SIGMET

SIGMET, or Significant Meteorological Information (AIM 7-1-6), is a severe weather advisory that contains meteorological information concerning the safety of all aircraft. Compared to AIRMETs, SIGMETs cover more severe weather. Today, according to the advancement of technology in civil aviation, the SIGMET is sent as IWXXM model.

Types
There are three main types of internationally recognized SIGMETs per ICAO:
 * Volcanic ash (VA or WV SIGMET)
 * Tropical Cyclone (TC SIGMET)
 * Other En-route weather phenomenon (WS SIGMET), which may consist of
 * Thunderstorm types
 * Turbulences types
 * Mountain waves
 * Icing/Sleet/Hail
 * Dust or sand storms
 * Radioactive Cloud

This information is usually broadcast on the ATIS at ATC facilities, as well as over VOLMET stations. They are assigned an alphabetic designator from N through Y (excluding S and T). SIGMETs are issued as needed, and are valid up to four hours. SIGMETS for hurricanes and volcanic ash outside the CONUS are valid up to six hours.

Convective SIGMETs
For airmen in the U.S., there is an additional category of SIGMET known as a convective SIGMET. These are issued for convection over the coterminous U.S.. There are three types of convective SIGMETs:

There are also a few special issuance convective SIGMETs to cover extreme weather more common in the U.S., including


 * Tornadoes
 * Hail greater than or equal to 3/4 inches in diameter
 * Wind gusts greater than or equal to 50 knots

Structure
SIGMETs are internationally used by ICAO and WMOs using standardized abbreviations

and are split into three lines:
 * Header
 * Summary
 * Main Body

In sum, a standardized SIGMET will have the following structure:

TTAAii CCCC YYGGgg [BBB] CCCC SIGMET [n][n]n VALID YYGGgg/YYGGgg CCCC- CCCC  FIR  OBS/FCST [AT GGggZ]   [MOV XXX xx KT/KMH] [INTSF/WKN/NC] =

Header
The header line consists of the following characters

TTAAii CCCC YYGGgg [CCx]

Summary
The first line of the broadcast is a summary line consisting of the following characters

CCCC SIGMET [n][n]n VALID YYGGgg/YYGGgg CCCC-

[a] – if it a convective SIGMET, then this will read SIG[E/C/W] CONVECTIVE SIGMET ##[E/C/W]. Where E/C/W indicates whether it's over the Eastern, Central, or Western United States, and ## indicates the number of the convective SIGMET issued for that region.
 * Notes

Body
The main body of a SIGMET can be much more variable, and consists of

CCCC [FIR/CTA list]  OBS/FCST [AT GGggZ]   [MOV XXX xx KT/KMH] [INTSF/WKN/NC] [FCST AT Z ]=

CCCC [FIR/CTA list] is again the 4-character ICAO location, followed by the affected flight or control regions.

 is a code describing the meteorological phenomena as follows:

If it is a convective SIGMET, the following codes may appear

OBS/FCST [AT GGggZ] indicates whether the phenomenon is observed (OBS) or forecasted (FCST), and the Zulu hour and minute that it was observed or will be forecasted.

 is a general description of location of the meteorological phenomenon, typically utilizing latitude and longitudinal coordinates.

 helps denote the altitude that the phenomenon will be occurred, and can be expressed as

[MOV XXX xx KT/KMH] if it is a moving front, the direction and rate of movement given as a compass direction (XXX, e.g. "N" or "WNW"), and the rate is given in KT (or KMH). Sometimes STNR (Stationary) may be used instead if no significant movement is expected.

[INTSF/WKN/NC] denotes the change in strength over time.

[FCST AT Z ] helps note where the front is expected to be at the end of the SIGMET's validity period.

SIGMET Renewal and Cancellation
If when the validity period is due to expire but the phenomenon is expected to persist, a new sequence number is added to the SIGMET to renew it.

If during the validity period of a SIGMET, the SIGMET is to be cancelled, the following replaces the SIGMET message

CNL SIGMET [n][n]n YYGGgg/YYGGgg

Examples
WSUS32 KKCI 071655 An en-route weather phenomenon in the U.S., issued by the Aviation Weather Center in Kansas City, MO on the 7th of August, at 16:55 UTC SIGC CONVECTIVE SIGMET 83C This is a convective weather pattern in the central region of the contiguous U.S. with sequence number 83C VALID UNTIL 1855Z That is valid until 18:55 UTC MI IN WI IL IA LM That covers Minnesota, Indiana, Wisconsin, Illinois, Iowa, and Lower Michigan flight regions FROM 30E GRR-30S GIJ-30SSW BDF-10ENE IOW-50ENE DBQ-30E GRR A description of the area of the storm, where GRR (Grand Rapids, MI airport), GIJ (Niles, MI VORTAC), BDF (Bradford, IL VORTAC), IOW (Iowa City, IA airport), and DBQ (Dubuque, IA airport) are ConTrol Areas (CTAs). AREA TS MOV FROM 25025KT. TOPS TO FL450. Thunderstorms moving from 250 degrees (roughly WSW) at 25 knots, covering an altitude from the cloud cover to 45,000 ft.