Severe thunderstorm watch
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A severe thunderstorm watch is issued when weather conditions are favorable for the development of severe thunderstorms. If the thunderstorms are forecasted to be so severe that they may produced 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.
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.
In the United States, the Storm Prediction Center (part of the National Weather Service) issues watches for areas likely to produce tornadoes and severe thunderstorms. The watch boxes 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.) In addition, a list of all counties included in its area of responsibility is now issued by each NWS forecast office for each watch.
A watch must not be confused with a severe thunderstorm warning.