Definition
In the U.S., the National Weather Service defines a severe thunderstorm as having large hail of at least 1 inch (2.5 cm), severe (storm-force) winds of 58 miles per hour (93 km/h) or greater, 1 or more tornadoes (though this would result in a tornado warning), or any combination of the three. Prior to January 2010, the size in which hail in a severe thunderstorm would be considered severe was ¾ of an inch; public complacency due to overly frequent issuances of severe thunderstorm warnings and recent studies stating that hail does not produce damage until it reaches one inch in diameter caused the upgrade. In Canada, a severe thunderstorm has wind gusts of >90 km/h (56 mph), hail with a diameter of >20 mm (0.8 inch), rainfall of >50 mm (2 inches) in an hour or >75 mm (3 inches) in three hours, or tornadoes.
A severe thunderstorm warning means there is significant danger for the warned area. Occasionally, severe thunderstorms can and do produce a tornado without warning. While not all severe thunderstorms produce tornadoes, they can produce serious straight line wind damage as severe as a tornado (and can actually cover a much wider area than a tornado). If a tornado is detected on radar or sighted, a tornado warning will be issued either in replacement of or concurrently to the existing severe thunderstorm warning. Generally, but not always, a severe thunderstorm watch or tornado watch will precede a warning.
If a Tornado Warning is issued, based on Doppler weather radar, it means strong rotation has been detected. Usually if a severe thunderstorm is producing weak rotation, it will only be a Severe Thunderstorm Warning; however, the public will usually be advised that this rotation has been detected, and the storm should be watched closely.
In the United States, local National Weather Service forecast offices, particularly those in the Great Plains or Southeastern U.S., sometimes include the wording "Severe thunderstorms can produce tornadoes with no advance warning..." or a similar reference in their severe thunderstorm warning products, usually when there is a tornado threat or especially when a tornado watch is in effect, though sometimes the wording is included even if the threat of severe weather does not include a threat of tornadic activity.
Some storms, especially in the Great Plains, may produce massive hailstones the size of baseballs (2.75 inches (7.0 cm)) or larger, falling fast enough to potentially kill a person by repeated blunt trauma.
In the United States, the National Weather Service issues warnings for tornadoes and severe thunderstorms in polygon shapes. Warnings are usually delineated in text by county or by a section of a county.
Read more about this topic: Severe Thunderstorm Warning
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