Severe Thunderstorm Watch

A severe thunderstorm watch (SAME code: SVA; sometimes referred to as "blue box" or "yellow box" by meteorologists and storm chasers) is issued when weather conditions are favorable for the development of severe thunderstorms. If the thunderstorms are forecast to be such that there is a significant risk that they may produce tornadoes, then a tornado watch (which also automatically implies a severe thunderstorm watch) is issued. A severe thunderstorm watch can also be upgraded to a tornado watch as conditions warrant (in which case the existing severe thunderstorm watch, or a portion of it, would be replaced). A watch must not be confused with a severe thunderstorm warning.

A watch does not mean that the severe weather is actually occurring, only that conditions have created a significant risk for it. If severe weather actually does occur, a severe thunderstorm warning or tornado warning will be issued, and urgent action should be taken. While a severe thunderstorm watch does not imply in its name the risk for tornadoes, the risk for tornadoes in such is not zero and they can and occasionally do strike even after only a severe thunderstorm watch is issued.

In the United States, the Storm Prediction Center (a national guidance center of the National Weather Service) issues watches for areas likely to produce tornadoes and severe thunderstorms. The watch boxes (or weather watches, WWs) are usually issued in the format of x miles north and south, or east and west, or either side of a line from y miles direction of city, state, to z miles another direction of another city, state. For example: "50 miles either side of a line from 10 miles northeast of Columbia, South Carolina to 15 miles south-southwest of Montgomery, Alabama". ("Either side" means perpendicular to the center line.) When displayed on a map, they are usually shown as either a blue or yellow outline, depending on the source. In addition, a list of all counties included in its area of responsibility is now issued by each NWS forecast office for each watch.

In the event that a severe thunderstorm watch is likely to lead to very destructive winds or hail (usually a major derecho event), enhanced wording with the words particularly dangerous situation (PDS) can be added to the watch. This is rare with severe thunderstorm watches (since the tornado threat has to remain low enough to only warrant a severe thunderstorm watch); it is far more common with tornado watches.

A similar warning is issued by Environment Canada's Meteorological Service of Canada from their offices in Vancouver, Edmonton, Toronto, Montreal and Dartmouth, Nova Scotia.