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.
There are at least three scenarios in which a Tornado Warning may be issued…
- Radar indicates that a severe thunderstorm is capable of producing a tornado.
- Radar indicates a hook echo and/or a debris ball. (strong tornado signature)
- 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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