 3rd Scheduled Issuance (UTC) 4th Scheduled Issuance (UTC) CONUS 0245 0845 1445 2045 Alaska 0415 (DT)/ 0515 (ST) 1215 (DT)/ 1315 (ST) 2015 (DT)/ 2115 (ST) None Hawaii 0400 1000 1600 2200 Note: DT—During Alaska Daylight Time, ST—During Alaska Standard Time AIRMETs are valid for 6 hours except for those in Alaska, which are valid for 8 hours. The valid period of an AIRMET message cannot exceed the valid time of the AIRMET bulletin. However, note that each AIRMET contains remarks concerning the continuance of the phenomenon during the 6 hours following the AIRMET ending time. Also, AIRMET bulletins can contain a separate outlook when conditions meeting AIRMET criteria are expected to occur during the 6-hour (8-hour for Alaska) period after the valid time of the AIRMET bulletin. 5.2.5 AIRMET Format and Example. An AIRMET message includes the following information as appropriate and in the order indicated: • Reference to appropriate active SIGMETs affecting the area at the time of AIRMET issuance (e.g., SEE SIGMET BRAVO SERIES). • Beginning time of the AIRMET phenomenon if different from the AIRMET beginning valid time. • AIRMET name (SIERRA, TANGO, or ZULU), update number, weather phenomenon, and ending valid time. (Note: The AIRMET number is reset to 1 after 0000 UTC each day.) ### Page 161 5-21 o AIRMET Sierra describes IFR (instrument flight rules) conditions and/or extensive mountain obscurations. Hawaii AIRMETs for mountain obscuration may be issued for an area less than 3,000 mi2. o AIRMET Tango describes moderate turbulence, sustained surface winds of 30 kts or greater, and Non-Convective low-level windshear. o AIRMET Zulu describes moderate icing and provides freezing-level heights. • List of affected states (CONUS only). • Location of phenomenon using VORs. • Description of phenomenon for the AIRMET issuance. • Vertical extent (bases and tops), as appropriate. • Ending time of phenomenon if different from the AIRMET ending time. • Remarks concerning the continuance of the phenomenon during the 6 hours following the AIRMET ending time. • CONUS and Hawaii AIRMETs: A separate AIRMET outlook is included in the AIRMET bulletin when conditions meeting AIRMET criteria are expected to occur during the 6-hour period after the valid time of the AIRMET bulletin. • Alaska AIRMETs: Outlook information is included in the appropriate area forecast (FA) zone when conditions are expected to occur during the 8-hour period after the valid time of the AIRMET bulletin. Figure 5-11. AIRMET Bulletin Decoding—Example ### Page 162 5-22 Table 5-6. Decoding an AIRMET Bulletin Line Content Description 1 BOS AIRMET area identifier S AIRMET series WA Product type 211945 Issuance UTC date/time 2 AIRMET Product type SIERRA AIRMET series UPDT 3 Update number FOR IFR AND MTN OBSCN Product description VALID UNTIL 220200 Ending UTC date/time 3A AIRMET IFR..ME NH VT MA CT RI NY NJ AND CSTL WTRS Product type/series… Phenomenon location (states) 3B AIRMET MTN OBSCN..ME NH VT MA NY PA 4A FROM CAR TO YSJ TO 150E ACK TO EWR TO YOW TO CAR Phenomenon location (VOR locations) 4B FROM CAR TO MLT TO CON TO SLT TO SYR TO CAR 5A CIG BLW 010/VIS BLW 3SM PCPN/BR. CONDS CONT BYD 02Z THRU 08Z. Phenomenon description 5B MTNS OBSCD BY CLDS/PCPN/BR. CONDS CONT BYD 02Z THRU 08Z. The AIRMET bulletin in Figure 5-11, AIRMET Bulletin Decoding—Example, is decoded as follows: (Line 1) AIRMET SIERRA issued for the Boston area at 1945Z on the 21st day of the month. “SIERRA” contains information on instrument flight rules (IFR) and/or mountain obscurations. (Line 2) This is the third updated issuance of this Boston AIRMET series as indicated by “SIERRA UPDT 3” and is valid until 0200Z on the 22nd. (Line 3A) The affected states within the BOS area are: Maine, New Hampshire, Vermont, Massachusetts, Connecticut, Rhode Island, New York, New Jersey and coastal waters. (Line 3B) The affected states within the BOS area are: Maine, New Hampshire, Vermont, Massachusetts, New York and Pennsylvania. (Line 4A) Within an area bounded by: Caribou, ME; to Saint Johns, New Brunswick; to 150 nautical miles east of Nantucket, MA; to Newark, NJ; to Ottawa, Ontario; to Caribou, ME. ### Page 163 5-23 (Line 4B) Within an area bounded by: Caribou, ME to Millinocket, ME to Concord, NH to Slate Run, PA to Syracuse, NY to Caribou, ME. (Line 5A) Ceiling below 1,000 feet/visibility below 3 statute miles, precipitation/mist. Conditions continuing beyond 0200Z through 0800Z. (Line 5B) Mountains Obscured by clouds, precipitation and mist. Conditions continuing beyond 0200Z through 0800Z. 5.2.5.1 AIRMET Updates and Amendments. If an AIRMET is amended, AMD is added after the date/time group on the FAA product line. The update number will be incremented, UPDT is added to end of the line containing the list of affected states (CONUS only). The issuance time of the AIRMET bulletin is updated to reflect the time of the amendment. The ending valid time remains unchanged. 5.2.5.2 AIRMET Corrections. AIRMETs containing errors are corrected by adding COR after the date/time group on the FAA product line. The issuance time of the AIRMET bulletin is updated to reflect the time of the correction. The ending valid time remains unchanged. 5.3 Graphical Airman’s Meteorological Advisory (G-AIRMET). The Graphical AIRMET (G-AIRMET) product is a decision-making tool based on weather “snapshots” displayed at short time intervals. The G-AIRMET identifies hazardous weather in space and time more precisely than text products, enabling pilots to maintain high safety margins while flying more efficient routes. The NWS’s goal is to maximize aviation safety and air space efficiency by providing the most accurate and timely weather information possible to enhance both pre-flight and in-flight decision making. For decades, NWS has issued text-based AIRMETs that have provided broad-scale descriptions of hazardous weather. Often referred to as a time “smear,” the text-based AIRMET requires meteorologists to describe hazardous weather over large geographical areas for 6-hour periods. G-AIRMET provides more precise and informative weather hazard depictions than the text-only AIRMET. Aviation weather users have found that pictures are worth a thousand contractions. G-AIRMETs provide a better path from the aviation meteorologist to the weather user, by providing precise, interactive, and easy-to-understand graphical displays. Meteorologists can put their energy into creating and updating G-AIRMET weather graphics. The traditional text AIRMET coincides with G-AIRMET information because texts AIRMETs are formatted from G-AIRMET information. G-AIR METs are available for the CONUS and adjacent coastal waters only and can be viewed on https://www.aviationweather.gov. ### Page 164 5-24 5.3.1 G-AIRMET Issuance. G-AIRMETs are issued by the AWC every 6 hours and are updated/amended as necessary, coinciding with the text AIRMET products. Table 5-7. G-AIRMET Issuance Schedule 1st Scheduled Issuance (UTC) 2nd Scheduled Issuance (UTC) 3rd Scheduled Issuance (UTC) 4th Scheduled Issuance (UTC) CONUS 0300 0900 1500 2100 5.3.2 G-AIRMET Content. G-AIRMET snapshots are graphical forecasts of en route weather hazards valid at discrete times no more than 3 hours apart for a period of up to 12 hours into the future (00, 03, 06, 09, and 12 hours). 00 hour represents the initial conditions, and the subsequent 3-hourly graphics depict the area affected by the particular hazard at that valid time. The AWC’s website provides several options to display the G-AIRMET. There is an interactive display as well as snapshots that may be viewed as static (single), combined, and looped images using the tools provided on the Web page. The following are examples of G-AIRMET snapshot static images. G-AIRMET depicts the following en route aviation weather hazards: • IFR Ceiling and Visibility (see Figure 5-12, G-AIRMET—Ceiling and Visibility (IFR) Snapshot Example). o Areas of cloud ceilings with bases less than 1000 ft AGL and/or areas of surface visibilities below 3 sm, including the weather causing the visibility restriction. The cause of the visibility restriction includes only PCPN, FU, HZ, BR, FG, and BLSN. ### Page 165 5-25 Figure 5-12. G-AIRMET—Ceiling and Visibility (IFR) Snapshot Example • M ountain Obscuration (see Figure 5-13, G-AIRMET—Mountain Obscuration Snapshot Example). o Areas of widespread mountain obscuration where Visual Meteorological Conditions (VMC) cannot be maintained, including the weather causing the obscuration. The weather causing the obscuration includes only CLDS, PCPN, FU, HZ, BR, and FG. Figure 5-13. G-AIRMET—Mountain Obscuration Snapshot Example ### Page 166 5-26 • Icing (see Figure 5-14, G-AIRMET—Icing Snapshot Example). o Areas of moderate airframe icing, other than convectively induced, including the vertical extent (base and top). Altitude variations in the base of icing layers may be denoted (e.g., “080 / 060” indicates the altitude varies between 6,000 and 8,000 ft MSL). Figure 5-14. G-AIRMET—Icing Snapshot Example • Fre ezing Level (see Figure 5-15, G-AIRMET—Freezing Level Snapshot Example). o Freezing level is defined as the lowest freezing level above the ground or at the SFC as appropriate. Freezing levels above the ground are delineated at 4,000 ft intervals MSL. Areas with multiple freezing levels above the ground are delineated including the vertical extent (base and top). ### Page 167 5-27 Figure 5-15. G-AIRMET—Freezing Level Snapshot Example • Turbulen ce (see Figure 5-16, G-AIRMET—Turbulence-High Snapshot Example, and Figure 5-17, G-AIRMET—Turbulence-Low Snapshot Example). o Areas of moderate turbulence, other than convectively induced, including the vertical extent (base and top). ### Page 168 5-28 Figure 5-16. G-AIRMET—Turbulence-High Snapshot Example F igure 5-17. G-AIRMET—Turbulence-Low Snapshot Example ### Page 169 5-29 • Low-Level Wind Shear (LLWS) (see Figure 5-18, G-AIRMET—Low-Level Wind Shear (LLWS) Snapshot Example). o LLWS is defined as windshear below 2,000 ft AGL, other than convectively induced, exceeding 10 kts per 100 ft (vector difference between two points in space). Figure 5-18. G-AIRMET—Low-Level Wind Shear (LLWS) Snapshot Example • Str ong Surface Winds (see Figure 5-19, G-AIRMET—Strong Surface Winds Snapshot Example). o Areas of sustained surface winds greater than 30 kts. The direction and speed of winds are not depicted; only the area where sustained surface winds greater than 30 kts are occurring or forecast to occur. ### Page 170 5-30 Figure 5-19. G-AIRMET—Strong Surface Winds Snapshot Example 5.3.3 Use. When using the G-AIRMET, users must keep in mind that if a 00-hour forecast shows no significant weather and a 03-hour forecast shows hazardous weather, they must assume a change is occurring during the period between the two forecasts. It should be taken into consideration that the hazardous weather starts immediately after the 00-hour forecast unless there is a defined initiation or ending time for the hazardous weather. The same would apply after the 03-hour forecast. The user should assume the hazardous weather condition is occurring between the snapshots unless informed otherwise. 5.4 Center Weather Advisory (CWA). A CWA is an aviation weather warning for conditions meeting or approaching national in-flight advisory (AIRMET, SIGMET, or Convective SIGMET) criteria. The CWA is primarily used for aircrews to anticipate and avoid adverse weather conditions in the en route and terminal environments. CWAs are available on https://www.aviationweather.gov. 5.4.1 CWA Issuance. CWAs are issued by the NWS Center Weather Service Units (CWSU). CWSU AORs are depicted on Figure 5-20, Center Weather Service Unit (CWSU) Areas of Responsibility. ### Page 171 5-31 Figure 5-20. Center Weather Service Unit (CWSU) Areas of Responsibility C WAs are valid for up to 2 hours and may include forecasts of conditions expected to begin within 2 hours of issuance. If conditions are expected to persist after the advisory’s valid period, a statement to that effect is included in the last line of the text. Additional CWAs will subsequently be issued as appropriate. Notice of significant changes in the phenomenon described in a CWA is provided by a new CWA issuance for that phenomenon. If the forecaster deems it necessary, CWAs may be issued hourly for Convective activity. 5.4.2 CWA Criteria. CWAs may be issued for the following situations: • There is no existing AWC or AAWU advisory in effect. • Any of the following conditions occur: o Conditions meeting Convective SIGMET criteria. o Icing—moderate or greater. o Turbulence—moderate or greater. o Heavy and extreme precipitation. o Freezing precipitation. o Conditions at or approaching low IFR (LIFR). o Surface wind gust at or above 30 kts. o LLWS (Surface—2,000 ft). o Volcanic ash, duststorms, or sandstorms. ### Page 172 5-32 o When a hazard has grown significantly outside of the boundary defined by the AWC or AAWU advisory. o To upgrade a thunderstorm advisory to include severe thunderstorms. o To upgrade an AIRMET to include isolated severe turbulence or icing. If greater than isolated severe turbulence or icing is occurring, then a new (non-supplementary) CWA should be issued. o To define a line of thunderstorms within a larger area covered by the AWC or AAWU advisory. o To better define hazards expected at a major terminal already within an AWC or AAWU advisory. o Anything that in the judgment of the CWSU forecaster will add value to an existing advisory. • In the forecaster’s judgment the conditions listed above, or any others, may adversely impact the safe flow of air traffic. 5.4.3 CWA Format and Example. Figure 5-21. Center Weather Advisory (CWA) Decoding Example ### Page 173 5-33 Table 5-8. Decoding a Center Weather Advisory (CWA) Line Content Description 1 ZDV ARTCC Identification 2 Phenomenon Number (single digit, 1-6) CWA Product Type (UCWA/CWA) 032140 Beginning and/or issuance UTC date/time 2 ZDV ARTCC Identification CWA Product Type 2 Phenomenon Number (single digit, 1-6) 02 Issuance Number (issued sequentially for each Phenomenon Number) VALID TIL 032340Z Ending valid UTC date/time 3 FROM FMN TO 10N FMN TO 20NE FMN TO 10E FMN TO FMN Phenomenon Location 4 ISOLD SEV TS NR FMN MOVG NEWD 10KTS. TOP FL410. WND GSTS TO 55KTS. HAIL TO 1 INCH RPRTD AT FMN. SEV TS CONTG BYD 2340Z Phenomenon Description The CWA in Figure 5-21, Center Weather Advisory (CWA) Decoding Example, is decoded as follows: (Line 1) Center Weather Advisory issued for the Denver ARTCC (ZDV) CWSU. The “2” after ZDV in the first line denotes this is the second meteorological event of the local calendar day. This CWA was issued/begins on the 3rd day of the month at 2140 UTC. (Line 2) The Denver ARTCC (ZDV) is identified again. The “202” in the second line denotes the phenomena number again (2) and the issuance number (02) for this phenomenon. This CWA is the valid until the 3rd day of the at 2340 UTC. (Line 3) From Farmington, New Mexico to 10 nautical miles north of Farmington, New Mexico to 20 nautical miles northeast of Farmington, NM to 10 nautical mile east of Farmington, New Mexico to Farmington, New Mexico. (Line 4) Isolated severe thunderstorms near Farmington moving northeastward at 10 knots. Tops to Flight Level 410. Wind gusts to 55 knots. Hail to one inch reported at Farmington. Severe thunderstorms continuing beyond 2340 UTC. 5.5 Meteorological Impact Statement (MIS). The MIS is a nontechnical plain language product intended primarily for FAA traffic managers and those involved in planning aircraft routing. MISs are issued by NWS Center Weather Service Units (CWSU). MISs are available at https://www.aviationweather.gov, as well as CWSU websites. ### Page 174 5-34 5.5.1 Valid Period. The MIS valid times are determined according to local policy. The MIS is limited to not exceed a 48-hour valid period. 5.5.2 Content. The MIS is a brief nontechnical discussion of meteorological events causing or expecting to cause the disruption of the safe flow of air traffic. This is followed by specifics such as what is causing the disruption, area, altitudes, and movement. The MIS may refer to an online graphic, especially for complex situations, using a specific Web address and provide a brief description of the weather that is included in the text MIS. MIS products are numbered sequentially beginning at midnight local time each day. The MIS is disseminated and stored as a “replaceable product.” If the expiration time of the MIS is after the closing time of the CWSU, then a “No updates available after ddhhmmZ” message should be included at the end of the MIS text, where dd = date, hh = hour, mm = minutes. 5.5.3 Format. The MIS format consists of a communication header line, the words “FOR ATC PLANNING PURPOSES ONLY,” and the text. • Header Line: zzz MIS ii Valid ddtttt-ddtttt Zzz is the ARTCC identification (i.e. ZJX), MIS is the product type, ii is the 2-digit sequential issuance number, and ddtttt is the valid beginning and ending date/time UTC. • The line immediately below the header line reads “FOR ATC PLANNING PURPOSES ONLY.” • The m aximum length of the MIS is four lines. The MIS is nontechnical in nature to convey expected weather and impacts in the clearest and simplest manner possible to the user. References to a graphical product on the local CWSU website or https://www.aviationweather.gov may be included. Example: **ZAB MIS 02 VALID 281300-290300 ...FOR ATC PLANNING PURPOSES ONLY...** AN UPPER-LVL DISTURBANCE OVER COLORADO COMBINED WITH A STRONG JET STREAM MOVING ACROSS THE SWRN U.S. IS FCST TO PRODUCE AREAS OF TURBULENCE ACROSS PORTIONS OF ZAB. THE TURBULENCE IS FCST TO SUBSIDE AFT 00Z AS THE DISTURBANCE AND JETSTREAM MOVE FURTHER EAST. MIS issued by the Albuquerque, New Mexico, CWSU. The second MIS issuance of the day, valid from the 28th day of the month at 1300 UTC until the 29th at 0300 UTC. For air traffic control (ATC) planning purposes only. (Brief discussion of the meteorological events to affect the ARTCC.) ### Page 175 5-35 5.6 Additional Products for Convection. In addition to the SIGMETs, Convective SIGMETs, and CWAs already discussed, the NWS offers more products informing the aviation community about the potential for Convective weather. 5.6.1 Convective Outlooks (AC). The NWS Storm Prediction Center (SPC) issues narrative and graphical Convective Outlooks to provide the CONUS NWS WFOs, the public, the media, and emergency managers with the potential for severe (tornado, wind gusts 50 kts or greater, or hail 1 in in diameter or greater) and non-severe (general) convection and specific severe weather threats during the following 8 days. The Convective Outlook defines areas of marginal risk (MRGL), slight risk (SLGT), enhanced risk (ENH), moderate risk (MDT), or high risk (HIGH) of severe weather based on a percentage probability, which varies for time periods from 1 day to 3 days, and then two probabilistic thresholds for days 4 through 8. The day 1, day 2, and day 3 Convective Outlooks also depict areas of general thunderstorms (TSTMS). The outlooks in graphical (see Figure 5-22, Day 1 Categorical Convective Outlook Graphic Example) and text formats are available on the SPC web site. 5.6.1.1 Issuance. Convective Outlooks are scheduled products issued at the following times: Table 5-9. Convective Outlook Issuance Schedule Convective Outlook Issuance Time (UTC) Valid Period (UTC) Day 1 0600 1200 – 1200 1300 1300 – 1200 1630 1630 – 1200 2000 2000 – 1200 0100 0100 – 1200 Day 2 0600 (Daylight Saving Time) 0700 (Standard Time) Day 2/1200 – 1200 1730 Day 2/1200 – 1200 Day 3 0730 (Daylight Saving Time) 0830 (Standard Time) Day 3/1200 – 1200 Day 4-8 0900 (Daylight Saving Time) 1000 (Standard Time) Day 4/1200 – Day 8/1200 (1 day intervals provided in graphic format on SPC Web page) SPC corrects outlooks for format and grammatical errors and amends outlooks when the current forecast does not or will not reflect the ongoing or future Convective development. ### Page 176 5-36 Figure 5-22. Day 1 Categorical Convective Outlook Graphic Example Fi gure 5-23. Categorical Outlook Legend for Days 1-3 Convective Outlook Graphic Example For mat and Example of the Categorical Convective Outlook Narrative. **DAY 1 CONVECTIVE OUTLOOK** NWS STORM PREDICTION CENTER NORMAN OK 1244 AM CDT WED MAR 25 2015 VALID 251200Z - 261200Z ...THERE IS AN ENH RISK OF SVR TSTMS LATE THIS AFTERNOON AND EVENING ACROSS PARTS OF NORTH CENTRAL AND NORTHEAST OKLAHOMA...ADJACENT SOUTHEAST KANSAS...SOUTHWEST MISSOURI AND NORTHWEST ARKANSAS... ...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE ENH RISK...ACROSS PARTS OF THE SOUTH CENTRAL PLAINS INTO THE OZARK PLATEAU... ...THERE IS A MRGL RISK OF SVR TSTMS ACROSS SURROUNDING AREAS OF THE **CENTRAL AND SOUTHERN PLAINS EAST NORTHEASTWARD INTO PARTS OF THE LOWER OHIO VALLEY...** ### Page 177 **5-37 ...SUMMARY...** SEVERE THUNDERSTORMS...MAINLY CAPABLE OF PRODUCING LARGE HAIL AND LOCALLY DAMAGING WIND GUSTS...ARE EXPECTED ACROSS PARTS OF THE SOUTH **CENTRAL PLAINS INTO THE OZARK PLATEAU LATE THIS AFTERNOON AND EVENING.** ...SYNOPSIS... SIGNIFICANT AMPLIFICATION IS ONGOING WITHIN A BRANCH OF MID-LATITUDE WESTERLIES ACROSS THE EASTERN PACIFIC INTO WESTERN NORTH AMERICA. AS LARGE-SCALE MID/UPPER RIDGING CONTINUES TO BUILD NEAR THE PACIFIC COAST DURING THIS PERIOD...MODELS INDICATE THAT LARGE-SCALE TROUGHING WILL BEGIN TO EVOLVE DOWNSTREAM...EAST OF THE ROCKIES...THROUGH THE VICINITY OF THE APPALACHIANS. WITHIN THIS LATTER REGIME...ONE NOTABLE SHORT WAVE IMPULSE IS FORECAST TO PIVOT RAPIDLY NORTHEASTWARD...FROM THE OHIO VALLEY THROUGH THE ST. LAWRENCE VALLEY BY LATE TONIGHT. AN ASSOCIATED SURFACE LOW MAY CONSOLIDATE WITH ANOTHER LOW OVER THE GREAT LAKES REGION...BEFORE CONTINUING INTO QUEBEC LATE IN THE PERIOD...WITH A **TRAILING COLD FRONT ADVANCING SOUTHEASTWARD ACROSS THE CENTRAL UNITED STATES.** INITIALLY STALLED OVER PARTS OF THE LOWER OHIO VALLEY/OZARK PLATEAU AND CENTRAL PLAINS...MODELS SUGGEST THAT THE SOUTHWESTERN FLANK OF THIS FRONT WILL BEGIN A MORE RAPID SOUTHWARD SURGE...IN RESPONSE TO ANOTHER SIGNIFICANT SHORT WAVE IMPULSE DIGGING SOUTHEAST OF THE CENTRAL ROCKIES THROUGH MUCH OF THE SOUTH CENTRAL PLAINS BY 12Z THURSDAY. THIS MAY BE ACCOMPANIED BY WIDESPREAD CONVECTIVE DEVELOPMENT...AND THE RISK FOR SEVERE STORMS. ...SOUTH CENTRAL PLAINS/OZARK PLATEAU INTO LOWER OHIO VALLEY... SOME BOUNDARY LAYER MOISTENING CONTINUES ALONG AND SOUTH OF THE FRONT...BUT A SUBSTANTIVE RETURN FLOW OF MOISTURE OFF THE GULF OF MEXICO STILL APPEARS UNLIKELY DURING THIS PERIOD. MODEL SURFACE DEW POINT FORECASTS FOR THIS AFTERNOON MAY STILL BE AT LEAST A BIT TOO HIGH. HOWEVER...NORTHEASTWARD ADVECTION OF WARM ELEVATED MIXED LAYER AIR IS EXPECTED TO AGAIN CONTRIBUTE TO VERY STEEP MID-LEVEL LAPSE RATES BY THIS AFTERNOON. THIS PROBABLY WILL SUPPORT SIZABLE CAPE /PERHAPS AT OR ABOVE 2000 J PER KG/ WITHIN AT LEAST A NARROW CORRIDOR ALONG THE FRONTAL ZONE. IN THE PRESENCE OF STRONG SHEAR BENEATH 40-50 KT WESTERLY 500 MB FLOW...CONDITIONS SHOULD BECOME CONDUCIVE TO THE DEVELOPMENT OF SEVERE STORMS FOR AT LEAST A PERIOD LATE THIS AFTERNOON INTO EARLY EVENING. **MODELS SUGGEST THAT FORCING FOR ASCENT ASSOCIATED WITH LOW-LEVEL** WARM ADVECTION MAY SUPPORT A RAPID INCREASE IN SEVERE STORM DEVELOPMENT BY 20-22Z NEAR THE CENTRAL AND EASTERN KANSAS/OKLAHOMA BORDER AREA. THIS MAY INITIALLY INCLUDE DISCRETE STORMS...BEFORE GROWING UPSCALE...WHICH SHOULD TEND TO TRACK EASTWARD ALONG THE FRONT INTO THE OZARK PLATEAU. ADDITIONAL SEVERE STORMS MAY FORM SHORTLY THEREAFTER ACROSS PARTS OF WEST CENTRAL OKLAHOMA. IT APPEARS THAT THIS LATTER ACTIVITY WILL OCCUR JUST AHEAD OF THE SOUTHWARD SURGING COLD FRONT...AND STRONGER STORMS PROBABLY WILL BE CAPABLE OF PRODUCING LARGE HAIL AND LOCALLY STRONG WIND GUSTS. AS LARGE-SCALE FORCING SUPPORTS INCREASINGLY WIDESPREAD THUNDERSTORM ACTIVITY THROUGH MUCH OF THE INSTABILITY AXIS...WHILE THE SURFACE **COLD FRONT BEGINS TO SURGE SOUTHWARD AND TENDS TO UNDERCUT** ### Page 178 5-38 CONVECTION...IT REMAINS UNCLEAR AT THE PRESENT TIME HOW LONG THE SEVERE WEATHER POTENTIAL WILL CONTINUE INTO THE EVENING HOURS. ..KERR/MOSIER.. 03/25/2015 5.6.2 Watch Notification Messages. The SPC issues Aviation Watch Notification Messages (SAW) to provide an area threat alert for the aviation meteorology community to forecast organized severe thunderstorms that may produce tornadoes, large hail, and/or Convective damaging winds as indicated in Public Watch Notification Messages within the CONUS. SPC issues three types of Watch Notification Messages: SAW, Public Severe Thunderstorm Watch Notification Message, and Public Tornado Watch Notification Message. They are available on the SPC website. The SAW was formerly known as the Alert Severe Weather Watch Bulletin (AWW). The NWS no longer uses that title or acronym for this product. The NWS uses the acronym SAW for the Aviation Watch Notification Message, but retains AWW in the product header for processing by weather data systems. The NWS uses the acronym AWW for their Aviation Weather Warning product, which is a completely different product from the SAW. The Severe Thunderstorm and Tornado Watch Notification Messages were formerly known as the Severe Weather Watch Bulletins (WW). The NWS no longer uses that title or acronym for this product but retains WW in the product header for processing by weather data systems. It is important to note the difference between a Severe Thunderstorm (or Tornado) Watch and a Severe Thunderstorm (or Tornado) Warning. A watch means severe weather is possible during the watch valid time, while a warning means that severe weather has been observed or is expected within the hour. Only the SPC issues Severe Thunderstorm and Tornado Watches, while only NWS WFOs issue Severe Thunderstorm and Tornado Warnings. 5.6.2.1 Aviation Watch Notification Message (SAW). The SPC issues SAW to provide an area threat alert for the aviation meteorology community to forecast organized severe thunderstorms that may produce tornadoes, large hail, and/or Convective damaging winds as indicated in Public Watch Notification Messages. The SA W product is an approximation of the area in a watch. For the official area covered by a watch, see the corresponding Public Watch product. To illustrate, Figure 5-24, Aviation Watch (polygon) Compared to Public Watch (shaded) Example, is an example of the Aviation Watch (polygon) compared to the P ublic Watch (shaded). Also, the SAW is easier to communicate verbally over the radio and telephone than reciting the entire Public Watch product. ### Page 179 5-39 Figure 5-24. Aviation Watch (polygon) Compared to Public Watch (shaded) Example 5.6.2.1.1 Issuance. The SPC will issue the SAW after the proposed Convective watch area has been collaborated with the affected NWS WFOs defining the approximate areal outline of the watch. Watch Notification Messages are nonscheduled, event-driven products valid from the time of issuance to expiration or cancellation time. Valid times are in UTC. SPC will correct watches for formatting and grammatical errors. When tornadoes or severe thunderstorms have developed, the local NWS WFO will issue the warnings for the storms. 5.6.2.1.2 Format and Example of an Aviation Watch Notification Message (SAW). SPC forecasters may define the area as a rectangle or parallelogram (X mi either side of the line from point A to point B) or (X miles north and south or east and west of the line from point A to point B). Distances of the axis coordinates should be in sm. The aviation coordinates referencing VOR locations and state distances will be in nautical miles (NM). Valid times will be in UTC. The watch half width will be in sm. The SAW will contain hail ### Page 180 5-40 size in inches or ½ in (forecaster discretion for tornado watches associated with hurricanes) surface and aloft, surface Convective wind gusts in knots, maximum cloud tops and the Mean Storm Motion Vector, and replacement information, if necessary. The SAW will refer users to the accompanying public product, known as the Watch Outline Update (WOU) message, which provides the names of all counties or parishes in the watch area. Note the letter “n” refers to the issuance number, e.g., 2 means the second issuance of the WOU message. WWUS30 KWNS 271559 SAW2 SPC AWW 271559 WW 568 TORNADO AR LA MS 271605Z - 280000Z AXIS..65 STATUTE MILES EAST AND WEST OF LINE.. 45ESE HEZ/NATCHEZ MS/ - 50N TUP/TUPELO MS/ ..AVIATION COORDS.. 55NM E/W /18WNW MCB - 60E MEM/ HAIL SURFACE AND ALOFT..3 INCHES. WIND GUSTS..70 KNOTS. MAX TOPS TO **550. MEAN STORM MOTION VECTOR 26030.** LAT...LON 31369169 34998991 34998762 31368948 THIS IS AN APPROXIMATION TO THE WATCH AREA. FOR A COMPLETE DEPICTION OF THE WATCH SEE WOUS64 KWNS FOR WOU2. ### Page 181 5-41 Table 5-10. Decoding an Aviation Weather Watch Notification Message Line(s) Content Description 1 WWUS30 KWNS 271559 Communication header with issuance date/time 2 SAW2 NWS product type (SAW) and issuance number (2) 3 SPC AWW 271559 Issuing office Product Type Issuance date/time 4 WW 568 **TORNADO** AR LA MS 271605Z - 280000Z Watch number Watch Type States affected Valid date/time period 5 AXIS..65 STATUTE MILES EAST AND WEST OF A LINE... Watch axis 6 45ESE HEZ/NATCHEZ MS/ - 50N TUP/TUPELO MS/ Anchor points 7 …AVIATION COORDS..55NM E/W /18WNW MCB - 60E MEM/ Aviation coordinates 8-9 HAIL SURFACE AND ALOFT…3 INCHES. WIND GUSTS..70 KNOTS. MAX TOPS TO 550. MEAN STORM MOTION VECTOR 26030. Type, intensity, max tops and mean storm motion using standard contractions. 10 (blank line) 11 LAT...LON 31369169 34998991 4998762 31368948 Latitude and longitude coordinates 12 (blank line) 13-15 THIS IS AN APPROXIMATION TO THE WATCH AREA. FOR A COMPLETE DEPICTION OF THE WATCH SEE WOUS64 KWNS FOR WOU2. Notice that this is an approximation of the watch area and for users to refer to the referenced product for the actual area 5.6.2.2 Public Severe Thunderstorm Watch Notification Message. The SPC issues a Public Severe Thunderstorm Watch Notification Message when forecasting six or more hail events of 1 in (quarter sized) diameter or greater or damaging winds of 50 kts (58 mph) or greater. The forecast event minimum threshold is at least 2 hours over an area of at least 8,000 mi2. Below these thresholds, the SPC, in collaboration with affected NWS offices, may issue a watch for smaller areas and for shorter periods of time when conditions warrant, and for Convective watches along coastlines, near the Canadian border, and near the Mexican border. ### Page 182 5-42 A Public Severe Thunderstorm Watch Notification Message contains three bulleted blocks of information: • The geographic area of the watch, • The valid time of the watch, and • A description of the primary threats anticipated within the watch. A plain text watch summary is included beneath the bulleted information followed by a more detailed description of the area and axis of the watch. The SPC includes the term “adjacent coastal waters” when the watch affects coastal waters adjacent to the Pacific/Atlantic coast, the Gulf of Mexico, or the Great Lakes. Adjacent coastal waters refers to a WFO’s near-shore responsibility (out to 20 NM for oceans), except for Convective watches, which include portions of the Great Lakes. The SPC issues a watch cancellation message when no counties, parishes, independent cities and/or marine zones remaining are in the watch area prior to the expiration time. The text of the message will specify the number and area of the canceled watch. 5.6.2.2.1 Format of Public Severe Thunderstorm Watch Notification Message. WWUS20 KWNS ddhhmm (communication header) URGENT - IMMEDIATE BROADCAST REQUESTED SEVERE THUNDERSTORM WATCH NUMBER nnnn NWS STORM PREDICTION CENTER NORMAN OK time am/pm time zone day mon dd yyyy THE STORM PREDICTION CENTER HAS ISSUED A * SEVERE THUNDERSTORM WATCH FOR PORTIONS OFPORTION(S) OF STATE(S) AND ADJACENT COASTAL WATERS (IF REQUIRED) * EFFECTIVE (TIME PERIOD) UNTIL hhmm am/pm time zone. ...THIS IS A PARTICULARLY DANGEROUS SITUATION (IF NECESSARY)... * PRIMARY THREATS INCLUDE... **HAIL TO X.X INCHES IN DIAMETER POSSIBLE** **THUNDERSTORM WIND GUSTS TO XX MPH POSSIBLE** **DANGEROUS LIGHTNING POSSIBLE SUMMARY...PLAIN TEXT DESCRIPTION OF WHY THE WATCH IS NEEDED.** NARRATIVE DESCRIPTION OF WATCH AREA USING A LINE AND ANCHOR POINTS. DISTANCES TO EITHER SIDE OF THE LINE WILL BE IN STATUTE MILES. THIS SECTION INDICATES THE WATCH IS AREA IS AN APPROXIMATION AND “FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOUn).” **PRECAUTIONARY/PREPAREDNESS ACTIONS...** ### Page 183 5-43 CALL TO ACTION STATEMENTS OTHER WATCH INFORMATION...OTHER WATCHES IN EFFECT AND IF THIS WATCH REPLACES A PREVIOUS WATCH. && AVIATION...BRIEF DESCRIPTION OF SEVERE WEATHER THREAT TO AVIATORS. HAIL SIZE WILL BE GIVEN IN INCHES AND WIND GUSTS IN KNOTS. MAXIMUM STORM TOPS AND A MEAN STORM VECTOR WILL ALSO BE GIVEN. $$ ..FORECASTER NAME.. MM/DD/YY To see a dynamic representation of this, please go to the following website: http://www.spc.noaa.gov/products/watch/2010/ww0018.html. 5.6.2.2.2 Example of a Public Severe Thunderstorm Watch Notification Message. WWUS20 KWNS 161711 (communication header) SEL2 SPC WW 161710 **URGENT - IMMEDIATE BROADCAST REQUESTED SEVERE THUNDERSTORM WATCH NUMBER 647** **NWS STORM PREDICTION CENTER NORMAN OK 1210 PM CDT FRI JUL 16 2011** THE NWS STORM PREDICTION CENTER HAS ISSUED A * SEVERE THUNDERSTORM WATCH FOR PORTIONS OF EASTERN IOWA **NORTHERN ILLINOIS** **NORTHWEST INDIANA** LAKE MICHIGAN * EFFECTIVE THIS FRIDAY AFTERNOON FROM 1210 PM UNTIL 500 PM CDT. * PRIMARY THREATS INCLUDE... **HAIL TO 2 INCHES IN DIAMETER POSSIBLE...** **THUNDERSTORM WIND GUSTS TO 70 MPH POSSIBLEDANGEROUS LIGHTNING** POSSIBLE THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 75 STATUTE MILES EITHER SIDE OF A LINE FROM 40 MILES SOUTHEAST OF SOUTH BEND INDIANA TO 35 MILES SOUTHWEST OF CEDAR RAPIDS IOWA. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU2). **PRECAUTIONARY/PREPAREDNESS ACTIONS...** REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE FAVORABLE FOR SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH AREA. **PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR THREATENING** WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS AND POSSIBLE WARNINGS. SEVERE THUNDERSTORMS CAN AND OCCASIONALLY DO PRODUCE TORNADOES. ### Page 184 **5-44 OTHER WATCH INFORMATION...CONTINUE...WW 646...** DISCUSSION...THUNDERSTORMS WILL CONTINUE TO INCREASE ACROSS WATCH AREA WHERE AIR MASS HAS BECOME STRONGLY UNSTABLE AND UNCAPPED. **VEERING SHEAR PROFILE SUPPORT STORMS EVOLVING INTO SHORT LINE** SEGMENTS ENHANCING WIND DAMAGE POTENTIAL AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT TO 2 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 500. MEAN STORM MOTION VECTOR 33025. ...NAME 5.6.2.3 Public Tornado Watch Notification Message. The SPC issues a Public Tornado Watch Notification Message when forecasting two or more tornadoes or any tornado that could produce EF2 or greater damage. The forecast event minimum thresholds are at least 2 hours over an area at least 8,000 mi2. Below these thresholds, the SPC, in collaboration with affected NWS offices, may issue for smaller areas and for shorter periods of time when conditions warrant, and for Convective watches along coastlines, near the Canadian border, and near the Mexican border. A Public Tornado Watch Notification Message contains the area description and axis, watch expiration time, the term “damaging tornadoes,” a description of the largest hail size and strongest thunderstorm wind gusts expected, the definition of the watch, a call-to-action statement, a list of other valid watches, a brief discussion of meteorological reasoning, and technical information for the aviation community. The SPC may enhance a Public Tornado Watch Notification Message by using the words “THIS IS A PARTICULARLY DANGEROUS SITUATION” when there is a likelihood of multiple strong (damage of EF2 or EF3) or violent (damage of EF4 or EF5) tornadoes. The SPC includes the term “adjacent coastal waters” when the watch affects coastal waters adjacent to the Pacific/Atlantic coast or the Gulf of Mexico. Adjacent coastal waters refers to a WFO’s near-shore responsibility (out to 20 NM for oceans), which include portions of the Great Lakes. The SPC issues a watch cancellation message whenever it cancels a watch prior to the expiration time. The text of the message will specify the number and area of the canceled watch. 5.6.2.3.1 Format of a Public Tornado Watch Notification Message. The format for a Public Tornado Watch Notification Message is the same as the Public Severe Thunderstorm Watch Notification Message. ### Page 185 5-45 5.6.2.3.2 Example of a Public Tornado Watch Notification Message. A Public Tornado Watch Notification Message contains three bulleted blocks of information: • The geographic area of the watch, • The valid time of the watch, and • A description of the primary threats anticipated within the watch. A plain text summary is included beneath the bulleted information. (Note this is a fictitious example) WWUS20 KWNS 050550 (communication header) SEL2 SPC WW 051750 **URGENT - IMMEDIATE BROADCAST REQUESTED TORNADO WATCH NUMBER 243** **NWS STORM PREDICTION CENTER NORMAN OK 1250 AM CDT MON MAY 5 2011** THE NWS STORM PREDICTION CENTER HAS ISSUED A * TORNADO WATCH FOR PORTIONS OF **WESTERN AND CENTRAL ARKANSAS** **SOUTHERN MISSOURI** FAR EASTERN OKLAHOMA * EFFECTIVE THIS MONDAY MORNING FROM 1250 AM UNTIL 600 AM CDT. ...THIS IS A PARTICULARLY DANGEROUS SITUATION... * PRIMARY THREATS INCLUDE NUMEROUS INTENSE TORNADOES LIKELY NUMEROUS SIGNIFICANT DAMAGING WIND GUSTS TO 80 MPH LIKELY NUMEROUS VERY LARGE HAIL TO 4 INCHES IN DIAMETER LIKELY THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 100 STATUTE MILES EAST AND WEST OF A LINE FROM 15 MILES WEST NORTHWEST OF FORT LEONARD WOOD MISSOURI TO 45 MILES SOUTHWEST OF HOT SPRINGS ARKANSAS. FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU2). REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH AREA. **PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR THREATENING** **WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS AND POSSIBLE WARNINGS.** OTHER WATCH INFORMATION...THIS TORNADO WATCH REPLACES TORNADO WATCH NUMBER 237. WATCH NUMBER 237 WILL NOT BE IN EFFECT AFTER 1250 AM CDT. CONTINUE...WW 239...WW 240...WW 241...WW 242... ### Page 186 **5-46 DISCUSSION...SRN MO SQUALL LINE EXPECTED TO CONTINUE EWD...WHERE** LONG/HOOKED HODOGRAPHS SUGGEST THREAT FOR EMBEDDED SUPERCELLS/POSSIBLE TORNADOES. FARTHER S...MORE WIDELY SCATTERED SUPERCELLS WITH A THREAT FOR TORNADOES WILL PERSIST IN VERY STRONGLY DEEP SHEARED/LCL ENVIRONMENT IN AR. AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT TO 4 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 70 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 500. MEAN STORM MOTION VECTOR 26045. . ..NAME To see a dynamic representation of this, please go to the following website: http://www.spc.noaa.gov/products/watch/2017/ww0020.html. 5.6.3 Traffic Flow Management (TFM) Convective Forecast (TCF). The TCF is a high-confidence graphical representation of forecasted convection meeting specific criteria of coverage, intensity, and echo top height. The TCF graphics are produced every 2 hours and are valid at 4, 6, and 8 hours after issuance time. Areas of convection in the TCF include any area of Convective cells containing (at a minimum): 1. Composite radar reflectivity of at least 40 decibels (dBZ); 2. Echo tops at or above FL250; 3. Coverage (1 and 2) of at least 25 percent of the polygon area; and 4. Forecaster confidence of at least 50 percent (high) that criteria (1, 2, and 3) will be met. Lines of convection in the TCF include any lines of Convective cells having (at a minimum): 1. Composite radar reflectivity of at least 40 dBZ having a length of at least 100 NM; 2. Linear coverage of 40 percent or greater; 3. Echo tops at or above FL250; and 4. Forecaster confidence of at least 50 percent (high) that criteria (1, 2, and 3) will be met. All four of the threshold criteria listed above for both areas and lines of convection are required for inclusion in the TCF. This is defined as the minimum TCF criteria. The TCF domain is the FIR covering the CONUS and adjacent coastal waters. It also includes the Canadian airspace south of a line from Thunder Bay, Ontario to Quebec City, Quebec. From March 1st through October 31st, the TCF is collaboratively produced by meteorologists at the AWC in Kansas City, MO, and embedded at the FAA Air Traffic Control System Command Center (ATCSCC) in Warrenton, VA; at the CWSU ### Page 187 5-47 embedded at the FAA’s ARTCC; at various airlines; and by other authorized participants. Automated routines will continue to make the TCF available from November 1st through February 28th. The TCF is issued 24 hours a day, 7 days a week at 30 minutes prior to the indicated issuance time. The issuance time supports the FAA’s Strategic Planning (SP) Webinar, which occurs 15 minutes following odd hours Eastern Time (ET). The Canadian portion of the forecast is available from April 1st through September 30th. However, Nav Canada may request the issuance of each forecast as early as March 1st and as late as October 31st. All available Canadian forecasts are incorporated into the TCF. During times the forecasts are not available for Canadian airspace, the TCF graphics will be annotated with “No Canadian TCF.” The TCF is intended to be used as an SP tool for air traffic flow management. It aids in the reduction of air traffic delays, reroutes, and cancellations due to significant convection. It is not intended to be used for tactical air traffic flow decisions, in the airport terminal environment, or for pilot weather briefing purposes. The graphical representation is subject to annual revision. Figure 5-25. TFM Convective Forecast (TCF)—Example ### Page 188 5-48 Figure 5-26. TFM Convective Forecast (TCF)—Example 5.6.3.1 Issuance. The TCF is issued by the AWC. The TCF is issued every 2 hours. The TCF product issuance time is approximately the bottom of the hour preceding the FAA ATCSCC’s SP telecom (i.e., the 1600 UTC issuance would be available at ~1530 UTC for the 1615 UTC SP telecom). The product can be found on https://www.aviationweather.gov. 5.6.3.2 Content. Data graphically displayed on the TCF consist of coverage of convection within a defined polygon, maximum tops, and confidence of Convective occurrence meeting the TCF minimum criteria. Additiona l information on the TCF product is available on the TCF website form under the INFO tab, on https://www.aviationweather.gov. ### Page 189 5-49 5.6.3.3 Coverage. The Convective coverage within the forecast polygon is represented by the amount of fill within the polygon (see Figure 5-27, TCF Product Legend). The polygon is then assigned as a sparse or medium coverage. • Medium coverage, defined by medium fill, indicates 40 to 100 percent of the polygon is forecast to contain convection. • Sparse coverage, represented by sparse fill, means 25 to 39 percent of the polygon is forecast to contain convection. Figure 5-27. TCF Product Legend 5.6.3.3.1 Confidence. The TCF is a high confidence (50 to 100 percent) graphical representation of forecasted convection that denotes the forecasters’ confidence that the represented convection will meet the minimum criteria. Confidence is not to be associated with probability of occurrence. **5.6.3.3.2 TOPS.** The word “TOPS” is used to depict the forecast maximum echo tops, in thousands of feet MSL, specified by four selected layers listed in Figure 5-27. The heights of the forecast echo tops must cover at least 25 percent of the polygon. The exact location of the highest echo top within the polygon cannot be determined. 5.6.3.4 Use. The TCF is an SP tool for air traffic flow management in the 2- to 8-hour forecast period. The TCF does not include a forecast for all convection. If the convection does not meet the threshold criteria, it is not included in the TCF. The TCF is not intended to be used as a tactical short-term decision product. The product is not intended to be used as a pilot weather briefing product; however, TCF is available to pilots and aircraft dispatchers as an additional product for SP. ### Page 190 5-50 5.6.4 Extended Convective Forecast Product (ECFP). The ECFP planning tool (see Figure 5-28, ECFP Example) is a graphical representation of the forecast probability of thunderstorms and is also intended to support the long-range planning for Convective constraints in the National Airspace System (NAS). The product identifies graphically where thunderstorms are expected through the next 72 hours over the CONUS. Although the ECFP uses TCF-style graphics to facilitate ease of interpretation, the ECFP does not forecast TCF criteria. The development of the ECFP planning tool was a response to FAA and industry needs in planning for weather hazards, specifically convection, 1 to 3 days in advance. To meet these planning needs, the ECFP provides traffic planners and collaborators a quick look at where the probability of convection is greatest. By utilizing TCF-style graphics, users familiar with TCF can easily determine where traffic constraints are most likely to occur over the next 3 days. 5.6.4.1 Issuance. The ECFP is an automated forecast product issued by the AWC. It is issued four times a day at approximately 0100, 0700, 1300, and 1900 UTC. Figure 5-28. ECFP Example 5.6.4.2 C ontent. This automated graphical forecast is created from the Short Range Ensemble Forecast (SREF) Calibrated Thunderstorm guidance. Probability of thunderstorm contours are depicted at 40, 60, and 80 percent probabilities, using TCF-like shading. Hashed areas represent 40 to 59 percent probability, ### Page 191 5-51 solid lined areas represent 60 to 79 percent probability, and solid, blue-filled areas represent greater than 80 percent probability. 5.6.4.3 Use. The ECFP is intended for FAA traffic managers at ARTCC, FAA ATCSCC, airline and aviation industry dispatch and flight planners, as well as commercial weather information providers supporting airlines and the FAA. 5.6.5 National Convective Weather Forecast (NCWF). The NCWF is a near real-time, high-resolution display of current and 1-hour extrapolated forecasts of selected hazardous Convective conditions for the CONUS. The NCWF may be used in conjunction with the report and forecast information contained within Convective SIGMETs. The NCWF is available for use by General Aviation (GA), aircraft dispatchers, and Traffic Management Units (TMU). 5.6.5.1 Issuance. The NCWF is issued by the AWC and is updated every 5 minutes. The product is available at the Convection Web page on https://www.aviationweather.gov. 5.6.5.2 Content. The NCWF displays current Convective hazard fields (detection), 1-hour extrapolated forecast polygons (see Figure 5-29, NCWF Example), forecast speed and directions, and echo tops. Past performance polygons can also be select ed for display. ### Page 192 5-52 Figure 5-29. NCWF Example 5.6.5.2.1 NCWF Hazard Scale. The Convective hazard scale uses six hazard levels (see Figure 5-30, NCWF Hazard Scale) to characterize hazardous convection conditions. The six hazard levels are determined by two factors: • Intensities and maximum tops of WSR-88D reflectivity data; and • Frequencies of cloud-to-ground lightning. Higher hazard levels are associated with higher radar reflectivity intensities and higher frequencies of lightning strikes. The six hazard levels are reduced to four-color codes for display on the NCWF. ### Page 193 5-53 Figure 5-30. NCWF Hazard Scale 5.6.5.2.2 One-Hour Extrapolated Forecast Polygons. One-hour extrapolated forecast polygons are high-resolution polygons outlining areas expected to be filled by selected Convective hazard fields in 1 hour. Extrapolated forecasts depict new locations for the Convective hazard fields based on their past movements. Extrapolation forecasts do not forecast the development of new Convective hazard conditions or the dissipation of existing conditions. Forecasts are provided only for Convective hazard scale Levels 3 or higher. The forecast polygons do not depict specific forecast hazard levels. In Figure 5-29, the light blue polygon denotes the location of the 1-hour forecast Convective hazard field. 5.6.5.2.3 Forecast Speed and Direction. Forecast speed and direction are assigned to current Convective hazard fields having a 1-hour extrapolated forecast. A line (or arrow on the AWC JavaScript product) is used to depict the direction of movement. The speed in knots is depicted by the first group of two numbers located near the current Convective hazard field. The second group of three numbers identifies echo tops. Forecast speed and direction is only updated every 10 minutes. The larger update time interval (compared to five-minute updates for the NCWF) smooths erratic forecast velocities. In Figure 5-29, the forecast direction (depicted by an arrow) is pointing to the southeast, and the speed is 25 kts. 5.6.5.2.4 Echo Tops. Echo tops are assigned to current Convective hazard fields having a 1-hour extrapolated forecast. Echo tops are depicted by a group of three numbers located near the current Convective hazard field and are plotted in hundreds of feet MSL. In Figure 5-31, NCWF One-Hour E xtrapolated Forecast Polygon, Forecast Movement Velocity and Echo Tops—Examp le, the echo tops are 45,000 ft MSL. ### Page 194 5-54 Figure 5-31. NCWF One-Hour Extrapolated Forecast Polygon, Forecast Movement Velocity and Echo Tops—Example 5.6.5.2.5 Past Performance Polygons. Past performance polygons are magenta polygons displaying the past hour’s extrapolated forecast polygons with the current Convective hazard fields. A perfect forecast would have the polygons filled with Convective hazard scale Levels 3 or higher data. Levels 1 and 2 would be outside the polygons. The display of past performance polygons allows the user to review the accuracy of the past hour’s forecast. Figure 5-32, NCWF Past Performance Polygons—Example, depicts current Convective hazard fields and past performance polygons (magenta) valid at 1500Z. The past performance polygons are the 1-hour extrapolated forecasts made at 1400Z. Although the polygons do not perfectly match the current Level 3 and higher hazard fields, the forecasts are still fairly accurate. Newly developed Convective hazard Levels 3 and higher do not have past performance polygons. Extrapolated forecasts do not forecast developing hazardous Convective. ### Page 195 5-55 Figure 5-32. NCWF Past Performance Polygons—Example 5.6.5.3 St rengths and Limitations. 5.6.5.3.1 Strengths. Strengths of the NCWF include: • Convective hazard fields that agree very well with radar and lightning data; • Updates every five minutes; • High-resolution forecasts of Convective hazards; and • Well-forecasted long-lived Convective precipitation. 5.6.5.3.2 Limitations. Limitations of the NCWF include: • Initiation, growth, and decay of Convective precipitation are not forecast; • Short-lived or embedded convection may not be accurately displayed or forecast; • Low-topped convection that contains little or no lightning may not be depicted; ### Page 196 5-56 • Erroneous motion vectors are occasionally assigned to storms; and • Convective hazard field scales are not identified within the forecast polygons. 5.6.5.4 Uses of the NCWF. The purpose of the NCWF is to produce a Convective hazard field diagnostic and forecast product based on radar data, echo top mosaics, and lightning data. The target audience includes the FAA and other government agencies, pilots, aircraft dispatchers, aviation meteorologists, and other interested aviation users in the general public. 5.7 Products for Tropical Cyclones. The NWS issues SIGMETs, Convective SIGMETs, and CWAs to inform the aviation community about the potential or existence of tropical cyclones. These above-listed products are the primary source of information. The NWS also issues other products pertaining to tropical cyclones. Tropical cyclones have a sustained wind speed of 34 kts or greater and are called tropical storm, hurricane, typhoon, and cyclone, depending on their intensity or region of the world. 5.7.1 Tropical Cyclone Advisory (TCA). The TCA is intended to provide tropical cyclone forecast guidance, through 24 hours, for international aviation safety and routing purposes. TCAs are issued by the World Meteorological Organization’s (WMO) TCACs for all ongoing tropical cyclone activity in their respective AORs. There are seven TCACs worldwide, with two in the United States, the NWS’ National Hurricane Center (NHC) in Miami, Florida, and the NWS’s Central Pacific Hurricane Center (CPHC) in Honolulu, Hawaii. The NHC is responsible for all ongoing tropical cyclone activity in the Atlantic and eastern Pacific, north of the equator, while the CPHC is responsible for the central Pacific, north of the equator. TCAs are available on their respective websites. 5.7.1.1 Issuance. The NHC and CPHC issue TCAs at 0300, 0900, 1500, and 2100 UTC and are valid from the time of issuance until the next scheduled issuance or update. The forecast position information in the TCA is interpolated from the official forecast data, which is valid at 0000, 0600, 1200, and 1800 UTC. 5.7.1.2 Content. TCAs list the current tropical cyclone position, motion, and intensity, and 6-, 12-, 18-, and 24-hour forecast positions and intensities. It is an alphanumeric text product produced by hurricane forecasters and consists of information extracted from the official forecasts. This forecast is produced from subjective evaluation of current meteorological and oceanographic data as well as output from Numerical Weather Prediction (NWP) models, and is coordinated with affected NWS offices, the NWS National Centers, and the Department of Defense (DOD). ### Page 197 5-57 5.7.1.3 Format. The format of the Aviation Tropical Cyclone Advisory is as follows: FKaa2i cccc ddhhmm (ICAO communication header) (TROPICAL CYCLONE TYPE) (NAME)ICAO ADVISORY NUMBER ## (ISSUING OFFICE CITY STATE) BBCCYYYY time UTC day of week mon dd yyyy TEXT $$ 5.7.1.4 Aviation Tropical Cyclone Advisory (TCA) Example. FKNT25 KNHC 210900 TCANT5 TROPICAL STORM ICAO ADVISORY NUMBER 27 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL092007 0900 UTC SUN OCT 21 2007 TC ADVISORY DTG: 20071021/0900Z TCAC: KNHC TC: ERNESTO NR: 027 PSN: N3000 W08012 MOV: N 13KT C: 0998HPA MAX WIND: 045KT FCST PSN + 06 HR: 211200 N3106 W07951 FCST MAX WIND + 06 HR: 045KT FCST PSN + 12 HR: 211800 N3206 W07930 FCST MAX WIND + 12 HR: 050KT FCST PSN + 18 HR: 220000 N3321 W07903 FCST MAX WIND + 18 HR: 045KT ### Page 198 5-58 FCST PSN + 24 HR: 220600 N3436 W07836 FCST MAX WIND + 24 HR: 040KT **RMK THE FORECAST POSITION INFORMATION IN** **THIS PRODUCT IS INTERPOLATED FROM OFFICIAL FORECAST DATA VALID AT 0000...** **0600...1200...AND 1800Z.** NXT MSG: 20071021/1500Z $$ 5.7.1.5 Additional Tropical Cyclone Information Products. The NHC, CPHC, and select WFOs issue many public tropical storm and hurricane/typhoon information products. Many are in Web-based graphic format. For more information on these public forecasts, see the websites of NHC, CPHC, WFO Guam, and WFOs along the Gulf Coast and the East Coast. 5.8 Volcanic Ash Forecasts. In addition to SIGMETs, the NWS issues forecasts to notify the aviation community of volcanic ash clouds. 5.8.1 Volcanic Ash Advisory Center (VAAC). A VAAC is a meteorological office designated by an ICAO regional air navigation agreement to provide advisory volcanic ash information to MWOs, World Area Forecast Centers (WAFC), area control centers, flight information centers, and international operational meteorological (OPMET) data banks regarding the lateral and vertical extent and forecast movement of volcanic ash in the atmosphere following a volcanic eruption. There are nine VAACs worldwide (see Figure 5-33, Volcanic Ash Advisory Centers (VAACs)). The duties of a VAAC include: • Monitoring relevant geostationary and polar-orbiting satellite data to detect the existence and extent of volcanic ash in the atmosphere in the area concerned; • Activating the volcanic ash numerical trajectory/dispersion model in order to forecast the movement of any ash cloud that has been detected or reported; and • Issuing advisory information regarding the extent and forecast movement of the volcanic ash cloud. ### Page 199 5-59 Figure 5-33. Volcanic Ash Advisory Centers (VAACs) AORs for VAACs include Anchorage, Montreal, Washington. Wellington (New Zealand), Buenos Aires (Argentina), London, Toulouse (France), Tokyo, and Darwin. The United States has two VAACs with responsibilities defined in ICAO Annex 3, Meteorological Service for International Air Navigation. The Washington VAAC is jointly managed by the National Environmental Satellite, Data, and Information Service (NESDIS) Satellite Analysis Branch (SAB) and the NWS National Centers for Environmental Prediction (NCEP) Central Operations (NCO). The Anchorage VAAC is managed by the NWS’s AAWU. The AORs for each VAAC are: • Washington VAAC. o FIRs in CONUS and adjacent coastal waters. o The Oakland Oceanic FIR over the Pacific Ocean. o The New York FIR over the western Atlantic Ocean. o FIRs over and adjacent to the Caribbean and Central and South America north of 10 degrees south latitude. • Anchorage VAAC. o The Anchorage FIR. o Russian FIRs north of 60 degrees north latitude and east of 150 degrees east longitude. 5.8.2 Volcanic Ash Advisory (VAA). The VAA is advisory information on volcanic ash cloud issued in abbreviated plain language, using approved ICAO abbreviations and numerical values of self-explanatory nature. ### Page 200 5-60 A graphical (e.g., chart) version of the VAA is also produced and is sometimes referred to as VAG. The VAG contains the same information as in the VAA but in a four-panel chart format. 5.8.2.1 VAA Issuance. Each VAAC issues VAAs to provide guidance to MWOs for SIGMETs involving volcanic ash. VAAs are issued as necessary, but at least every 6 hours until such time as the volcanic ash cloud is no longer identifiable from satellite data, no further reports of volcanic ash are received from the area, and no further eruptions of the volcano are reported. VAAs in both text and graphical format are available from the websites of the VAAC. 5.8.2.2 VAA Format. The VAA format conforms to the “Template for advisory message for volcanic ash” included in ICAO Annex 3. 5.8.2.3 VAA Example. **FVAK21 PAWU 190615** VOLCANIC ASH ADVISORY ISSUED: 20030419/0615Z VAAC: ANCHORAGE VOLCANO: CHIKURACHKI, 900-36 LOCATION: N5019 E15527 AREA: KAMCHATKA NORTHERN KURIL ISLANDS SUMMIT ELEVATION: 7674 FT (2339 M) ADVISORY NUMBER: 2003-02 INFORMATION SOURCE: SATELLITE AVIATION COLOR CODE: NOT GIVEN ERUPTION DETAILS: NEW ERUPTION OCCURRED APPROX 190500 UTC. HEIGHT IS ESTIMATED AT FL300. ESTIMATE IS BASED ON OBSERVEDAND MODEL WINDS. MOVEMENT APPEARS TO BE E AT 75 KTS. OBS ASH DATA/TIME: 19/0500Z OBS ASH CLOUD: VA EXTENDS FM NEAR VOLCANO EWD TO N50 E160. FCST ASH CLOUD +6HR: 30NM EITHER SIDE OF LN FM NIPPI N49 E159 - N50 E175. FCST ASH CLOUD +12HR: 30NM EITHER SIDE OF LN FM N50 E168 - N50 E180. FCST ASH CLOUD +18HR: 30NM EITHER SIDE OF LN FM N51 E175 - N50 E185. NEXT ADVISORY: 20030419/1500Z REMARKS: UPDATES AS SOON AS INFO BECOMES AVAILABLE. ### Page 201 5-61 5.8.2.4 VAA Graphic Example. Figure 5-34. Volcanic Ash Advisory in Graphical Format 5.9 G raphical Forecast for Aviation (GFA). The GFAs are a set of Web-based displays which are expected to provide the necessary aviation weather information to give users a complete picture of the weather that may impact flights in the CONUS. These displays are updated continuously and provide forecasts, observational data, and warnings of weather phenomena that can be viewed from 14 hours in the past to 15 hours in the future. This product covers the surface up to FL420 (or 42,000 ft MSL). Wind, icing, and turbulence forecasts are available in 3,000-ft increments from the surface up to 18,000 ft MSL, and in 6,000-ft increments from 18,000 ft MSL to FL420. Turbulence forecasts are also broken into low (below 18,000 ft MSL) and high (above 18,000 ft MSL) graphics. A maximum icing graphic and maximum wind velocity graphic (regardless of altitude) are also available. The graphic below is an example of an aviation forecast for clouds. ### Page 202 5-62 Figure 5-35. Aviation Forecast for Clouds—Example 5.9.1 Time Selection and Slider. Available weather product tabs are configured based on the time selection. Users may choose between Forecast and Observations/Warnings (Obs/Warn). The specific hour may then be chosen by the time slider. Figure 5-36. Time Selection and Slider 5.9.1.1 Forecast Weather Products Tabs. Weather products with forecast information are available by selecting one of the tabs at the top of the display above the time selector. The selected product will be highlighted. The default is TAF. ### Page 203 5-63 Figure 5-37. Forecast Weather Products Tabs 5.9.1.2 Forecast Weather Product Elements Tabs. The following table shows which weather elements are displayed on each forecast product tab. The acronyms in this table are decoded for each product by clicking the products tab to review the NWS Hazard and Code and the AWC SIGMET and Code charts found at https://www.aviationweather.gov/gf a/help?page=products. Table 5-11. Weather Elements on Forecast Product Tabs Product Gridded Product Weather Data Overlay SIGMET NWS Warnings AIRMET Vertical Levels TAF - - All All - - CIG/VIS LAMP Flight Category, Ceiling, or Visibility NDFD Precipitation/ Weather Type and Intensity Symbol C-SIG TC TS BD BS VA WS BZ Sierra (IFR) - Clouds RAP Model Cloud Coverage, Tops, or Bases RAP Model Cloud Base, Layers, and Tops C-SIG TC TS VA - Mountain Obscuration - PCPN/WX NDFD Precipitation/ Weather Type and Likelihood NDFD Precipitation/ Weather Type and Intensity **C-SIG TC TS BD BS VA** TO TS WS BZ IS LS ST - - TS NDFD Thunderstorm Coverage NDFD Thunderstorm Type and Intensity C-SIG TC TS TO TS - - Winds RAP/NDFD Wind Speed RAP/NDFD Wind and Gust Barb C-SIG TC TS WS BZ IS ST GA HW LLWS, Strong Surface Winds SFC, MAX, every 3,000 ft from FL030 to FL300; every 6,000 ft from FL300 to FL480 Turb Graphical Turbulence Guidance (GTG) - C-SIG TC TS TB - Turbulence LO, HI, every 3,000 ft from FL030 to FL300; every 6,000 ft from FL300 to FL480 ### Page 204 5-64 Product Gridded Product Weather Data Overlay SIGMET NWS Warnings AIRMET Vertical Levels Ice FIP @ “SFC” NDFD Winter Precipitation/ Weather Type and Likelihood @ “SFC” NDFD Winter Precipitation/ Weather Type and Intensity C-SIG TC TS IC @ “SFC” WS BZ IS LS Icing SFC, MAX, every 3,000 ft from FL030 to FL300; every 6,000 ft from FL300 to FL480 5.9.1.3 Observations/Warnings (Obs/Warn). The Observation/Warning option provides an option to display weather data for the current time and the previous fourteen hours (rounded to the nearest hour). Users may advance through time using the arrow buttons or by clicking on the desired hour. Figure 5-38. Observations/Warnings Tab 5.9.1.4 Observations/Warnings Weather Product Tabs. The following table shows what is displayed on each Observations/Warnings weather product tab. Each product in this menu is overlaid on an infrared (IR) satellite image with a radar loop of the five most recent images up to the selected hour. Table 5-12. Products Displayed on Observations/Warnings Weather Products Tab Product Weather Data Overlay SIGMET NWS Warnings Vertical Levels METAR METAR Station Plot All All - PCPN/WX METAR Precipitation Weather Symbol **C-SIG TC TS BD BS VA** TO TS WS BZ IS LS ST - CIG/VIS FLT CAT: Non-VFR Color-Coded METAR Flight Category Symbol CIG: Number (ft/100) VIS: Number (SM) **C-SIG TC TS BD BS VA** WS BZ - PIREP PIREP symbols All - ALL, every 3,000 ft from FL030 to FL300; every 6,000 ft from FL300 to FL480 RAD/SAT - All All - 5.9.2 GFA Static Images. Some users with limited Internet connectivity may access static images via the AWC at https://www.aviationweather.gov/gfa/plot. There are two static graphical images ### Page 205 5-65 available titled, “Aviation Cloud Forecast” and “Aviation Surface Forecast.” The Aviation Cloud Forecast provides cloud coverage, bases, layers, and tops with AIRMET Sierra for mountain obscuration and AIRMET Zulu for icing overlaid. The Aviation Surface Forecast provides visibility, weather phenomena, and winds (including wind gusts) with AIRMET Sierra for IFR conditions and AIRMET Tango for sustained surface winds of 30 kts or more overlaid. These images are presented on 10 separate maps providing forecast views for the entire CONUS on one, and 9 regional views which provide more detail for the user. They are updated every 3 hours and provide forecast snapshots for 3, 6, 9, 12, 15, and 18 hours in the future. Figure 5-39. GFA Static Cloud Forecast ### Page 206 5-66 Figure 5-40. GFA Surface Forecast 5.9.3 Strengths and Limitations (GFA). 5.9.3.1 Strengths. • One-stop shop for multiple data fields. • Focused on low-altitude flights common to GA community. • Simplified display for non-meteorologist users. • Available 24 hours a day, 7 days a week. • Updated continuously. • Resolution of 2.2 km. • Available 6 hours in the past up to 15 hours in the future. 5.9.3.2 Limitations. Caution should be applied as users configure the GFA for use. Users can turn on and turn off certain functions, i.e., AIRMET’s and SIGMET’s overlays for a given geographical area. This can lead to hidden areas of hazardous weather for a given flight path. ### Page 207 5-67 5.9.4 Recommended Action for Users. 5.9.4.1 For more information on how to use this product, all users are encouraged to visit the GFA interactive web tool at https://www.aviationweather.gov/gfa. 5.9.4.2 A short video explaining how to navigate the GFA can be found at https://www.youtube.com/watch?v=kLe6Eu3fwS0. 5.9.4.3 Users may provide feedback for the Aviation Surface Forecast and the Aviation Cloud Forecast graphics at https://www.aviationweather.gov/ areafcst. 5.10 Localized Aviation Model Output Statistics (MOS) Program (LAMP). The LAMP weather product is a statistical model program that provides specific point forecast guidance on sensible weather elements (perceivable elements such as temperature, wind, sky cover, etc.). LAMP weather product forecasts are provided in both graphical and coded text format, and are currently generated for more than 1,500 locations. The LAMP weather product is entirely automated and may not be as accurate as a forecast generated with human involvement. However, information from the LAMP weather product can be used in combination with Terminal Aerodrome Forecasts (TAF), and other weather reporting and forecasting products and tools, to provide additional information and enhance situational awareness regarding a particular location. 5.10.1 Strengths and Limitations (LAMP). 5.10.1.1 Strengths. • Updated hourly, incorporating the latest surface conditions to produce hourly forecasts of weather elements reaching up to 25 hours in the future. • Users can access fresh forecast information for the next 24 hours. • Provides both categorical and probabilistic forecast guidance on various elements, allowing users to extract the type of information they want. • Categorical ceiling/visibility forecast information is presented, along with forecasts dependent on the occurrence of precipitation. This data attempts to account for some of the temporary fluctuations that occur in flight. 5.10.1.2 Limitations. May not be as accurate as a forecast generated with human involvement. 5.10.2 Recommended Action for Users. • A list of LAMP airports can be found at http://www.nws.noaa.gov/mdl/gfslamp/gfslamp.shtml. ### Page 208 5-68 Note: Once you have selected the above link, look at the left-side menu under “Text Forecasts” and select “By Station” to get a list of the LAMP stations by state. • A detailed descriptio n of the LAMP can be accessed at http://www.nws.noaa.gov/mdl/gfslamp/docs/LAMP_description.shtml. • The NOAA/NWS LAMP home page can be accessed at http://www.nws.noaa.gov/mdl/lamp/index.shtml. 5.11 Area Forecasts (FA). An FA is an abbreviated plain language forecast concerning the occurrence or expected occurrence of specified en route weather phenomena. The FA (in conjunction with AIRMETs, SIGMETs, Convective SIGMETs, CWAs, etc.) is used to determine forecast en route weather over a specified geographic region. FAs cover an 18- to 24-hour period, depending on the region, and issued three to four times daily, depending on the region, and are updated as needed. The exact phenomenon contained in FAs also varies by region. FAs are not intended for use as a substitute for a TAF for those air carrier operations that require weather forecasts at specific airports (i.e., origin, destination, and any alternate). In these operations, a TAF or equivalent forecast that is generated for the specific airport in question, is required to meet the 14 CFR regulatory requirements that specify certain weather conditions and minima at the origin, destination, and alternate airports. 5.11.1 FA—Gulf of Mexico, Caribbean, Hawaii, and Alaska. 5.11.1.1 FA Issuance. The following offices issue FAs for the following areas: • The AWC: o Gulf of Mexic o: The northern Gulf of Mexico, including the Houston Oceanic FIR, the Gulf of Mexico portion of the Miami Oceanic FIR, and the coastal waters west of 85W longitude (see Figure 5-41, AWC Area Forecast (FA) Region and WMO Header—Gulf of Mexico). o Caribbean Sea: Portions of the Gulf of Mexico (south of the Houston Oceanic FIR to approximately 22N latitude), the Caribbean Sea, and adjacent portions of the North Atlantic (see Figure 5-42, AWC Area Forecast (FA) Region and WMO Header—Caribbean). o Hawaii: The main Hawaiian Islands and adjacent coastal waters extending out 40 NM from the coastlines (see Figure 5-43, WFO Honolulu Area Forecast (FA) Region and WMO Header— Hawaii ). ### Page 209 5-69 • The AAWU: o Alaska: S even FAs containing a total of 25 zones (see Table 5-11, Weather Elements on Forecast Product Tabs), covering separate geographic al areas of Alaska and the adjacent coastal waters, including the Pribilof Islands and Southeast Bering Sea (see Figure 5-44, AAWU Flight Advisory and Area Forecast (FA) Zones—Alaska). Figure 5-41. AWC Area Forecast (FA) Region and WMO Header—Gulf of Mexico ### Page 210 5-70 Figure 5-42. AWC Area Forecast (FA) Region and WMO Header—Caribbean F igure 5-43. WFO Honolulu Area Forecast (FA) Region and WMO Header—Hawaii ### Page 211 5-71 Figure 5-44. AAWU Flight Advisory and Area Forecast Zones—Alaska Tabl e 5-13. AAWU Area Forecast (FA) Zones—Alaska 1 Arctic Coast Coastal 2 North Slopes of the Brooks Range 3 Upper Yukon Valley 4 Koyukuk and Upper Kobuk Valley 5 Northern Seward Peninsula-Lower Kobuk Valley 6 Southern Seward Peninsula-Eastern Norton Sound 7 Tanana Valley 8 Lower Yukon Valley 9 Kuskowim Valley 10 Yukon-Kuskowim Delta 11 Bristol Bay 12 Lynn Canal and Glacier Bay 13 Central Southeast Alaska 14 Southern Southeast Alaska 15 Coastal Southeast Alaska 16 Eastern Gulf Coast 17 Copper River Basin 18 Cook Inlet-Susitna Valley 19 Central Gulf Coast 20 Kodiak Island 21 Alaska Peninsula-Port Heiden to Unimak Pass 22 Unimak Pass to Adak 23 St. Lawrence Island-Bering Sea Coast 24 Adak to Attu 25 Pribilof Islands and Southeast Bering Sea ### Page 212 5-72 5.11.1.2 FA Standardization. All FAs follow these standards: • All heights or altitudes are referenced to MSL, unless otherwise noted (i.e., prefaced by AGL or CIG), and annotated using the height in hundreds of feet, consisting of three digits (e.g., 040). For heights at or above 18,000 ft, the level is preceded by FL to represent FLs (e.g., FL180). Tops are always referenced to MSL. • References to latitude and longitude are in whole degrees and minutes following the model: Nnn[nn] or Snn[nn], Wnnn[nn], or Ennn[nn] with a space between latitude and longitude and a hyphen between successive points. Example: N3106 W07118 – N3011 W7209. • Messages are prepared in abbreviated plain language using contractions from Order 7340.2 for domestic products and ICAO Document 8400 for products issued for Oceanic FIRs. A limited number of non-abbreviated words, geographical names, and numerical values of a self-explanatory nature may also be used. • Weather and obstructions to visibility are described using the weather abbreviations for surface weather observations (METAR/SPECI). 5.11.1.3 FA Issuance Schedule. FAs are scheduled products issued at the following times (see Table 5-14, Area Forecast (FA) Issuance Schedule). Table 5-14. Area Forecast (FA) Issuance Schedule Gulf of Mexico (UTC) Caribbean (UTC) Hawaii (UTC) Alaska (UTC) 1st Issuance 0130 0330 0340 0415 (DT) 0515 (ST) 2nd Issuance 1030 0930 0940 1215 (DT) 1315 (ST) 3rd Issuance 1830 1530 1540 2015 (DT) 2115 (ST) 4th Issuance None 2130 2140 None Note: DT—During Alaska Daylight Time, ST—During Alaska Standard Time, UTC—Coordinated Universal Time. 5.11.1.4 FA Amendments. Amendments are issued whenever the weather significantly improves or deteriorates based upon the judgment of the forecaster. AMD is included after the date/time group on the FAA product line. The date/time group on the WMO and FAA lines is updated to indicate the time of the correction. The ending valid time remains unchanged. ### Page 213 5-73 5.11.1.5 FA Corrections. FAs containing errors will be corrected. COR is included after the date/time group on the FAA product line. The date/time group on the WMO and FAA lines is updated to indicate the time of the correction. The ending valid time remains unchanged. 5.11.1.6 FA—Gulf of Mexico (FAGX). FAs issued for the Gulf of Mexico (FAGX) cover the airspace between the surface and 45,000 ft MSL and include the following elements with each geographical section having an entry, even if it is negative: • Synopsis: A brief discussion of the significant synoptic weather affecting the FAGX area during the entire 24-hour valid period. • Significant Clouds and Weather: A description of the significant clouds and weather for the first 12 hours of the forecast period, including the following elements: o Cloud amount (SCT, BKN or OVC) for clouds with bases below FL180. o Cloud bases and tops associated with the above bullet. o Precipitation and thunderstorms. o Visibility below 7 sm and obstruction(s) to visibility. o Sustained surface winds greater than or equal to 20 kts. o 12- to 24-hour categorical outlook (LIFR, IFR, MVFR or VFR). • Icing and Freezing Level: Moderate or severe icing and freezing level. • Turbulence: Moderate or greater turbulence. 5.11.1.6.1 FA—Gulf of Mexico (FAGX) Example. FAGX20 KKCI 091812 (ICAO product header) OFAGX (NWS AWIPS Communication header) SYNOPSIS VALID TIL 101900Z FCST...091900Z-100700Z OTLK...100700Z-101900Z **INTERNATIONAL OPERATIONS BRANCH** **AVIATION WEATHER CENTER KANSAS CITY MISSOURI .** CSTL WATERS FROM COASTLINE OUT TO HOUSTON OCEANIC FIR AND GLFMEX MIAMI OCEANIC FIR AND W OF 85W. HOUSTON OCEANIC FIR AND GLFMEX MIAMI OCEANIC FIR. . **TS IMPLY SEV OR GTR TURB SEV ICE LLWS AND IFR CONDS. HGTS MSL.** . **01 SYNOPSIS...HIGH PRES OVR NRN GLFMEX.** . **02 SIGNIFICANT CLD/WX...** . **CSTL WATERS...** **SCT020. OTLK...VFR.** ### Page 214 5-74 . HOUSTON OCEANIC FIR… SCT020. OTLK...VFR. . **GLFMEX MIAMI OCEANIC FIR...** **SCT020. OTLK...VFR.** . **03 ICE AND FRZLVL...** CSTL WATERS…SEE AIRMETS ZULU WAUS44 KKCI AND WAUS42 KKCI. HOUSTON OCEANIC FIR… NO SGFNT ICE EXP OUTSIDE CNVTV ACT. GLFMEX MIAMI OCEANIC FIR…NO SGFNT ICE EXP OUTSIDE CNVTV ACT. FRZLVL…140 THRUT. . **04 TURB...** CSTL WATERS…SEE AIRMETS TANGO WAUS44 KKCI AND WAUS42 KKCI. HOUSTON OCEANIC FIR… NO SGFNT TURB EXP OUTSIDE CNVTV ACT. GLFMEX MIAMI OCEANIC FIR…NO SGFNT TURB EXP OUTSIDE CNVTV ACT. 5.11.1.7 FA—Caribbean (FACA). FAs issued for the Caribbean (FACA) cover the airspace between the surface and 24,000 ft MSL and include the following elements. Each geographical section will have an entry even if it is negative. • Synopsis: A brief discussion of the significant synoptic weather affecting the FACA area during the 24-hour valid period. • Significant Clouds and Weather: A description of the significant clouds and weather for the first 12 hours of the forecast period, including the following elements. o Cloud amount (SCT, BKN, or OVC) for cloud bases below FL180. o Cloud bases and tops associated with the above bullet. o Precipitation and thunderstorms. o Visibility below 7 sm and obstruction(s) to visibility. o Sustained surface winds greater than or equal to 20 kts. o 12- to 24-hour categorical outlook (IFR, MVFR or VFR). • Icing and Freezing Level: Moderate or greater icing and freezing level. • Turbulence: Moderate or greater turbulence. 5.11.1.7.1 FA—Caribbean (FACA) Example. FACA20 KKCI 121530 (ICAO product header) OFAMKC (NWS AWIPS Communication header) **INTERNATIONAL OPERATIONS BRANCH** AVIATION WEATHER CENTER KANSAS CITY MISSOURI VALID 121600-130400 OUTLOOK...130400-131600 . **ATLANTIC S OF 32N W OF 57W...CARIBBEAN...GULF OF MEXICO BTN 22N AND 24N.** . ### Page 215 **5-75 TS IMPLY SEV OR GTR TURB SEV ICE LLWS AND IFR CONDS. SFC TO 400 MB.** . SYNOPSIS...WK CDFNT EXTDS FM NR 28N60W TO 23N63W TO THE MONA PASSAGE. CDFNT WL MOV EWD AND WKN TODAY. EXP NARROW BAND OF CLDS WITH ISOL SHRA INVOF CDFNT. . **SIGNIFICANT CLD/WX...** **ERN MONTERREY FIR...NRN MERIDA FIR SCT025 SCT060. OTLK...VFR.** . ATLC SWRN NEW YORK FIR...SAN JUAN FIR NW OF CDFNT...SCT025 SCT060. LYR OCNL BKN. TOP 120. ISOL SHRA. **OTLK...VFR.** VCNTY CDFNT...SCT025 BKN060. OCNL BKN025. TOP 120. WDLY SCT SHRA. ISOL TSRA TIL 20Z. OTLK...VFR SHRA. **SE OF CDFNT...SCT025 SCT060. ISOL SHRA. OTLK...VFR.** . **ATLC MIAMI FIR SCT025 SCT060. LYR OCNL BKN. TOP 120. ISOL SHRA. OTLK...VFR.** . WRN PIARCO FIR...NRN MAIQUETIA FIR...CURACAO FIR BTN 61W-63W...SCT025 BKN060. OCNL BKN025. TOP 120. WDLY SCT SHRA. OTLK...VFR SHRA. **RMNDR...SCT025 SCT060. ISOL SHRA. OTLK...VFR.** . SANTO DOMINGO FIR...PORT-AU-PRINCE FIR SCT025 SCT060. LYR OCNL BKN. TOP 120. ISOL SHRA. OTLK...VFR. . NRN BARRANQUILLA FIR...NRN PANAMA FIR SCT025 SCT060. ISOL SHRA. SFC WND NE 20-25KT. OTLK...VFR. . **KINGSTON FIR...NERN CNTRL AMERICAN FIR...HABANA FIR SCT025 SCT060. ISOL SHRA. OTLK...VFR.** . **ICE AND FRZLVL...** **NO SGFNT ICE EXP OUTSIDE CNVTV ACT.** **FRZLVL... 145-170.** . **TURB...** **NO SGFNT TURB EXP OUTSIDE CNVTV ACT.** 5.11.1.8 FA—Hawaii. FAs issued for Hawaii cover the airspace between the surface and 45,000 ft MSL and include the following elements: • Synopsis: A brief discussion of the significant synoptic weather affecting the FA area during the 18-hour valid period. • Significant Clouds and Weather: A description of the significant clouds and weather for the first 12 hours of the forecast period, including the following elements: o Cloud amount (SCT, BKN, or OVC) with bases and tops. o Visibilities of 6 sm or less with obstruction(s) to visibility. o Precipitation and thunderstorms. ### Page 216 5-76 o Sustained surface winds of 20 kts or greater. o 12- to 18-hour categorical outlook (IFR, MVFR, or VFR), including expected precipitation and/or obstructions to visibility. 5.11.1.8.1 FA—Hawaii Example. FAHW31 PHFO 080940 (ICAO product header) FA0HI (NWS AWIPS Communication header) . HNLC FA 080940 (Area forecast region, product type, issuance date/time) SYNOPSIS AND VFR CLD/WX SYNOPSIS VALID UNTIL 090400 CLD/WX VALID UNTIL 082200...OUTLOOK VALID 082200-090400 . **SEE AIRMET SIERRA FOR IFR CLD AND MT OBSC.** **TS IMPLY SEV OR GREATER TURB SEV ICE LOW LEVEL WS AND IFR COND.** **NON MSL HGT DENOTED BY AGL OR CIG.** . **SYNOPSIS...SFC HIGH FAR N PHNL NEARLY STNR.** . **BIG ISLAND ABOVE 060.** **SKC. 20Z SCT090. OUTLOOK...VFR.** . BIG ISLAND LOWER SLOPES...COAST AND ADJ WATERS FROM UPOLU POINT TO CAPE KUMUKAHI TO APUA POINT. SCT030 BKN050 TOPS 080 ISOL BKN030 VIS 3-5SM -SHRA BR. 21Z SCT030 SCT-BKN050 TOPS 080 ISOL BKN030 5SM -SHRA. OUTLOOK...VFR. . BIG ISLAND LOWER SLOPES…COAST AND ADJ WATERS FROM APUA POINT TO SOUTH CAPE TO UPOLU POINT. SKC. 21Z SCT-BKN060 TOPS 080. 23Z SCT030 SCT- BKN060 TOPS 080 ISOL BKN030 -SHRA. OUTLOOK...VFR. . BIG ISLAND LOWER SLOPES…COAST AND ADJ WATERS FROM SOUTH CAPE TO PHKO TO UPOLU POINT. **SCT050 ISOL BKN050 TOPS 080. 18Z FEW050. 23Z SCT-BKN050 TOPS 080.** **OUTLOOK...VFR.** . **N AND E FACING SLOPES...COAST AND ADJ WATERS OF THE REMAINING ISLANDS.** SCT020 BKN045 TOPS 070 TEMPO BKN020 VIS 3-5SM -SHRA...FM OAHU EASTWARD ISOL CIG BLW 010 AND VIS BLW 3SM SHRA BR WITH TOPS 120. 22Z SCT025 SCT-BKN050 TOPS 070 ISOL BKN025 3-5SM -SHRA. OUTLOOK...VFR. . **REST OF AREA.** SCT035 SCT-BKN050 TOPS 070 ISOL BKN030 -SHRA. 20Z SCT050 ISOL SCT030 BKN045 TOPS 070 -SHRA. OUTLOOK...VFR 5.11.1.9 FA—Alaska. FAs issued for Alaska cover the airspace between the surface and 45,000 ft MSL and include the following elements: • Synopsis: A brief description of the significant synoptic weather affecting the FA area during the first 18 hours of the forecast period. ### Page 217 5-77 • Significant Clouds and Weather: A description of the significant clouds and weather for each geographical zone during the first 12 hours of the forecast period, including the following elements: o AIRMET information for IFR ceiling and visibility, mountain obscuration, and strong surface winds. o Cloud amount (SCT, BKN, or OVC) with bases and tops. o Visibility below 7 sm and obstruction(s) to visibility. o Precipitation and thunderstorms. o Surface wind greater than 20 kts. o Mountain pass conditions using categorical terms (for selected zones only). o 12- to 18-hour categorical outlook (VFR, MVFR and IFR). • Icing and Freezing Level: A description of expected icing conditions including the following elements: o AIRMET information for icing and freezing precipitation. o Icing not meeting SIGMET or AIRMET criteria during the 6- to 12-hour period. o Freezing level. o If no significant icing is forecast, NIL SIG will be entered, followed by the freezing level. • Turbulence: A description of expected turbulence conditions, including the following elements: o AIRMET information for turbulence or LLWS. o Turbulence not meeting SIGMET or AIRMET criteria during the 6- to 12-hour period. o If no significant turbulence is forecast, NIL SIG will be entered. 5.11.1.9.1 FA—Alaska Example. FAAK47 PAWU 222010 (ICAO product header) FA7H (NWS AWIPS Communication header) JNUH FA 222015 (Area forecast region, product type, issuance date/time) EASTERN GULF COAST AND SE AK... . AIRMETS VALID UNTIL 230415 CB IMPLY POSSIBLE SEV OR GREATER TURB SEV ICE LLWS AND IFR CONDS. **NON MSL HEIGHTS NOTED BY AGL OR CIG.** . ### Page 218 5-78 SYNOPSIS VALID UNTIL 231400 989 MB LOW 275 NM SE KODIAK IS WILL MOV SE WARD TO ABOUT 350 NM S PASI BY 14Z WHILE FILLING TO 998 MB. ASSOCD OCFNT ARCING E AND SE FM LOW WILL MOV ONSHR SE AK AND DSIPT BY END OF PD. . **LYNN CANAL AND GLACIER BAY JB...VALID UNTIL 230800 ...CLOUDS/WX...** **FEW025 SCT050 BKN100 TOP 120.** **OTLK VALID 230800-231400...VFR.** **PASSES...WHITE...CHILKOOT...VFR.** ...TURB... **NIL SIG.** ...ICE AND FZLVL... **NIL SIG. FZLVL 020.** . **CNTRL SE AK JC...VALID UNTIL 230800 ...CLOUDS/WX...** **FEW025 SCT050 BKN100 TOP 120.** **AFT 03Z ISOL BKN050 -SHRA.** **OTLK VALID 230800-231400...VFR.** ...TURB... **AFT 05Z SW PAFE ISOL MOD TURB BLW 040.** ...ICE AND FZLVL... **NIL SIG. FZLVL 025.** . **SRN SE AK JD...VALID UNTIL 230800 ...CLOUDS/WX...** **FEW025 SCT050 BKN100 TOP 120.** **AFT 00Z OCNL BKN050 -RA. ISOL BKN025 -RA.** **AFT 03Z SFC WND SE 25G35KT.** **OTLK VALID 230800-231400...VFR.** ...TURB... **AFT 02Z CLARENCE STRAIT SW ISOL MOD TURB BLW 040.** ...ICE AND FZLVL... **AFT 02Z OUTER CST PAHY S ISOL MOD ICEIC 040-100. FZLVL 025.** . 5.12 Alaskan Graphical Area Forecasts (FA). NWS’s AAWU produces a series of graphical forecasts to complement the text-based FA for Alaska (see paragraph 5.11.1). These forecasts are available from the AAWU’s Web page under Graphical FA. ### Page 219 5-79 • Flying Weather. • Surface Forecast. • Icing Forecast. • Turbulence Forecast. • Convective Outlook. Additional products may be available. Some of these may be labeled experimental and, thus, the contents and format are subject to change. 5.12.1 Issuance. The AAWU issues the Alaskan Graphical FAs. The flying weather, icing, and turbulence forecasts are issued three times a day, approximately at 0530, 1330, and 2130 UTC during Alaska Standard Time (AKST), and 0430, 1230, and 2030 UTC during Alaska Daylight Time (AKDT). Surface Forecasts are issued four times a day, approximately at 0315, 0915, 1530, and 2130 UTC during AKST, and 0215, 0815, 1430, and 2030 UTC during AKDT. The Convective Outlook is a seasonal product and only issued from May 1st through September 30th at 1230 UTC with updates at 0430 UTC. All forecasts are amended as needed. ### Page 220 5-80 5.12.2 Content. 5.12.2.1 Flying Weather. The Flying Weather graphic (see Figure 5-45, Alaskan Graphical Area Forecast (FA)—Flying Weather Example) illustrates areas of prevailing Marginal VFR (MVFR) or IFR conditions, strong winds (30 kts sustained or greater), and any active volcano in Alaska. This product consists of two 6-hour forecasts valid for a total of 12 hours. Each forecast specifies where such conditions can be expected within the 6-hour valid time. Areas of occasional or continuous MVFR/IFR are represented by shaded regions (red for IFR, blue for MVFR), whereas areas of predominately VFR weather are not shaded. MVFR/IFR conditions are possible outside these shaded regions, but only isolated in coverage. Strong surface winds are shown in a circle hatch overlay. Active volcanoes are denoted by a volcano symbol at the location of the volcano. Note: This forecast is also referred to as IFR/MVFR graphic on their website. Figure 5-45. Alaskan Graphical Area Forecast (FA)—Flying Weather Example ### Page 221 5-81 5.12.2.2 Surface Forecast. The Surface Forecast graphic (see Figure 5-46, Alaskan Graphical Area Forecast (FA)—Surface Chart Example) illustrates prominent surface features, including sea-level pressure, areas of high and low pressure, fronts and troughs, and precipitation. Each forecast shows the surface weather that can be expected within 1 hour of the designated time. Areas of high pressure are depicted along with the maximum sea-level pressure. Areas of low pressure are depicted with the minimum sea-level pressure. The mean 12-hour motion of low pressure systems are also shown. Areas of occasional or continuous precipitation and/or fog are represented by shaded regions (green for precipitation, yellow for fog), whereas isolated or scattered precipitation is not shaded. This product is issued every 6 hours with forecasts valid for 00Z, 06Z, 12Z, and 18Z. Figure 5-46. Alaskan Graphical Area Forecast (FA)—Surface Chart Example 5.12.2.3 Ici ng Forecast. The Icing Forecast graphic (see Figure 5-47, Alaskan Graphical Area Forecast (FA)—Icing For ecast Example) provides information about freezing levels and the potential for significant icing at specified valid times. ### Page 222 5-82 Freezing level heights are blue-filled contours (every 2,000 ft). Areas of isolated (ISOL) moderate (MOD) icing are shaded yellow, occasional (OCNL) or continuous (CONS) moderate icing are shaded orange, and red is used for moderate with isolated severe (SEV) icing (refer to SIGMETs for occasional or greater severe icing). These forecasts are issued every 8 hours and amended as needed. Figure 5-47. Alaskan Graphical Area Forecast (FA)—Icing Forecast Example 5.12.2.4 Turb ulence Forecast. The Turbulence Forecast graphic (see Figure 5-48, Alaskan Graphical Area Forecast (FA)—Tur bulence Forecast Example) depicts areas of significant turbulence at specified valid times. Areas of isolated (ISOL) moderate (MOD) turbulence are shaded yellow, occasional (OCNL) or continuous (CONS) moderate turbulence are shaded orange, and red is used for moderate with isolated severe (SEV) turbulence (refer to SIGMETs for occasional or greater severe turbulence). Separate graphics are provided for low-level (defined for this product as FL180 and below) and high-level (defined for this product as above FL180) turbulence. ### Page 223 5-83 Figure 5-48. Alaskan Graphical Area Forecast (FA)—Turbulence Forecast Example 5.12.2.5 Conve ctive Outlook. The Convective Outlook graphic (see Figure 5-49, Alaskan Graphical Area Forecast (FA)—Convec tive Outlook) is a seasonal product that provides information about Convective activity at specific valid times. Each forecast indicates where conditions are favorable for the development of towering cumulus and thunderstorms. Locations of towering cumulus are depicted in yellow. Locations of isolated (ISOL), scattered (SCT), and widespread (WDSPRD) thunderstorms (TS) are depicted in orange, red, and dark red, respectively. Cloud bases and tops are also depicted. ### Page 224 5-84 Figure 5-49. Alaskan Graphical Area Forecast (FA)—Convective Outlook 5.12.3 Use. The Alaska Graphical FAs are a series of forecasts intended to complement, not replace, the text-based FA for Alaska. 5.13 Terminal Aerodrome Forecast (TAF). A TAF is a concise statement of the expected meteorological conditions significant to aviation for a specified time period within 5 sm of the center of the airport’s runway complex (terminal). The TAFs use the same weather codes found in METAR weather reports (see paragraph 3.1) and can be viewed on https://www.aviationweather.gov. 5.13.1 Responsibility. TAFs are issued by NWS WFOs. ### Page 225 5-85 5.13.2 Generic Format of the Forecast Text of an NWS-Prepared TAF. Table 5-15. Generic Format of NWS TAFs TAF or TAF AMD or TAF COR Type of report CCCC YYGGggZ Y1Y1G1G1/Y2Y2G2G2 dddffGfmfmKT Location identifier Date/time of forecast origin group Valid period Wind group VVVV w'w' or NSW NsNsNshshshs or VVhshshs or SKC WShwshwshws/dddftKT Visibility group Significant weather group Cloud and vertical obscuration groups Non-Convective Low-Level Wind Shear (LLWS) group TTGGgg Forecast change indicator groups FMY1Y1GGgg TEMPO Y1Y1GG/YeYeGeGe PROB30 Y1Y1GG/YeYeGeGe From group Temporary group Probability group ### Page 226 5-86 5.13.2.1 Type of Report (TAF or TAF AMD or TAF COR). The report-type header always appears as the first element in the TAF and is produced in three forms: a routine forecast, TAF, an amended forecast, TAF AMD, or a corrected forecast, TAF COR. TAFs are amended whenever they become, in the forecaster’s judgment, unrepresentative of existing or expected conditions, particularly regarding those elements and events significant to aircraft and airports. An amended forecast is identified by TAF AMD (in place of TAF) on the first line of the forecast text. 5.13.2.2 Location Identifier (CCCC). After the line containing either TAF, TAF AMD, or TAF COR, each TAF begins with its four-letter ICAO location identifier. Examples: KDFW Dallas-Fort Worth PANC Anchorage, Alaska PHNL Honolulu, Hawaii 5.13.2.3 Date/Time of Forecast Origin Group (YYGGggZ). The date/time of forecast origin group (YYGGggZ) follows the terminal’s location identifier. It contains the day of the month in two 2 digits (YY) and the time in four digits (GGgg in hours and minutes) in which the forecast is completed and ready for transmission, with a Z appended to denote UTC. This time is entered by the forecaster. A routine forecast, TAF, is issued 20 to 40 minutes before the beginning of its valid period. Examples: 061737Z The TAF was issued on the 6th day of the month at 1737 UTC. 121123Z The TAF was issued on the 12th day of the month at 1123 UTC. 5.13.2.4 Valid Period (Y1Y1G1G1/Y2Y2G2G2). The TAF valid period (Y1Y1G1G1/Y2Y2G2G2) follows the date/time of forecast origin group. Scheduled 24- and 30-hour TAFs are issued four times per day, at 0000, 0600, 1200, and 1800Z. The first two digits (Y1Y1) are the day of the month for the start of the TAF. The next two digits (G1G1) are the starting hour (UTC). Y2Y2 is the day of the month for the end of the TAF, and the last two digits (G2G2) are the ending hour (UTC) of the valid period. A forecast period that begins at midnight UTC is annotated as 00. If the end time of a valid period is at midnight UTC, it is annotated as 24. For example, ### Page 227 5-87 a 00Z TAF issued on the 9th of the month and valid for 24 hours would have a valid period of 0900/0924. Whenever an amended TAF (TAF AMD) is issued, it supersedes and cancels the previous TAF. That is, users should not wait until the start of the valid period indicated within the TAF AMD to begin using it. Examples: 1512/1612 The TAF is valid from the 15th day of the month at 1200 UTC until the 16th day of the month at 1200 UTC. 2306/2412 This is a 30-hour TAF valid from the 23rd day of the month at 0600 UTC until the 24th day of the month at 1200 UTC. 0121/0218 This is an amended TAF valid from the 1st day of the month at 2100 UTC until the 2nd day of the month at 1800 UTC. 0600/0624 This TAF is valid from the 6th day of the month at 0000 UTC until the 6th day of the month at 2400 UTC (or 7th day of the month at 0000 UTC). 5.13.2.5 Wind Group (dddffGfmfmKT). The initial time period and any subsequent FM groups begin with a mean surface wind forecast (dddffGfmfmKT) for that period. Wind forecasts are expressed as the mean three-digit direction (ddd—relative to true north) from which the wind is blowing, rounded to the nearest 10 degrees, and the mean wind speed in knots (ff) for the time period. If wind gusts are forecast (gusts are defined as rapid fluctuations in wind speeds with a variation of 10 kts or more between peaks and lulls), they are indicated immediately after the mean wind speed by the letter G, followed by the peak gust speed expected. KT is appended to the end of the wind forecast group. Any wind speed of 100 kts or more will be encoded in three digits. Calm winds are encoded as 00000KT. The prevailing wind direction is forecast for any speed greater than or equal to 7 kts. When the prevailing surface wind direction is variable (variations in wind direction of 30 degrees or more), the forecast wind direction is encoded as VRBffKT. Two conditions where this can occur are very light winds and Convective activity. Variable wind direction for very light winds must have a wind speed of 1 to 6 kts inclusive. For Convective activity, the wind group may be encoded as VRBffGfmfmKT, where Gfmfm is the maximum expected wind gusts. VRB is not used in the Non-Convective LLWS group. Squalls are forecast in the wind group as gusts (G) but must be identified in the significant weather group with the code SQ. ### Page 228 5-88 Examples: 23010KT Wind from 230 degrees “true” (southwest) at 10 knots. 28020G35KT Wind from 280 degrees “true” (west) at 20 knots gusting to 35 knots. VRB05KT Wind variable at 5 knots. This e xample depicts a forecast for light winds that are expected to variable in direction. VRB15G30KT Wind variable at 15 knots gusting to 30 knots. This example depicts winds that are forecast to be variable with Convective activity. 00000KT Wind calm 090105KT W ind from 90 degrees at 105 knots 5.13.2.6 Visibility Group (VVVV). The initial time period and any subsequent FM groups include a visibility forecast (VVVV) in statute miles appended by the contraction sm. When the prevailing visibility is forecast to be less than or equal to 6 sm, one or more significant weather groups are included in the TAF. However, drifting dust (DRDU), drifting sand (DRSA), drifting snow (DRSN), shallow fog (MIFG), partial fog (PRFG), and patchy fog (BCFG) may be forecast with prevailing visibility greater than or equal to 7 sm. When a whole number and a fraction are used to forecast visibility, a space is included between them (e.g., 1 1/2SM). Visibility greater than 6 sm is encoded as P6SM. If the visibility is not expected to be the same in different directions, prevailing visibility is used. When volcanic ash (VA) is forecast in the significant weather group, visibility is included in the forecast, even if it is unrestricted (P6SM). For example, an expected reduction of visibility to 10 sm by volcanic ash is encoded in the forecast as P6SM VA. Although not used by the NWS in U.S. domestic TAFs, the contraction “CAVOK” (Ceiling and Visibility OK) may replace the visibility, weather, and sky condition groups if all of the following conditions are forecast: Visibility of 10 kilometers (km) (6 sm) or more, no clouds below ### Page 229 5-89 1500 meters (m) (5,000 ft) or below the highest minimum sector altitude (whichever is greater), no cumulonimbus, and no significant weather phenomena. Examples: P6SM Visibility unrestricted 1 1/2SM Visibility 1 and ½ sm 4SM Visibility 4 sm 5.13.2.7 Significant Weather Group (w’w’ or NSW). The significant weather group (w’w’ or NSW) consists of the appropriate qualifier(s) and weather phenomenon contraction(s) or NSW (no significant weather). If the initial forecast period and subsequent FM groups are not forecast to have explicit significant weather, the significant weather group is omitted. NSW is not used in the initial forecast time period or FM groups. One or more significant weather group(s) is (are) required when the visibility is forecast to be 6 sm or less. The exceptions are: volcanic ash (VA), low drifting dust (DRDU), low drifting sand (DRSA), low drifting snow (DRSN), shallow fog (MIFG), partial fog (PRFG), and patchy fog (BCFG). Obstructions to vision are only forecast when the prevailing visibility is less than 7 sm or, in the opinion of the forecaster, is considered operationally significant. Volcanic ash (VA) is always forecast when expected. When VA is included in the significant weather group, visibility is included in the forecast as well, even if the visibility is unrestricted (P6SM). NSW is used in place of significant weather only in a TEMPO group to indicate when significant weather (including in the vicinity (VC)) included in a previous subdivided group is expected to end. Multiple precipitation elements are encoded in a single group (e.g., -TSRASN). If more than one type of precipitation is forecast, up to three appropriate precipitation contractions can be combined in a single group (with no spaces) with the predominant type of precipitation being first. In this single group, the intensity refers to the total precipitation and can be used with either one or no intensity qualifier, as appropriate. In TAFs, the intensity qualifiers (light, moderate, and heavy) refer to the intensity of the precipitation and not to the intensity of any thunderstorms associated with the precipitation. ### Page 230 5-90 Intensity is coded with precipitation types, except ice crystals and hail, including those associated with thunderstorms and those of a showery nature (SH). No intensity is ascribed to blowing dust (BLDU), blowing sand (BLSA), or blowing snow (BLSN). Only moderate or heavy intensity is ascribed to sandstorm (SS) and duststorm (DS). 5.13.2.7.1 Exception for Encoding Multiple Precipitation Types. When more than one type of precipitation is forecast in a time period, any precipitation type associated with a descriptor (e.g., FZRA) is encoded first in the precipitation group, regardless of the predominance or intensity of the other precipitation types. Descriptors are not encoded with the second or third precipitation type in the group. The intensity is associated with the first precipitation type of a multiple precipitation type group. For example, a forecast of moderate snow and light freezing rain is coded as -FZRASN, although the i ntensity of the snow is greater than the freezing rain. Examples: Combinations of one prec ipitation and one non-precipitation weather phenomena: -DZ FG Light drizzle and fog (obstruction that reduces visibility to less than 5/8 sm) RA BR Moderate rain and mis t -SHRA FG L ight rain showers and fog +SN FG Heavy snow a nd fo g Combinations of more than one type of precipitation: -RASN FG HZ Light rain and snow (light rain predominant), fog and haze TSSNRA Thunderstorm with moderate snow and rain (moderate snow predominant) FZRASNPL Moderate freezing rain, snow and ice pellets (freezing rain mentioned first due to the descriptor, followed by other precipitation types in order of predominance) SHSNPL Moder ate snow showers and ice pellets ### Page 231 5-91 5.13.2.7.2 Thunderstorm Descriptor. The TS descriptor is treated differently than other descriptors in the following cases: • When non-precipitating thunderstorms are forecast, TS may be encoded as the sole significant weather phenomenon; and • When forecasting thunderstorms with freezing precipitation (FZRA or FZDZ), the TS descriptor is included first, followed by the intensity and weather phenomena. Example: TS -FZRA When a thunderstorm is included in the significant weather group (even using vicinity—VCTS), the cloud group (NsNsNshshshs) includes a forecast cloud type of CB. See the following example for encoding VCTS. Example: -FZRA VCTS BKN010CB 5.13.2.7.3 Fog Forecast. A visibility threshold must be met before a forecast for fog (FG) is included in the TAF. When forecasting a fog-restricted visibility from 5/8 sm to 6 sm, the phenomena is coded as BR (mist). When a fog-restricte d visibility is forecast to result in a visibility of less than 5/8 sm, the code FG is used. The forecaster never encodes weather obstruction as mist (BR) when the forecast visibility is greater than 6 sm (P6SM). The follow ing fog-related terms are used as described below: ### Page 232 5-92 Table 5-16. TAF Fog Terms TERM DESCRIPTION Freezing Fog (FZFG) Any fog (visibility less than 5/8 SM) consisting predominantly of water droplets at temperatures less than or equal to 32 ˚F / 0 ˚C, whether or not rime ice is expected to be deposited. FZBR is not a valid significant weather combination and will not be used in TAFs. Shallow Fog (MIFG) The visibility at 6 feet above ground level is greater than or equal to 5/8 sm and the apparent visibility in the fog layer is less than 5/8 sm. Patchy Fog |(BCFG) Fog patches covering part of the airport. The apparent visibility in the fog patch or bank is less than 5/8 sm, with the foggy patches extending to at least 6 feet above ground level. Partial Fog (PRFG) A substantial part of the airport is expected to be covered by fog while the remainder is expected to be clear of fog (e.g., a fog bank). Note: MIFG, PRFG and BCFG may be forecast with prevailing visibility of P6SM. Examples: 1/2SM FG Fog is reducing visibilities to less than 5/8 sm, therefore FG is used to encode the fog. 3SM BR Fog is reducing visibilities to between 5/8 and 6 sm, therefore BR is used to encode the fog. 5.13.2.8 Vicinity (VC). In the United States, vicinity (VC) is defined as a donut-shaped area between 5 and 10 sm from the center of the airport’s runway complex. The FAA requires TAFs to include certain meteorological phenomena, which may directly affect flight operations to and from the airport. Therefore, NWS TAFs may include a prevailing condition forecast of fog, showers, and thunderstorms in the airport’s vicinity. A prevailing condition is defined as a greater than or equal to 50 percent probability of occurrence for more than half of the subdivided forecast time period. VC is not included in TEMPO or PROB groups. The significant weather phenomena in Table 5-17, TAF Use of Vicinity (VC), are valid for use in prevailing portions of NWS TAFs in combination with VC: ### Page 233 5-93 Table 5-17. TAF Use of Vicinity (VC) PHENOMENON CODED Fog* VCFG Shower(s)** VCSH Thunderstorm VCTS *Always coded as VCFG regardless of visibility in the obstruction, and without qualification as to intensity or type (frozen or liquid) **The VC group, if used, should be the last entry in any significant weather group (w’w’) 5.13.2.9 Cloud and Vertical Obscuration Groups (NsNsNshshshs or VVhshshs or SKC). The initial time period and any subsequent FM groups include a cloud or obscuration group (NsNsNshshshs or VVhshshs or SKC), used as appropriate to indicate the cumulative amount (NsNsNs) of all cloud layers in ascending order and height (hshshs), to indicate vertical visibility (VVhshshs) into a surface-based obstructing medium, or to indicate a clear sky (SKC). All cloud layers and obscurations are considered opaque. 5.13.2.9.1 Cloud Group (NsNsNshshshs). The cloud group (NsNsNshshshs) is used to forecast cloud amount in Table 5-18, TAF Sky Cover. Table 5-18. TAF Sky Cover SKY COVER CONTRACTION SKY COVERAGE SKC 0 oktas FEW 0 to 2 oktas SCT 3 to 4 oktas BKN 5 to 7 oktas OVC 8 oktas ### Page 234 5-94 When 0 oktas of sky coverage is forecast, the cloud group is replaced by SKC. The contraction CLR, which is used in the METAR code, is not used in TAFs. TAFs for sites with ASOS/AWOS contain the cloud amount and/or obscurations, which the forecaster expects, not what is expected to be reported by an ASOS/AWOS. Heights of clouds (hshshs) are forecast in hundreds of feet AGL. The lowest level at which the cumulative cloud cover equals 5/8 or more of the celestial dome is understood to be the forecast ceiling. For example, VV008, BKN008, or OVC008 all indicate an 800 ft ceiling. 5.13.2.9.2 Vertical Obscuration Group (VVhshshs). The vertical obscuration group (VVhshshs) is used to forecast, in hundreds of feet AGL, the vertical visibility (VV) into a surface-based total obscuration. VVhshshs is this ceiling at the height indicated in the forecast. TAFs do not include forecasts of partial obscurations (i.e., FEW000, SCT000, or BKN000). Example: 1SM BR VV008 Ceiling is 800 ft due to vert ical visibility into fog 5.13.2.9.3 Cloud Type (CB). The only cloud type included in the TAF is CB. CB follows cloud or obscuration height (hshshs) without a space whenever thunderstorms are included in significant weather group (w’w’), even if thunderstorms are only forecast in the vicinity (VCTS). CB can be included in the cloud group (NsNsNshshshs) or the vertical obscuration group (VVhshshs) without mentioning thunderstorm in the significant weather group (w’w’). Therefore, situations may occur where nearly identical NsNsNshshshs or VVhshshs appear in consecutive time periods, with the only change being the addition or elimination of CB in the forecast cloud type. Examples : 1/2SM TSRA OVC010CB Thunderstorms are forecast at the airport 5.13.2.10 Low-Level Wind Shear Alert System (LLWAS). The LLWAS system was originally developed by the FAA in the 1970s to detect large-scale wind shifts (sea breeze fronts, gust fronts, and cold and warm fronts). It was developed by the FAA in response to an accident at JFK Airport in New York. The aircraft (Eastern 66) landed during a wind shift caused by interacting sea breeze and thunderstorm outflows. ### Page 235 5-95 Figure 5-50. Microburst Effect on Landing Aircraft Artist rendition of a microburst and its effect on a landing aircraft (Image Credit: National Center for Atmospheric Research (NCAR)). This Phase-1 LLWAS was very simple. It compared a center field wind to five other sensors around the airport. When there was a 15-knot vector difference, it would flash the wind data to the air traffic controller and the controller would read the raw winds (e.g., 120/35 (120 degrees at 35 kts), 110/20, 350/15, etc.) from each sensor to the pilot landing or about to take off and the pilot had to do the vector addition in his head to determine the headwind/tailwind components. This simple system worked for large-scale weather features, but it also had a serious false alarm problem and the sensors were too far apart to capture small, but intense windshear events important to aircraft. Also, the center field wind could be variable and this would trigger windshear alarms at all the outer sensors since all the other sensor winds were compared to center field. Research conducted at the NCAR in the 1980s indicated that microburst windshear was very dangerous to aircraft below 1,000 ft. Several major accidents during the 1980s also implicated windshear as a factor. 5.13.2.10.1 LLWAS History. In 1983, the FAA asked NCAR to develop a version of LLWAS that could detect microbursts. Between 1983 and 1988, NCAR developed and tested a new LLWAS system, called enhanced LLWAS, or LLWAS–Network Expansion, that detected microbursts, determined the strength in terms of headwind/tailwind gains or losses (in knots), and located the event (on the runway, at 1, 2, or 3 NM on departure or arrival). The system was designed to provide alerts specific to each runway operation. It was designed to have a probability of detection of 90 percent or greater and a false alarm rate of 10 percent or less. ### Page 236 5-96 This system was later improved and is now called the Phase-3 LLWAS. A typical Phase-3 LLWAS will have enough sensors to be spaced 2 km apart (∼1 NM apart) and cover out to 2 NM from the end of each major runway. The largest LLWAS is at Denver International Airport, which has 32 wind sensors. Most Phase-3 systems have between 12 and 16 wind sensors. A siting evaluation is done for each airport to determine the network geometry since it depends on terrain, number of runways, obstructions, etc. The Phase-3 LLWAS alert information is described below. If a pilot is landing on runway 08, and there is a microburst on his path, the controller would have a display that reads: 08A MBA 30K–3MF 350/25. This is read to a pilot arriving on runway 08 (08A) by a final controller as “microburst alert (MBA), expect a thirty knot loss (30K–) at three miles final (3MF), threshold wind three–five–zero at 25 (knots).” If there was a windshear with a wind speed gain at 1 mile departure (headwind gain), for a pilot departing runway 25 left, the final controller’s LLWAS display would show: 25LD WSA 15k+ 1MD. This would be read as “windshear alert, expect a fifteen knot gain at one mile departure.” There are Phase-3 LLWAS systems at 9 U.S. airports and Phase-2 LLWAS at more than 100 airports. Taiwan, Korea, Singapore, Saudi Arabia, and Kuwait are now implementing Phase-3 LLWAS systems. Note, the FAA also has Terminal Doppler Weather Radars (TDWR) for windshear detection at 45 airports and has Airport Surveillance Radar-9 (ASR-9)-based windshear detection systems at another 37 airports. The FAA originally had 110 Phase-1 LLWAS systems, which were upgraded to Phase-2 systems. A Phase-2 LLWAS has the same number of sensors (5 to 6) as a Phase-1 system (described above), but the windshear algorithm was upgraded to significantly decrease the number of false alarms. As mentioned above, a Phase-1 or Phase-2 LLWAS was not designed to detect microbursts per se, but if the flow is large and strong, it may send an alert. When NCAR developed the Phase-3 LLWAS, it gave the specifications to the FAA. The University Corporation for Atmospheric Research (UCAR) Foundation owned the intellectual property for the windshear algorithm during the lifetime of the patent. A license agreement was required for companies to implement LLWAS technology until early 2013 when the patent exclusion expired. A license from UCAR Foundation is no longer required to utilize the LLWAS algorithm. The UCAR Foundation does, however, provide technical materials such as test datasets, test airport configuration files, test alert outputs, etc. to aid companies in the implementation and testing of the Phase-3 LLWAS algorithm. ### Page 237 5-97 5.13.2.10.2 Frequently Asked Questions (FAQ). 1. Q. What is LLWAS? A. A typical Phase-3 LLWAS system includes a network of anemometers (wind sensors) atop tall poles located around the airport (i.e., remote stations) out to no more than 3 NM from the end of the runways, a master station that processes system data and communicates with the remote stations, an archiving system, operator console, alphanumeric alarm displays, and in some instances, graphical displays. 2. Q. How does it work? A. The wind data at each remote station is processed every 10 seconds to determine if there is divergence or convergence within the network, or station-to-station wind differences between stations aligned with the runways. The divergence/convergence information is processed and if the intensity of the event is large enough, the system will calculate the strength of the along-runway wind losses or gains and generate windshear or microburst alerts (depending on strength), and identify the location of the event. 3. Q. What is a microburst? A. A microburst is an intense windshear. By definition: Microburst n: A small, very intense downdraft that descends to the ground resulting in a strong wind divergence. The size of the event is typically less than 4 km across. Microbursts are capable of producing winds of more than 100 mph, causing significant damage. The life span of a microburst is around 5 to 15 minutes. 4. Q. What causes microbursts? A. Microbursts are strong windshears (greater than 30-knot wind speed losses over distances of 1 to 4 km) that are primarily generated by evaporative cooling and rain loading. When precipitation (rain or snow) descends below the cloud base or is mixed with dry air, it begins to evaporate and this evaporation process cools the air. The cool air descends and accelerates as it approaches the ground. When the cool air approaches the ground, it spreads out in all directions and this divergence of the wind is the signature of the microburst. In humid climates, microbursts can also generate from heavy precipitation. The weight and drag associated with the falling precipitation can result in a downdraft that will descend to the ground and spread out. 5. Q. Why is it a problem for airplanes? A. Windshear is a rapid change of wind speed or direction over a short distance. In general, windshear becomes a hazard for aircraft if the wind changes more than 20 kts over a distance of 1 to 4 km (0.5 to 2.5 NM). On either takeoff or landing, aircraft are near stall speeds. When going through a windshear, the headwind decreases ### Page 238 5-98 resulting in a loss of lift. If the aircraft is near stall, then a little loss of lift can make all the difference to whether the aircraft can continue the flight. 6. Q. How and what kind of decisions do airports or airlines make using LLWAS? A. LLWAS provides information on windshear type, location, and intensity. Windshear alerts are issued via radio to arriving and departing aircraft by final air traffic controllers. In the United States, most airlines require that pilots not continue their arrival or departure if there is a microburst alert valid for their operation. Although most aviation authorities do not close the runways when microbursts are occurring, the air traffic controllers will work with the pilots to reroute aircraft away from the event to a runway that is not impacted by the windshear. 7. Q. How and what kind of decisions do airports make using LLWAS? A. LLWAS provides information on windshear type, location, and intensity. The pilots get the alert from the controller and the pilot is supposed to determine if they feel comfortable continuing the operation. In the United States, airlines require that pilots do not continue if there is a microburst alert. 8. Q. Where have LLWAS systems been implemented? A. Phase-3 LLWAS systems have been implemented throughout the United States and at numerous international airports including, but not limited to, Taiwan, Korea, Saudi Arabia, Hong Kong, Kuwait, Italy, Singapore, and Spain. Additional LLWAS procurements are in progress. 9. Q. What are some of the windshear problems associated with airports? A. Any airport that has thunderstorms will be exposed to Convective windshear. Any airport near mountains will experience terrain-induced windshear from time to time. Any airport near a coast will experience windshear due to sea breezes. In the drier climates, even a light shower (or virga) can produce severe windshears, so in some places, the convection does not have to be as strong as a thunderstorm (e.g., Denver, Phoenix, Reno, Albuquerque, etc.). Discussion Credit: NCAR. For m ore information readers can go to: https://www.ral.ucar.edu/projects/low-level-wind-shear-alert-system-llwas. ### Page 239 5-99 5.13.2.11 Non-Convective Low-Level Wind Shear (LLWS) Group (WShwshwshws/dddffKT). Windshear (WS) is defined as a rapid change in horizontal wind speed and/or direction, with distance and/or a change in vertical wind speed and/or direction with height. A sufficient difference in wind speed, wind direction, or both can severely impact airplanes, especially within 2,000 ft AGL because of limited vertical airspace for recovery. Forecasts of LLWS in the TAF refer only to Non-Convective LLWS from the surface up to and including 2,000 ft AGL. LLWS is always assumed to be present in Convective activity. LLWS is included in TAFs on an “as-needed” basis to focus the aircrew’s attention on LLWS problems that currently exist or are expected. Non-Convective LLWS may be associated with the following: frontal passage, inversion, low-level jet, lee side mountain effect, sea breeze front, Santa Ana winds, etc. When LLWS conditions are expected, the Non-Convective LLWS code WS is included in the TAF as the last group (after cloud forecast). Once in the TAF, the WS group remains the prevailing condition until the next FM change group or the end of the TAF valid period if there are no subsequent FM groups. Forecasts of Non-Convective LLWS are not included in TEMPO or PROB groups. The format of the Non-Convective LLWS group is: WShwshwshws/dddffKT WS Indicator for Non-Convective LLWS Hwshwshws Height of the top of the WS layer in hundreds of feet AGL Ddd True direction in 10-degree increments at the indicated height -- VRB is not used for direction in the Non-Convective LLWS forecast Ff Speed in knots of the forecast wind at the indicated height KT Unit indicator for wind Example: TAF…13012KT…WS020/27055KT Windshear from the surface to 2,000 ft. Surface winds from 130 (southeast) at 12 kts changes to 270 (west) at 55 knots at 2,000 ft. In this example, the indicator WS is followed by a three-digit number that is the top of the windshear layer. LLWS is forecast to be present from the surface to this level. After the solidus (/), the five-digit wind group is the wind direction and speed at the top of the windshear layer. It is not a value for the amount of shear. ### Page 240 5-100 A Non-Convective LLWS forecast is included in the initial time period or a FM group in a TAF whenever: • One or more Pilot Weather Reports (PIREP) are received of Non-Convective LLWS within 2,000 ft of the surface, at or in the vicinity of the TAF airport, causing an indicated air speed loss or gain of 20 kts or more, and the forecaster determines the report(s) reflect a valid Non-Convective LLWS event rather than mechanical turbulence; or • When Non-Convective vertical WS of 10 kts or more per 100 ft in a layer more than 200 ft thick are expected or reliably reported within 2,000 ft of the surface at, or in the vicinity of, the airport. 5.13.2.12 Forecast Change Indicator Groups. Forecast change indicator groups are contractions that are used to subdivide the forecast period (24 hours for scheduled TAFs; less for amended or delayed forecasts) according to significant changes in the weather. The forecast change indicators FM, TEMPO, and PROB are used when a change in any or all of the elements forecast is expected. 5.13.2.12.1 From (FM) Group (FMYYGGgg). The change group FMYYGGgg (voiced as “from”) is used to indicate when prevailing conditions are expected to change significantly over a period of less than 1 hour. In these instances, the forecast is subdivided into time periods using the contraction FM, followed, without a space, by six digits, the first two of which indicate the day of the month and the final four, which indicate the time (in hours and minutes Z) the change is expected to occur. While the use of a four-digit time in whole hours (e.g. 2100Z) is acceptable, if a forecaster can predict changes and/or events with higher resolution, then more precise timing of the change to the minute will be indicated. All forecast elements following FMYYGGgg relate to the period of time from the indicated date and time (YYGGgg) to the end of the valid period of the terminal forecast, or to the next FM if the terminal forecast valid period is divided into additional periods. The FM group will be followed by a complete description of the weather (i.e., self-contained), and all forecast conditions given before the FM group are superseded by those following the group. All elements of the TAF (surface wind, visibility, significant weather, clouds, obscurations, and when expected, Non-Convective LLWS) will be included in each FM group, regardless if they are forecast to change or not. For example, if forecast cloud and visibility changes warrant a new FM group but the wind does not, the new FM group will include a wind forecast, even if it is the same as the most recently forecast wind. The only exception to this involves the significant weather group. If no significant weather is expected in the FM time period group, then significant ### Page 241 5-101 weather group is omitted. A TAF may include one or more FM groups, depending on the prevailing weather conditions expected. In the interest of clarity, each FM group starts on a new line of forecast text, indented five spaces. Examples: TAF KDSM 022336Z 0300/0324 20015KT P6SM BKN015 FM030230 29020G35KT 1SM +SHRA OVC005 TEMPO 0303/0304 30030G45KT 3/4SM -SHSN FM030500 31010G20KT P6SM SCT025... A change in the prevailing weather is expected on the 3rd day of the month at 0230 UTC and the 3rd day of the month at 0500 UTC. TAF KAPN 312330Z 0100/0124 13008KT P6SM SCT030 FM010320 31010KT 3SM -SHSN BKN015 FM010500 31010KT 1/4SM +SHSN VV007... Note the wind in the FM010500 group is the same as the previous FM group, but is repeated since all elements are required to be included in a FM group. 5.13.2.12.2 TEMPO (YYGG/YeYeGeGe) Group. The change-indicator group TEMPO YYGG/YeYeGeGe is used to indicate temporary fluctuations to forecast meteorological conditions that are expected to: • Have a high percentage (greater than 50 percent) probability of occurrence; • Last for one hour or less in each instance; and • In the aggregate, cover less than half of the period YYGG to YeYeGeGe. The first two digits (YY) are the day of the month for the start of the TEMPO. The next two digits (GG) are the starting hour (UTC). After the solidus (/), the next two digits (YeYe) are the ending day of the month, while the last two digits (GeGe) are the ending hour (UTC) of the TEMPO period. Each TEMPO group is placed on a new line in the TAF. The TEMPO identifier is followed by a description of all the elements in which a temporary change is forecast. A previously forecast element that has not changed during the TEMPO period is understood to remain the same and will not be included in the TEMPO group. Only those weather elements forecast to temporarily change are required to be included in the TEMPO group. ### Page 242 5-102 TEMPO groups will not include forecasts of either significant weather in the vicinity (VC) or Non-Convective LLWS. Examples: TAF KDDC 221130Z 2212/2312 29010G25KT P6SM SCT025 TEMPO 2215/2217 30025G35KT 1 1/2SM SHRA BKN010... In the example, all forecast elements in the TEMPO group are expected to be different than the prevailing conditions. The TEMPO group is valid on the 17 from 1500 UTC to 1700 UTC. TAF KSEA 091125Z 0912/1012 19008KT P6SM SCT010 BKN020 OVC090 TEMPO 0912/0915 -RA SCT010 BKN015 OVC040... In this example the visibility is not forecast in the TEMPO group. Therefore, the visibility is expected to remain the same (P6SM) as forecast in the prevailing conditions group. Also, note that in the TEMPO 0912/0915 group, all three cloud layers are included, although the lowest layer is not forecast to change from the initial time period. 5.13.2.12.3 PROB30 (YYGG/YeYeGeGe) Group. The probability group, PROB30 YYGG/YeYeGeGe, is only used by NWS forecasters to forecast a low probability occurrence (30 percent chance) of a thunderstorm or precipitation event and its associated weather and obscuration elements (wind, visibility, and/or sky condition) at an airport. The PRO B30 group is the forecaster’s assessment of probability of occurrence of the weather event that follows it. The first two digits (YY) are the day of the month for the start of the PROB30. The next two digits (GG) are the starting hour (UTC). After the solidus (/), the next two digits (YeYe) are the ending day of the month, while the last two digits (GeGe) are the ending hour (UTC) of the PROB30 period. PROB30 is the only PROB group used in NWS TAFs. Note that U.S. military and international TAFs may use the PROB40 (40 percent chance) group as well. The PROB30 group is located within the same line of the prevailing condition group, continuing on the line below if necessary. NWS TAFs do not use the PROB30 group in the first 9 hours of the TAF’s valid period, including amendments. Also, only one PROB30 group may be used in the initial forecast period and in any subsequent FM groups. Note that U.S. military and international TAFs do not have these restrictions. ### Page 243 5-103 PROB30 groups do not include forecasts of significant weather in the vicinity (VC) or Non-Convective LLWS. Example: FM012100 18015KT P6SM SCT050 PROB30 0123/0201 2SM TSRA OVC020CB In this example, the PROB30 group is valid on the 1st day of the month at 2300 UTC to the 2nd day of the month at 0100 UTC. 5.13.2.12.4 TAF Examples. TAF KPIR 111140Z 1112/1212 13012KT P6SM BKN100 WS020/35035KT TEMPO 1112/1114 5SM BR FM111500 16015G25KT P6SM SCT040 BKN250 FM120000 14012KT P6SM BKN080 OVC150 PROB30 1200/1204 3SM TSRA BKN030CB FM120400 14008KT P6SM SCT040 OVC080 TEMPO 1204/1208 3SM TSRA OVC030CB TAF Terminal Aerodrome Forecast KPIR Pierre, South Dakota 111140 Prepared on the 11th at 1140 UTC 1112/1212 Valid from the 11th at 1200 UTC until the 12th at 1200 UTC 13012KT Wind 130 at 12 kts P6SM Visibility greater than 6 sm BKN100 Ceiling 10,000 broken WS020/35035KT Windshear at 2,000 ft, wind from 350 at 35 kts TEMPO 1112/1114 Temporary conditions between the 11 th day of the month at 1200 UTC and the 11th day of the month at 1400 UTC 5SM Visibility 5 statute miles BR Mist FM111500 From the 11th day of the month at 1500 UTC 16015G25KT Wind 160 at 15 kts gusting to 25 kts P6SM Visibility greater than 6 sm SCT040 BKN250 4,000 scattered, ceiling 25,000 broken FM120000 From the 12th day of the month at 0000Z 14012KT Wind 140 at 12 kts P6SM Visibility greater than 6 sm BKN080 OVC150 Ceiling 8,000 broken, 15,000 overcast PROB30 1200/1204 30 percent probability between the 12 th day of the month at 0000 UTC and the 12th day of the month at 0400 UTC 3SM Visibility 3 sm TSRA Thunderstorm with moderate rain showers ### Page 244 5-104 BKN030CB Ceiling 3,000 broken with cumulonimbus FM120400 From the 12th day of the month at 0400 UTC 14008KT Wind 140 at 8 kts P6SM Visibility greater than 6 sm SCT040 OVC080 4,000 scattered, ceiling 8,000 overcast TEMPO 1204/1208 Temporary conditions between the 12 th day of the month at 0400 UTC and the 12th day of the month at 0800 UTC 3SM Visibility 3 sm TSRA Thunderstorms with moderate rain showers OVC030CB Ceiling 3,000 overcast with cumulonimbus TAF AMD KEYW 131555Z 1316/1412 VRB03KT P6SM VCTS SCT025CB BKN250 TEMPO 1316/1318 2SM TSRA BKN020CB FM131800 VRB03KT P6SM SCT025 BKN250 TEMPO 1320/1324 1SM TSRA OVC010CB FM140000 VRB03KT P6SM VCTS SCT020CB BKN120 TEMPO 1408/1412 BKN020CB TAF AMD Amended Terminal Aerodrome Forecast KEYW Key West, Florida 131555Z Prepared on the 13th at 1555 UTC 1316/1412 Valid from the 13th at 1600 UTC until the 14th at 1200 UTC VRB03KT Wind variable at 3 kts P6SM Visibility greater than 6 sm VCTS Thunderstorms in the vicinity SCT025CB BKN250 2,500 scattered with cumulonimbus, ceiling 25,000 broken TEMPO 1316/1318 Temporary conditions between the 13 th day of the month at 1600 UTC and the 13th day of the month at 1800 UTC 2SM Visibility 2 sm TSRA Thunderstorms with moderate rain showers BKN020CB Ceiling 2,000 broken with cumulonimbus FM131800 From the 13th day of the month at 1800 UTC VRB03KT Wind variable at 3 knots P6SM Visibility greater than 6 sm SCT025 BKN250 2,500 scattered, ceiling 25,000 broken TEMPO 1320/1324 Temporary conditions between the 13 th day of the month at 2000 UTC and the 14th day of the month at 0000 UTC 1SM Visibility 1 sm TSRA Thunderstorms with moderate rain showers OVC010CB Ceiling 1,000 overcast with cumulonimbus FM140000 From the 14th day of the month at 0000 UTC VRB03KT Variable wind at 3 knots P6SM Visibility greater than 6 sm VCTS Thunderstorms in the vicinity ### Page 245 5-105 SCT020CB BKN120 2,000 scattered with cumulonimbus, ceiling 12,000 broken TEMPO 1408/1412 Temporary conditions between the 14 th day of the month at 0800 UTC and the 14th day of the month at 1200 UTC BKN020CB Ceiling 2,000 broken with cumulonimbus 5.13.3 Issuance. Scheduled TAFs prepared by NWS offices are issued four times a day, every 6 hours, according to the following schedule: Table 5-19. TAF Issuance Schedule SCHEDULED ISSUANCE VALID PERIOD ISSUANCE WINDOW 0000 UTC 0000 to 2400 or 0600 UTC 2320 to 2340 UTC 0600 UTC 0600 to 0600 or 1200 UTC 0520 to 0540 UTC 1200 UTC 1200 to 1200 or 1800 UTC 1120 to 1140 UTC 1800 UTC 1800 to 1800 or 2400 UTC 1720 to 1740 UTC 5.13.3.1 Minimum Observational Requirements for Routine TAF Issuance and Continuation. The NWS forecaster must have certain information for the preparation and scheduled issuance of each individual TAF. Although integral to the TAF writing process, a complete surface (METAR/SPECI) observation is not required. Forecasters use the “total observation concept” to write TAFs with data including nearby surface observations, radar, satellite, radiosonde, model data, aircraft, and other sources. If information sources, such as surface observations, are missing, unreliable, or not complete, forecasters will append AMD NOT SKED to the end of a TAF. The use of AMD NOT SKED indicates the forecaster has enough data, using the total observation concept, to issue a forecast but will not provide updates. This allows airport operations to continue using a valid TAF. In rare situations where observations have been missing for extended periods of time (i.e., more than one TAF cycle of 6 hours), and the total observation concept cannot provide sufficient information, the TAF may be suspended by the use of NIL TAF. 5.13.3.2 Sites With Scheduled Part-Time Observations. For TAFs with less than 24-hour observational coverage, the TAF will be valid to the end of the routine scheduled forecast period even if observations cease prior to that time. The time observations are scheduled to end and/or ### Page 246 5-106 resume will be indicated by expanding the AMD NOT SKED statement. Expanded statements will include the observation ending time (AFT Y1Y1HHmm, e.g., AFT 120200), the scheduled observation resumption time (TIL Y1Y1HHmm, e.g., TIL 171200Z) or the period of observation unavailability (Y1Y1HH/YeYehh, e.g., 2502-2512). TIL will be used only when the beginning of the scheduled TAF valid period coincides with the time of the last observation or when observations are scheduled to resume prior to the next scheduled issuance time. When used, these remarks will immediately follow the last forecast group. If a routine TAF issuance is scheduled to be made after observations have ceased, but before they resume, the remark AMD NOT SKED will immediately follow the valid period group of the scheduled issuance. After sufficient data using the total observation concept has been received, the AMD NOT SKED remark will be removed. Examples of Scheduled Part-Time Observations TAFs. TAF AMD KRWF 150202Z 1502/1524 {TAF text} AMD NOT SKED 1505Z-1518Z= No amendments will be available between the 15th day of the month at 0500 UTC and the 15th day of the month at 1800 UTC due to lack of a complete observational set between those times. TAF AMD KPSP 190230Z 1903/1924 {TAF text} AMD NOT SKED= Amendments are not scheduled. 5.13.3.3 Automated Observing Sites Requiring Part-Time Augmentation. TAFs for automated stations without present weather and obstruction to vision information and have no augmentation or only part-time augmentation, are prepared using the procedures for part-time manual observation sites detailed in the previous paragraph, with one exception. This exception is the remark used when the automated system is unattended. Specifically, the time an augmented automated system is scheduled to go into unattended operation and/or the time augmentation resumes is included in a remark unique to automated observing sites: AMD LTD TO CLD VIS AND WIND (AFT YYHHmm, or TIL YYhhmm, or YYHH-YYhh), where YY is the date, HHmm is the time, in hours and minutes, of last augmented observation and hhmm is the time, in hours and minutes, the second complete observation is expected to be received. This remark, which does not preclude amendments for other forecast elements, is appended to the last scheduled TAF issued prior to the last augmented observation. It will also be appended to all subsequent amendments until augmentation resumes. ### Page 247 5-107 The AMD LTD TO (elements specified) remark is a flag for users and differs from the AMD NOT SKED AFT Z remark for part-time manual observation sites. AMD LTD TO (elements specified) means users should expect amendments only for those elements and the times specified. Example: TAF AMD KCOE 150202Z 1502/1524 text AMD LTD TO CLD VIS AND WIND 1505-1518= The amended forecast indicates that amendments will only be issued for wind, visibility, and clouds, between the 15th day of the month at 0500Z and the 15th day of the month at 1800Z. An amendment includes forecasts for all appropriate TAF elements, even those not reported when the automated site is not augmented. If unreported elements are judged crucial to the TAF and cannot be adequately determined (e.g., fog versus moderate snow), the TAF will be suspended (i.e., an amended TAF stating “AMD NOT SKED”). AWOS systems with part-time augmentation, which the forecaster suspects are providing unreliable information when not augmented, will be reported for maintenance and treated the same as part-time manual observation sites. In such cases, the AMD NOT SKED AFT YY/aaZ remark will be used. 5.13.3.4 Non-Augmented Automated Observing Sites. The TAF issued for automated observing stations with no augmentation may be suspended in the event the forecaster is notified of, or strongly suspects, an outage or unrepresentative data. Forecasters may also suspend TAF amendments when an element the forecaster judges to be critical is missing from the observation and cannot be obtained using the total observation concept. The term AMD NOT SKED will be appended, on a separate line and indented five spaces, to the end of an amendment to the existing TAF when appropriate. 5.14 Route Forecasts (ROFOR). 5.14.1 Issuance. ROFORs are only issued by the NWS WFO in Honolulu, HI, twice per day for two routes: San Francisco, CA (KSFO), to Honolulu, HI (PHNL), and Santa Barbara, CA (KSBA), and PHNL. ROFORs are available on the NWS WFO Honolulu website. 5.14.2 ROFOR Amendments. ROFORs are not required to be amended. 5.14.3 ROFOR Corrections. ROFOR corrections are issued as soon as possible when erroneous data has been transmitted. ### Page 248 5-108 5.14.4 ROFOR Content. ROFORs contain some or all of the following forecast parameters, which are valid for the specified times: • Winds and temperatures aloft at flight levels (FL) 050, 100, 180 and 240 (true direction in tens of degrees, speed in knots, air temperature in whole degrees Celsius with P for a value above 0 and M for a value below zero). • Significant en route weather. • Zone weather, in abbreviated code, for numbered zones (zone map available on NWS WFO Honolulu website). o Cloud amount in oktas, cloud types, cloud bases. and tops (MSL or FL). o Coverage and type of precipitation and weather along with visibility when below 7 sm. • Overall wind factor component at FLs 050, 100, 180 and 240. • Weather Synopsis. Note: See Appendi ces A and B for definitions of common terms used in ROFORs. 5.14.5 Example. 5.14.5.1 Santa Barbara and San Francisco to Honolulu Route ROFOR Example. **FRPN31 PHFO 301857 RFRKSF** **WINDS/TEMPERATURES AND WEATHER BY ZONE FOR ROUTE SFO/HNL VIA 31.3N/140W VALID AT 311200Z** FLIGHT LEVELS ZONE FL050 FL100 FL180 FL240 ZONE WEATHER 25 3315 P16 3208 P11 3109 M07 3216 M19 6-8 STSC 010/030 26 3316 P13 3211 P09 3117 M06 3023 M18 4-6 STSC 015/045 27 3013 P12 3212 P09 3020 M06 3024 M18 6-8 MERGING LYR TO 200 ISOL VIS 3-5SM RA ISOL TCU TOPS FL220 28 3008 P14 3008 P08 2815 M06 2918 M18 DO 29 9905 P14 9905 P08 2609 M06 2612 M18 4-6 CUSC 020/050 30 0506 P14 9905 P08 9905 M06 2406 M18 DO 31 0818 P15 0613 P09 0307 M06 9905 M18 4-6 CUSC 020/080 **ISOL -SHRA 32 0822 P15 0719 P09 0711 M05 9905 M17 DO** **OVERALL COMPONENTS P4 P2 M4 M10 ROUTE SBA/HNL VIA 29.5N/140W VALID AT 311200Z** FLIGHT LEVELS ZONE FL050 FL100 FL180 FL240 ZONE WEATHER 25 3509 P17 3108 P11 3011 M07 3015 M19 6-8 STSC 010/030 26 3416 P14 3312 P09 3218 M05 3123 M18 4-6 STSC 015/045 27 0111 P13 3510 P10 3017 M05 3021 M18 2-4 CUSC 020/045 28 0307 P14 3606 P09 2713 M05 2717 M18 DO 29 0406 P14 9905 P08 2507 M05 2610 M18 4-6 CUSC 020/050 30 0815 P15 0610 P09 9905 M05 9905 M17 DO The text continues at https://openairdocs.org/ac/00-45.md?from=249 --- From OpenAirDocs.org, an independent open copy of FAA data. Only the Federal Register edition on govinfo.gov is the official legal text of an AD.