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Source: http://www.photolib.noaa.gov/htmls/nssl0226.htm, Photo by:  Sean Waugh NOAA/NSSL


What Makes a Thunderstorm “Severe”?

When you watch the local weather person on the news and she says, “there is a severe thunderstorm moving through the area,” what does that mean? What makes a thunderstorm severe? In order to be classified as a severe thunderstorm, one of three criteria must be met.  First, any storm that produces a tornado is always considered severe.  Next, a storm that has winds of 50 knots (57.5 mph) or more constitutes as severe.  The third and final qualifier is hail at least one inch in diameter, roughly the size of a U.S. quarter.

Tornado Producing Storm

This one seems like a no-brainer.  Any storm that ultimately produces a tornado is considered severe by the National Weather Service.  Obviously tornadoes can do lots of damage to property in addition to being devastating and life-threatening to people.  According to the National Severe Storms Laboratory, there are approximately 1200 tornados that hit the U.S. annually.  They are most common in the Southern Plains between May and June.

Winds of 58MPH or More

Winds of this speed are known as damaging winds.  In fact, damaging winds cause more property destruction each year than tornadoes.  Why though, is the limit set at 58 miles per hour?  That figure actually come from the old aviation severe thunderstorm classification.  Today, we follow this guidance as power lines and tree damage starts in this range.  Even higher winds can cause devastating property damage.  For those wondering, most home weather stations can detect wind speeds up to 100-150mph.

Hail – 1” Diameter or More

The reason one inch diameter hail become the qualifier is because this is approximately the size hailstone that starts to do property damage.  One inch hail will cause damage to shingles on older roofs.  By the time hail reaches 1.25”, major destruction in the area will likely be reported.

Yes, believe it or not, it doesn’t matter how much rainfall or lightning there is.  Unless there is a tornado, high winds, or large hail, it’s not officially considered “severe”.  Obviously heavy rain storms can still product flash flooding that you need to look out for.

Hopefully the question of “what makes a thunderstorm severe?” has been answered.  More importantly, hopefully you’ll heed the warning in the future.  One major consideration in designating severe thunderstorms was making sure they didn’t “cry wolf” too often that it became ignored.  “It’s better to be safe than sorry.”


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