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A severe thunderstorm warning (SAME code: SVR) is issued when trained storm spotters or a Doppler weather radar indicate that a thunderstorm is producing or will soon produce dangerously large hail or high winds, capable of causing significant damage.〔 In the United States, severe thunderstorm warnings do not account for lightning, a significant hazard in any thunderstorm, or flooding caused by a thunderstorm's extreme rainfall (a flood advisory or Flash Flood Warning is issued in these cases).〔 A similar warning is issued by Environment Canada's Meteorological Service of Canada from their offices in Vancouver, Edmonton, Toronto, Montreal and Dartmouth.〔 Skywarn issues the severe thunderstorm warnings for the United Kingdom.〔(【引用サイトリンク】 url=http://beta.skywarn.org.uk/criteria.html )〕 ==Definition== In the United States, the National Weather Service (NWS) defines a severe thunderstorm as having large hail of at least , surface wind speeds of or greater, or any combination of the those two criteria.〔(【引用サイトリンク】title=one inch hail severe thunderstorm warning criteria )〕 Prior to January 2010, the hail size for which a thunderstorm would be considered severe was ; public complacency due to overly frequent issuances of severe thunderstorm warnings and recent studies stating that hail did not produce significant damage on the ground until it reached one inch in diameter caused the upgrade in hail criteria. In Canada, a severe thunderstorm is defined as having wind gusts of greater than , hail with a diameter of greater than , rainfall of greater than in an hour or greater than in three hours, or tornadoes.〔 A severe thunderstorm warning indicates that there is impending danger for the warned area. Occasionally, severe thunderstorms can and do produce tornadoes without warning.〔 While not all severe thunderstorms produce tornadoes, they can produce serious straight line wind damage as severe as a tornado, which can actually cover a much wider area than a tornado usually does.〔 If a tornado is detected on radar or is sighted visually, 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 that strong rotation has been detected within a thunderstorm. Usually, if a thunderstorm is producing only weak rotation, it will only yield hazardous weather warranting a severe thunderstorm warning. However, the public will usually be advised that this type of rotation has been detected and that the storm in question should be watched closely in the near future for further intensification. In the United States, local NWS 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 warnings, usually when there is a tornado threat or especially when a tornado watch is in effect. Some storms, especially in the Great Plains, may produce massive hailstones the size of baseballs () or larger which may fall fast enough to potentially kill a person by repeated blunt trauma. Weather Forecast Offices (WFO) of the National Weather Service issue warnings for tornadoes and severe thunderstorms in polygon shapes, based on the projected path of a storm at the time of the warning's issuance as estimated by Doppler radar. Warnings were issued on a per-county basis before October 2007,〔 and they are now usually delineated on maps in polygon shapes and in text by a sections of counties, although entire counties are sometimes included, especially if they are small. Storm Prediction Center and other NWS products, as well as severe weather alert displays used by some U.S. television stations, highlight severe thunderstorm warnings with a yellow or orange polygon or filled county/parish outlines. Depending on the severity of the storm, some NWS offices may also request activation of the Emergency Alert System. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「severe thunderstorm warning」の詳細全文を読む スポンサード リンク
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