Definition
A Convective SIGMET (WST) is an inflight weather advisory issued by the Aviation Weather Center for hazardous convective weather affecting 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, or tornadoes; embedded thunderstorms; lines of thunderstorms; or thunderstorms covering 40 percent or more of an area of at least 3,000 square miles. Convective SIGMETs are issued for the contiguous 48 states in three regional areas (Eastern, Central, and Western), valid for up to two hours, and reissued hourly at 55 minutes past the hour.
Plain English
An urgent warning to pilots that severe or widespread thunderstorm activity is happening or expected in a specific area, and that all aircraft -- regardless of size -- should take it seriously.
Context Anchor
Seen in inflight weather briefings, aviation weather displays, and preflight planning when thunderstorm activity may affect the route.
Derivation
SIGMET comes from 'SIGnificant METeorological information.' The leading 'W' in WST is the international identifier code for the convective (thunderstorm) version of a SIGMET, distinguishing it from the non-convective SIGMET, which uses other identifiers. Knowing the 'W' marks it as convective helps pilots quickly recognize that a WST is specifically about thunderstorm hazards rather than turbulence, icing, or volcanic ash.
Why Pilots Care
Identifies weather that can produce loss of aircraft control, structural damage, or sudden severe conditions requiring immediate route changes.
Grounding Statement
A WST means thunderstorm weather is not just possible in theory; it has been identified as a serious aviation hazard in a specific area and time period.
Intuition Check
Do not read “significant” as meaning merely “worth noting.” In this term, it means the weather is important enough to affect flight safety and route decisions.
Example Sentence 1
During the preflight briefing, the pilot noted a Convective SIGMET (WST) for a line of thunderstorms across central Texas and chose to delay the departure.
Example Sentence 2
An updated WST warned of hail and severe turbulence, prompting the crew to request a deviation around the affected area.