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.
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.
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
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.
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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.