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 (19mm) in diameter, and high winds as being 55 miles per hour (88km/h) or greater.

A severe thunderstorm warning means there is significant danger for the warned area. In rare instances, severe thunderstorms can and do produce a tornado without warning. Frequently, a severe thunderstorm will produce serious wind damage as severe as a tornado. Some storms, especially in the Great Plains, may produce massive hailstones the size of baseballs or even grapefruit, 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 tornados and severe thunderstorms on a per-county basis.

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