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AIRMETs & SIGMETs (RP)




AIRMET, SIGMET, Convective SIGMET are in-flight weather advisories that have detailed information on potential hazardous weather to all in-flight aircraft. These three advisories are in conjunction with the Area Forecasts.


AIRMET:

AIRMETs (WA) are issued every six hours, they include surface winds of 30 knots or greater, ceilings less than 1,000 feet and visibility less than 3 miles.


AIRMETs are broken down into three categories:

  • Sierra – IFR and mountain obstructions. (Ceilings less than 1,000 feet AGL and visibility is less than 3SM.)

  • Tango – Turbulence, low-level wind shear, strong surface winds.

  • Zulu – Icing, freezing levels.


Synopsis gives a detailed summary of the location, movement and types of weather pressure systems, fronts, and circulation patterns.


SIGMETs:

SIGMETs (WS) are issued for non-convective weather that can be hazardous to all aircraft. They are unscheduled weather forecasts valid for 4 hours. If a SIGMET is for a hurricane or typhoon than the SIGMET is valid for 6 hours. These reports are issued for:

  • Severe icing not associated with a thunderstorm.

  • Severe or extreme turbulence or CAT (clear air turbulence) not associated with thunderstorms.

  • Dust storms, sandstorms.

  • Volcanic ash.



Convective SIGMETs:

Convective SIGMETs (WST) are in-flight weather advisories issued for hazardous convective weather that affects the safety of flight. They are issued 55 minutes past the hour and are valid for 2 hours. They can be issued at any time as needed. Convective SIGMETs cover the areas of the lower 48 states not including Alaska and Hawaii.


They are broken up into three areas of the U.S.:

  • E- eastern

  • C-central

  • W-western

They are numbered from 1-99 starting at time 0000Z. These reports are issued for:

  • Severe thunderstorms with surface winds of 50 knots or greater.

  • Tornadoes

  • Hail at the surface greater than or equal to 3/4 inches in diameter.

  • Embedded thunderstorms, or thunderstorms with heavy precipitation that affects 40% or more of a 3,000 square mile or greater region.


In the example below, this Convective SIGMET is the 14th one issued for the Eastcoast (14E).

(Convective SIGMET number 14 in the E-eastern region on the U.S.)



References:

Aviation Weather Handbook FAA-H-8083-28 (NEW Publication Dec. 22, 2022)




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