Chapter 26
Advisories
26.2.5.3 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 to, where possible, the latitude and longitude and FLs (altitude) covering the affected area during the SIGMET valid time. SIGMETs for volcanic ash clouds and tropical cyclones contain the positions of the ash cloud, tropical cyclone center, and radius of convection at the start of the valid time of the SIGMET.
- Movement toward or expected movement using 16 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 for intensifying (INTSF), weakening (WKN), or no change (NC).
- Forecast position of the volcanic ash cloud or the center of the tropical cyclone at the end of the validity period of the SIGMET message.
The SIGMET in Figure 26-7 is decoded as the following:
- (Line 1) The WMO product header is WSPA07. Issued by the Honolulu MWO on the first day of the month at 0410 UTC.
- (Line 2) The NWS Advanced Weather Interactive Processing System (AWIPS) communication header is SIGPAT.
- (Line 3) For the Oakland ARTCC (KZAK). This is the second 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 MWO.
- (Line 4) Concerning the Oakland Oceanic FIR, frequent thunderstorms observed and forecast within 200 NM of 30° and 6 minutes north; 140° and 12 minutes west; to 20° and 12 minutes north, 150° and 16 minutes west, cumulonimbus tops to FL400 moving west at 10 kt, weakening.
26.2.5.4 SIGMETs for Volcanic Ash (VA)—Outside the CONUS
A SIGMET for a 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 PIREPs in the absence of a 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.
SIGMETs for a volcanic ash cloud are valid up to six hours and provide an observed or forecast location of the ash cloud at the beginning of the SIGMET. A six-hour forecast position for the ash cloud, valid at the end of the validity period of the SIGMET message, is also included.
26.2.5.5 SIGMETs for Tropical Cyclone (TC)—Outside the CONUS
SIGMETs for a tropical cyclone (which includes a hurricane, typhoon (western Pacific), and tropical storm) may be issued for non-frontal synoptic-scale cyclones meeting the following criteria:
- Originates over tropical or subtropical waters with organized convection and definite cyclonic surface wind circulation.
- Wind speeds reach 35 kt independent of the wind averaging time used by the TCAC.
SIGMETs for tropical cyclones will be valid up to six hours. SIGMETs for tropical cyclones will include two positions. The first position will be the observed position of the center of the tropical cyclone, taken from the TCA. The second position will be the forecast position of the center of the tropical cyclone valid at the end of the SIGMET period, which will coincide with the six-hour forecast position in the TCA.
In addition to the two storm positions, SIGMETs will include associated convection when applicable. SIGMETs will be reissued at least every six hours while the tropical cyclone winds remain or are expected to remain above 34 kt.
26.2.5.6 SIGMET Cancellation—Outside the CONUS
SIGMETs are cancelled when the phenomenon is no longer occurring or expected to occur.
26.2.5.7 SIGMET Amendments—Outside the CONUS
SIGMET amendments will not be issued. Instead, the next SIGMET in the series is issued to accomplish the update. The valid time of the new SIGMET is reset to reflect the new four-hour valid period (six-hour period for volcanic ash and tropical cyclone SIGMETs).