SIGMET (SIGnificant METeorlogical Information) advises of weather
potentially hazadous to all aircraft other than convective activity. In
the conterminous U.S., items covered are:
In Alaska and Hawaii, SIGMETs are also issued for the following events:
For the lower 48 states and adjacent coastal waters, Convective SIGMETs are issued hourly for Thunderstorm-related aviation hazards.
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 affect 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.