When a Severe Thunderstorm Warning is issued, what does that mean? There are only three criteria that makes a storm severe. When a Severe Thunderstorm Warning is issued, one or more of the three criteria are possible for the warned area. The three criteria are... 1. Hail to 1" in diameter 2. Winds gusting to 58 mph or higher 3. A tornado might also sometimes spin up. As you can see, there is no threshold for how much rain or lightning are occurring in the storm. So a storm may be the loudest storm you have ever heard because of the thunder and lightning, or may be the heaviest rain you have ever seen - but unless it is capable of producing large hail, high winds or a tornado, it is not considered severe. Storms that can still be noisy and cause disruptions but aren't always considered severe can sometimes have a significant weather advisory issued for those. The criteria for that is winds up to 40 mph and intense lightning. Of course, if a tornado is possible, a Tornado Warning is usually issued but sometimes they spin up so quick the National Weather Service does not have time to issue a tornado warning. So the next time you find yourself under a Severe Thunderstorm Warning, know that large hail and high winds are possible and sometimes if the complex of storms or if it is a single suprcell a tornado cannot be ruled out if the severe storm is strong enough and rotation tightens up enough.
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