Thunderstorms often produce frequent thunder and lightning as well as heavy rain. But hundred of vivid lightning flashes or inches of rain do not make a thunderstorm severe.
The National Weather Service uses wind speed and hail size to define severe thunderstorms. Meteorologists from the National Weather Service will declare that a thunderstorm is severe if any one or more of the below criteria are met:
- wind gusts reach 58 mph or faster
- hail is three quarters of an inch in diameter or larger
- the thunderstorm produces a tornado or tornadoes.
Severe thunderstorm warnings will only be issued if strong winds or large hail are occurring with a thunderstorm or are imminent. Tornado warnings will be issued if a tornado is spotted on the ground or Doppler radar indicates rotation and that a tornado could form at anytime.
For thunderstorms producing localized heavy downpours that may cause flooding, flash flood warnings or advisories will be issued by the National Weather Service to warn of the heavy rain and expected flooding.
Even though lightning is not a criteria used in defining a severe thunderstorm and therefore no warning will be issued, lightning is a very dangerous part of any thunderstorm and causes many injuries and deaths each year.