A SIGMET advises of weather, other than convective activity, that is potentially hazardous to all aircraft. SIGMETs are issued (for the lower 48 states and adjacent coastal waters) for the following weather-impacted reasons:
If the total area affected during the forecast period is very large, only a small portion of this total area may 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. if these conditions are occurring or expected to occur:
Special issuance criteria include:
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. 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.
CWAs are advisories issued by the Center Weather Service Units (CWSUs) that are for conditions just below severe criteria. CWAs are issued for:
An AIRMET advises of weather potentially hazardous to all aircraft but that does not meet SIGMET criteria.
AIRMETs are issued by the National Weather Service's Aviation Weather Center (for the lower 48 states and adjacent coastal waters) for the following weather impacted reasons:
If the total area affected during the forecast period is very large, only a small portion of this total area may be affected at any one time.
AIRMETs are routinely issued for 6 hour periods beginning at 0245 UTC. AIRMETS are also amended as necessary due to changing weather conditions or issuance cancellation of a SIGMET.