We often refer to any type of severe weather as “severe,” but the word has a very specific meaning when it comes to thunderstorms.
According to the National Oceanic and Atmospheric Administration, about 100,000 thunderstorms occur each year in the United States, but only 10% of them intensify into severe thunderstorms.
The storm threat must be severe enough to check at least one of three criteria boxes before the National Weather Service issues a severe thunderstorm warning or tornado warning. Some storms check two or all boxes at once.
This first criterion is the simplest. Creating a tornado of any strength, size, or duration automatically intensifies the storm.
We need to dig a little deeper into the ferocity of the storm’s hail and winds.
If a thunderstorm produces hail at least the size of a quarter, it’s officially severe.
This 1-inch diameter is the size at which hailstones can begin to cause damage to cars, such as dents, sidewalls, and cracked windshields, especially when the force of impact is increased by high winds.
When they reach the size of a golf ball, especially a baseball or softball, the damage is even more severe.
Forecasters describe hail larger than 2.75 inches as “devastating.” A National Weather Service warning of this severity automatically triggers a wireless emergency alert on your smartphone.
Until 2010, the threshold for severe hail was lower, at around the size of a penny, when the Bureau of Meteorology determined that it was usually not damaging.
Although all storms produce sudden high winds, 58 mph is the key criterion for a severe thunderstorm warning.
Very strong winds may cause minor damage to trees, power lines, and structures. The impact is even greater when wind gusts reach 80 mph. These winds can destroy unstructured buildings, cause significant damage to mobile homes, and strip roof materials from even well-built homes.
Winds above this speed are considered “destructive” and will also result in a radio emergency warning. Only about 10% of severe thunderstorms in the United States reach this level each year.
Some of the most powerful severe storms can have wind gusts in excess of 160 miles per hour, causing damage similar to a strong EF1 or EF2 tornado, causing significant damage or destruction to the roofs of mobile homes and solid structures.
This is rare and usually occurs in long-lasting, widespread storms called derechos.
One of the most powerful derechos in recent memory produced wind gusts of 190 to 140 mph in Iowa on August 10, 2020.
The derecho’s 14-hour journey spanned a wide area from southeastern South Dakota to Ohio. The path of destruction caused $13.8 billion in damage, according to Climate Central.
