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. A warning must not be confused with a severe thunderstorm watch.
In the U.S., the National Weather Service defines large hail as being at least ¾ 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
- hail with a diameter of >20 mm
- rainfall of >50 mm in an hour or >75 mm 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. However, 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 polygon shapes. Warnings are usually deliniated by county or by a section of a county.
A similar warning is issued by Environment Canada's Meteorological Service of Canada from their offices in Vancouver, Edmonton, Toronto, Montreal and Dartmouth.
