What factors are required for a thunderstorm to be announced as severe?

Dear Tom,
What factors are required for a thunderstorm to be announced as severe?
Margaret Robertson, Chicago
Dear Margaret,
The criteria for a severe thunderstorm are: A thunderstorm that produces a tornado, a thunderstorm that produces winds of 58 miles (50 knots) per hour or greater, or a thunderstorm that produces hail one inch in diameter or greater. Any of those three elements is sufficient for a thunderstorm to be classified as severe. These are the criteria for severe thunderstorms established by the National Weather Service and which we follow. A thunderstorm containing winds of 40 miles per hour (35 knots) or greater and/or hail at least one-half inch in diameter is defined as approaching severe limits. A severe thunderstorm warning will be issued when it is believed a thunderstorm possesses one or more of those criteria or has been reported to contain them.

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