SIGMET (SIGnificant METeorlogical Information)

A SIGMET (SIGnificant METeorlogical Information) advises of weather potentially hazadous to all aircraft other than convective activity.

SIGMETs are issued (for the lower 48 states and adjacent coastal waters) for the following weather-impacted reasons:

  • Severe Icing
  • Severe or Extreme Turbulence
  • Duststorms and sandstorms lowering visibilities to less that three (3) miles
  • Volcanic Ash
These SIGMET items are considered to be widespread because they must be affecting or be forecast to affect an area of at least 3000 square miles at any one time. However, if the total area to be affected during the forecast period is very large, it could be that only a small portion of this total area would be affected at any one time.

SIGMETs are issued for 6 hour periods for conditions associated with hurricanes and 4 hours for all other events. If conditions persist beyond the forecast period, the SIGMET is updated and reissued. Convective SIGMETS are issued hourly for Thunderstorm-related aviation hazards.


CONVECTIVE SIGMETs are issued in the conterminous U.S. for:

  • Severe surface weather including:
    • surface winds greater than or equal to 50 knots
    • hail at the surface greater than or equal to 3/4 inches in diameter
    • tornadoes
  • Embedded thunderstorms
  • Line of thunderstorms
  • Thunderstorms greater than or equal to VIP level 4 affecting 40% or more of an area at least 3000 square miles
Any Convective SIGMET implies severe or greater turbulence, severe icing, and low level wind shear. A Convective SIGMET may be issued for any convective situation which the forecaster feels is hazardous to all categories of aircraft.

Convective SIGMET bulletins are issued for the Eastern (E), Central (C), and Western (W) United States. The areas separate at 87 and 107 degrees west longitude with sufficent overlap to cover most cases when the phenomenon crosses the boundaries. Bulletins are issued hourly at Hour+55. The text of the bulletin consists of either an observation and a forecast or just a forecast. The forecast is valid for up to 2 hours.

Concept Mapping Toolkit
Insitute for Human and Machine Cognition
The University of West Florida