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.
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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.
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.
Figure 4-19. NWS NCEP Central Operations Model Analyses and Guidance Website DOC NOAA NWS NCEP Centers: AWC CPC EMC NCO NHC OPC SPC SWPC WPC NCEP Home ≥ NCO Home ≥ Systems Integration Branch ≥ Model Analyses and Guidance Model Analyses and Guidance Information + Website Check here for the Latest News Select Model Guidance, Observations and Analyses, or Tropical Guidance
MODEL GUIDANCE OBSERVATIONS AND ANALYSES TROPICAL GUIDANCE Upcoming Changes T User's Guide I Frequently Asked Questions I Product Description Document MAG V3.0.0 NOAA/ National Weather Service Disclaimer Privacy Policy National Centers for Environmental Prediction Credits About Us 5830 University Research Court Glossary Career Opportunities College Park, MD 20740 NCEP Internet Services Team Page last modified: May 20 2013 17:08 PM UTC.
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, 500 MB Height, Temperature, and Wind, 00-hour Forecast
(i.e., Model's Analyses from the NWS's North American Model [NAM] Example)) available on NCO's Model Analyses and Guidance website.
See paragraph 5.17.1 for additional information.
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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.
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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.
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).
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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.
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).
Figure 5-1. AWC SIGMET Areas of Responsibility-Continental U.S.
San Francisco Salt Lake City Chicago Boston Dallas/Fort Worth Miami Figure 5-2. AWC SIGMET Areas of Responsibility-Atlantic Basin 40
NEW YORK OCEANIC FIR 30
HOUSTON MIAMI,
OCEANIC OCEANIC Q I MIAMI
FIR FIR OCEANIC EIR
SAN JUAN → FIR -90 - 80 ·260 - 50 -40 5-2
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.
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• 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