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particularly dangerous situation
A Particularly Dangerous Situation (PDS) is a type of enhanced wording first used by the Storm Prediction Center (a national guidance center of the National Weather Service, the meteorological agency of the United States) on certain severe weather watches. It is issued at the discretion of the forecaster composing the watch and implies that there is an enhanced risk of very severe and life-threatening weather, usually a major tornado outbreak or (much less often) a long-lived, extreme derecho event, but possibly another weather hazard.
PDS watches are quite uncommon; less than 3% of watches issued by the SPC from 1996 to 2005 were PDS watches, or an average of 24 each year. When a PDS watch is issued, there are often more PDS watches issued for the same weather system, even on the same day during major outbreaks, so the number of days per year that a PDS watch is issued is significantly lower.
==Background==
The first PDS tornado watch was issued by Robert H. Johns for the April 2, 1982 tornado outbreak across the southern and central Great Plains. While historically applied only to severe thunderstorm, tornado and flash flood watches (i.e., Severe Local Storm "polygonal" events), PDS wording could theoretically be applied to other types of weather watches (such as winter storm, high wind, hurricane, or fire weather watches) when an enhanced threat for such conditions exists. These watches have generally (but not always) been issued during a high risk or an upper-end moderate risk either of severe storms from the SPC's convective outlooks or of flash flooding from the Weather Prediction Center's Excessive Rainfall Outlooks.
On April 24, 2011, the National Weather Service Weather Forecast Office in Memphis, Tennessee issued the first PDS flash flood watch to highlight the threat for widespread, significant and potentially life-threatening flash flooding due to repeated rounds of severe thunderstorms.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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