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
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.
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.
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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.
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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:
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• 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
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:
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• 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)
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.