Plain English Meaning
A WST is an urgent weather warning for pilots about dangerous thunderstorm activity — things like tornadoes, severe storms, large hail, or lines of storms. It tells pilots where these storms are, where they are heading, and that conditions are bad enough to threaten any aircraft.
Full Definition
A Convective SIGMET (WST) is an in-flight weather advisory issued by the Aviation Weather Center for hazardous convective weather that affects the safety of all aircraft. It is issued for severe thunderstorms with surface winds of 50 knots or greater, hail at the surface 3/4 inch in or greater, tornadoes, , , or thunderstorms covering 40% or more of an of at least 3,000 miles. are issued hourly for the eastern, central, and western , are valid for two hours, and implicitly include the of severe or greater , , and .