First, here is the definition of a severe thunderstorm. A severe thunderstorm is a thunderstorm with one inch diameter hail (the size of a quarter) and/or a 58 mph wind gust and/or a tornado. So how were these definitions developed? A tornado is obvious. Meteorologists consider hail the size of quarters to be “severe” because it is the minimum size required to produce damage relevant to most property in the United States. In particular, roof damage. The wind damage threshold, on the other hand, is a legacy that is less about scientific research and more about preventing airplane crashes. Nevertheless, winds that gust to 58 mph can certainly cause tree and power line damage, as well as blow around loose objects outdoors and prove dangerous to some high-profile vehicles.
Below are the different categories of severe thunderstorm probabilities and their definitions from the NWS Storm Prediction Center.
For the latest information on severe thunderstorm potential please see The Storm Prediction Center at spc.noaa.gov.
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