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 WA SIGMET)
* Tropical Cyclone (TC or WC 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. on a scheduled basis, hourly at 55 minutes past the hour. The Convective SIGMETs are valid for 2 hours or until superseded by the next SIGMET. If it is determined that the conditions do not meet the Convective SIGMET criteria within a given region, a CONVECTIVE SIGMET... NONE is issued. Additionally, a 2 to 6-hour outlook is attached at the end of each regional bulletin.

There are three types of convective SIGMETs:

When shown in a graphical depiction, the convective SIGMET polygon is a "snapshot" that delineates the region of thunderstorm at the issuance time of 55 past each hour.

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
* Indications of rapidly changing conditions not sufficiently described in existing convective SIGMETs.

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 BB''
CCCC SIGMET n]n VALID YYGGgg/YYGGgg CCCC-
CCCC FIR OBS/FCST T GGggZ OV XXX xx KT/KMH NTSF/WKN/NC=

Header

The header line consists of the following characters

TTAAii CCCC YYGGgg Cx''

Summary

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

CCCC SIGMET n]n VALID YYGGgg/YYGGgg CCCC-

::Notes
    /small> – if it a convective SIGMET, then this will read SIG /C/WCONVECTIVE SIGMET ## /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.

Body

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

CCCC IR/CTA list OBS/FCST T GGggZ OV XXX xx KT/KMH NTSF/WKN/NC CST AT Z

CCCC IR/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 T 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

OV 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.

NTSF/WKN/NC'' denotes the change in strength over time.

CST 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 YYGGgg/YYGGgg

In the United States, non-convective SIGMETs have a valid period no more than 4 hours. If the phenomena continue, a new SIGMET will be reissued at least every 4 hours. If an amendment is needed, a new SIGMET will be issued using the next series number. Corrections to SIGMETs are issued as necessary, and are identified with COR.

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.

See also

* AIRMET
* PIREP
* IWXXM
* VOLMET

References

{{Reflist

External links

IWXXM 1.0
Official WMO/ICAO representation of SIGMET information in XML

Aviation meteorology
Earth sciences data formats
Weather warnings and advisories
Aviation publications