Definition
A weather advisory issued by the National Weather Service warning of hazardous convective weather significant to all aircraft. A WST 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, or thunderstorms covering 40 percent or more of an area of at least 3,000 square miles.
Plain English
A WST is an urgent weather warning telling pilots that strong, dangerous thunderstorms are happening or expected in a specific area. These storms are bad enough to be a serious threat to any aircraft, big or small.
Context Anchor
Seen in aviation weather briefings, preflight planning tools, and weather product lists when convective weather is affecting an area.
Derivation
The letters come from the message header used on weather teletype circuits: 'W' for weather advisory and 'ST' for severe thunderstorm/storm. The 'WST' header identifies the message as a Convective SIGMET when it goes out over the wire.
Why Pilots Care
These warnings identify zones where flight through the area can quickly become unsafe, often requiring route changes or ground delays.
Grounding Statement
If a WST is active along your route, picture serious thunderstorm weather that may require changing the flight plan before takeoff or while en route.
Intuition Check
WST is not a waypoint, airport, or radio station. It is a weather product code for a Convective SIGMET.
Example Sentence 1
During his preflight briefing, the pilot saw a WST covering most of his planned route and chose to delay departure until the line of thunderstorms moved east.
Example Sentence 2
After checking the latest WST, the pilot delayed departure until the line of storms had passed.