Plain English Meaning
An urgent weather alert about thunderstorms or related severe weather that is dangerous to every aircraft, no matter the size. It warns pilots about things like big thunderstorms, tornadoes, large hail, or thunderstorm lines.
Full Definition
A weather advisory issued for hazardous convective weather that is significant to all aircraft. A Convective SIGMET is issued for severe thunderstorms with surface winds of 50 knots or greater, hail at the surface 3/4 inch or greater in diameter, tornadoes, embedded thunderstorms, lines of thunderstorms, and thunderstorms covering 40 percent or more of an area of at least 3,000 miles. are issued for the contiguous 48 states only and are valid for 2 hours.