Dear Tom,
What are the criteria for a severe thunderstorm?
Thomas Ono, Chicago

Dear Thomas,
The criteria for a severe thunderstorm are: A thunderstorm that produces a tornado, a thunderstorm that produces winds of at least 58 miles per hour (50 knots), or a thunderstorm that produces hail one inch in diameter or greater. Any of those elements is sufficient for a thunderstorm to classified as severe. These are the criteria established by the National Weather Service and which we follow. A severe thunderstorm warning is issued when it is believed a thunderstorm possesses one or more of those criteria or has been reported to contain them. A thunderstorm containing winds of 40 miles per hour or greater and/or hail at least one-half in diameter is defined as approaching severe limits.