Categories
Severe thunderstorms can be assessed in three different categories. These are "approaching severe", "severe", and "significantly severe".
Approaching severe is defined as hail between 1⁄2 to 1 inch (13 to 25 mm) diameter or winds between 50 and 58 M.P.H. (50 knots). In the United States, such storms will usually warrant a Significant Weather Alert.
Severe is defined as hail 1 inch (25 mm) diameter or larger, winds 58 M.P.H. or stronger, or a tornado.
Significant severe is defined as hail 2 inches (51 mm) in diameter or larger, winds 75 M.P.H. (65 knots) or stronger, a tornado of strength EF2 or stronger, the occurrence of flash flood phenomena by heavy precipitation, or extreme temperatures.
Both severe and significant severe events warrant a severe thunderstorm warning from the United States National Weather Service (excludes flash floods), the Environment Canada, the Australian Bureau of Meteorology, or the Meteorological Service of New Zealand if the event occurs in those countries. If a tornado is occurring (a tornado has been seen by spotters) or is imminent (Doppler weather radar has observed strong rotation in a storm, indicating an incipient tornado), the severe thunderstorm warning will be superseded by a tornado warning in the United States and Canada.
A severe weather outbreak is typically considered to be when 10 or more tornadoes, some will likely be long tracked and violent, and many large hail or damaging wind reports. Severity is also dependent on the size of the geographic area affected, whether it covers hundreds or thousands of square kilometers.
Read more about this topic: Severe Weather
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