What is a Severe Thunderstorm?



Most know what a thunderstorm is, but what exactly is a severe thunderstorm?

Thunderstorms are often accompanied by heavy rain, lightning, strong winds and even hail or tornadoes on occasion. Once a thunderstorm becomes strong enough, it can be classified as a severe thunderstorm. There is specific criteria that determines if a thunderstorm is severe.

To be a severe thunderstorm, a storm must have at least one of the following…

  • Winds of at least 58 MPH. (usually causes damage)
  • Large hail of at least 1″ in diameter. (can cause damage)
  • A tornado. (it’s commonly accepted that any thunderstorm with a tornado is severe)

These are not necessarily severe thunderstorms…

  • A storm with flooding rainfall. Flooding can be severe, but flooding alone does not make a thunderstorm severe by the National Weather Service (NWS) definition.
  • A storm with frequent/vivid lightning. A thunderstorm can have as much lightning as one can imagine, but if it does not have damaging winds and/or hail, it’s not officially considered a severe thunderstorm.

Severe Thunderstorm Watch:
This is issued with the Storm Prediction Center (SPC) believes there is a potential for severe thunderstorms across a given area. Severe thunderstorms can sometimes produce tornadoes, but the SPC will not issue a Tornado Watch unless they believe there is an enhanced potential for tornadoes. (sometimes they will use wording in a Severe Thunderstorm Watch to say that isolated tornadoes are possible.)

  • This type of Watch generally covers a large area and often includes multiple states.
  • A Watch is usually in effect for several hours.

Severe Thunderstorm Warning:
This is issued with a local NWS office believes that a thunderstorm is capable of producing damaging winds and/or large hail.

  • A Severe Thunderstorm Warning is usually a relatively small area, covering one to perhaps a few counties. If a squall line of thunderstorms develops, it is not uncommon for multiple warnings to be issued side by side.
  • A Warning is usually in effect for less than an hour, unless a storm moves at a slow speed.
  • The NWS will use a combination of storm reports (i.e. someone reported downed trees and power lines) and radar imagery to determine if a Warning should be issued.

Tornado Warning:
There are at least three scenarios in which a Tornado Warning may be issued…

  1. Radar indicates that a severe thunderstorm is capable of producing a tornado.
  2. Radar indicates a hook echo and/or a debris ball. (strong tornado signature)
  3. A trained weather spotter reports a funnel cloud or tornado to the NWS.

So remember, if a thunderstorm has damaging winds and/or large hail, it is severe. Otherwise, it’s just a “regular” thunderstorm.

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Quincy is a meteorologist and storm chaser who travels around the country documenting and researching severe weather. He has on-air experience with stations such as WTNH-TV in New Haven, CT and WREX-TV in Rockford, IL. He was most recently a digital meteorologist for weather.com. After achieving his B.S. degree in Meteorology at Western Connecticut State University (WCSU) in 2009, he returned as a University Assistant to help produce weather broadcasts. He also gave guest lectures and worked on website design. He has over nine years of professional weather forecasting experience and his forecasts have been featured in newspapers and on radio stations in multiple states.

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