Definition
A weather advisory issued by the Aviation Weather Center for hazardous convective weather that is significant to the safety of 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 in diameter or greater, tornadoes, embedded thunderstorms, lines of thunderstorms, or thunderstorm areas covering at least 40 percent of an area of 3,000 square miles or greater with thunderstorms reaching above a specified intensity level. Convective SIGMETs are issued hourly for the eastern, central, and western United States and are valid for two hours or until superseded.
Plain English
A weather alert that warns pilots about thunderstorms or thunderstorm activity bad enough to be dangerous to any aircraft. It tells you where the worst storms are, how strong they are, and how long the warning is good for.
Context Anchor
Pilots encounter WST — Convective SIGMETs during weather briefings, flight planning, and in-flight weather updates when thunderstorm activity may affect a route.
Derivation
Convective' comes from the Latin 'convehere,' meaning 'to carry together' — referring to the rising and sinking air currents inside thunderstorms. 'SIGMET' is a contraction of 'Significant Meteorological Information.' The 'WST' header code is the international identifier the National Weather Service places on the teletype message, distinguishing it from regular SIGMETs (WS) and AIRMETs (WA).
Why Pilots Care
Pilots must avoid these areas because they contain hazards like severe turbulence, lightning, and hail that can damage aircraft or cause loss of control.
Grounding Statement
A WST — Convective SIGMET marks an area where thunderstorm weather is serious enough that pilots should not treat it as ordinary rain or clouds.
Intuition Check
Do not read “SIGMET” as just another weather note. A WST — Convective SIGMET is specifically about hazardous thunderstorm-type weather that can affect all aircraft, including larger and better-equipped airplanes.
Example Sentence 1
The briefer told us a Convective SIGMET was active along our route, so we delayed departure until the line of storms moved east.
Example Sentence 2
A new convective SIGMET was issued covering the destination airport due to reported large hail.