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A flash flood warning (SAME code: FFW) is issued when a flash flood is imminent or occurring in the warned area. A flash flood is a sudden, violent flood after a heavy rain, or occasionally after a dam break. Rainfall intensity and duration, topography, soil conditions, and ground cover contribute to flash flooding. Most flash floods occur when there is a heavy amount of precipitation falling in an area and that water is then channeled through streams or narrow gullies. Flash floods may take minutes or hours to develop. It is possible to experience a flash flood without witnessing any rain. In this case, there would be heavy rain in areas upstream of the warned location. ==Flash flood alerts== There are two types of alerts for flash floods which are issued by the National Weather Service. One is a flash flood watch, which means that conditions are favorable for flash flooding, and the other is a flash flood warning, meaning that a flash flood is occurring or one will occur imminently and is usually issued when there are strong weather radar echoes for an area that is prone to flash flooding. Flash floods can also occur because of a dam or levee failure, or because of a sudden release of water held by an ice jam. Residents are usually urged to do the following when flash flooding is imminent:〔(【引用サイトリンク】url=http://www.noaawatch.gov/floods.php )〕 * Be aware of any signs of heavy rain * Move to higher ground if rapidly rising water is seen or heard * Not attempt to cross the flowing water In addition, some NWS Weather Forecast Offices have instituted an enhanced flash flood warning, referred to as a flash flood emergency (or as termed by the Albany, New York office as a flash flood warning emergency), which indicates a severe flooding situation in densely populated areas, similar to the procedure for declaring a tornado emergency. 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「Flash flood warning」の詳細全文を読む スポンサード リンク
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