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 diameter or greater, tornadoes, embedded thunderstorms, lines of thunderstorms, or thunderstorms covering 40% or more of an area of at least 3,000 square miles. Convective SIGMETs are issued hourly for the eastern, central, and western United States, are valid for two hours, and implicitly include the hazards of severe or greater turbulence, severe icing, and low-level wind shear.
Plain English
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.
Context Anchor
Seen during preflight weather briefings, in aviation weather products, and while checking for thunderstorm hazards along a planned route.
Derivation
The 'W' identifier comes from the product code used by the National Weather Service for SIGMETs (W-series text products), and 'ST' stands for 'storm' — the convective focus. The word 'convective' comes from Latin 'convectus,' meaning 'carried together,' which describes the rising and falling air currents that build thunderstorms.
Why Pilots Care
These advisories identify areas pilots must avoid to prevent aircraft damage or loss of control.
Intuition Check
Do not read “information” as casual background information. A WST is an official hazard advisory, and “significant” means significant to flight safety, not merely interesting or noticeable.
Example Sentence 1
During the preflight briefing, the pilot noted a Convective SIGMET (WST) for a line of thunderstorms across her planned route and chose to delay departure by two hours.
Example Sentence 2
The pilot checked the latest WST before departure and adjusted the flight path accordingly.