Severe thunderstorm warning

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See Severe weather terminology for a comprehensive article on this term and related weather terms.

A severe thunderstorm warning is issued when trained spotters or doppler radar indicate a strong thunderstorm is producing dangerously large hail or high winds, capable of causing significant damage. It does not account for lightning or flooding.

In the U.S., the National Weather Service defines large hail as being at least 3/4 inch (19 mm) in diameter, and high winds as being 58 miles per hour (93 km/h) or greater.

In Canada, a severe thunderstorm has one or more of the following:

wind gusts of 90 km/h or greater
hail with a diameter of 20 mm or greater
rainfall of 50 mm or more in an hour or 75 mm or more in three hours
tornadoes

A severe thunderstorm warning means there is significant danger for the warned area. Occasionally, severe thunderstorms can and do produce a tornado without warning. Frequently, a severe thunderstorm will produce serious wind damage as severe as a tornado. A severe thunderstorm warning can also be upgraded to a tornado warning if a tornado is detected on radar or actually spotted. Generally, but not always, a severe thunderstorm watch or tornado watch will precede a warning.

Some storms, especially in the Great Plains, may produce massive hailstones the size of baseballs or even grapefruits, falling fast enough to kill a person by repeated blunt trauma. Everyone in the path of such a storm should take cover immediately, as it is very dangerous and possibly life-threatening.

In the United States, the National Weather Service issues warnings for tornadoes and severe thunderstorms in the shapes of parallelograms, with yellow meaning severe thunderstorm and red signaling tornado.

A warning must not be confused with a severe thunderstorm watch.