SIGMETs are issued in the U.S. for any of the following:
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.