Tornado watch

A tornado watch (SAME code: TOA) is issued when weather conditions are favorable for the development of severe thunderstorms called a Supercell that are capable of producing tornadoes. A tornado watch therefore implies that it is also a severe thunderstorm watch. A tornado watch must not be confused with a tornado warning. In most cases, the potential exists for large hail and/or damaging winds in addition to tornadoes.

A watch does not mean that the severe weather is actually occurring, only that atmospheric conditions have created a significant risk for it. If severe weather actually does occur, a tornado warning or severe thunderstorm warning would then be issued. Note that a watch is not required for a warning to be issued; tornado warnings are occasionally issued when a tornado watch is not active (i.e. when a severe thunderstorm watch is active, or when no watches are in effect), if a severe thunderstorm develops and has a confirmed tornado or strong rotation.

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). In addition, a list of all counties included in its area of responsibility is now issued by each NWS forecast office for each watch.

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