A flash flood warning (FFW) is a severe weather alert issued by agencies like the U.S. National Weather Service to warn the public that a flash flood is imminent or already occurring in a specific area. These sudden, violent floods can be triggered by intense rainfall, dam or levee failures, or the sudden release of water from ice jams, and they may develop in minutes or hours—sometimes even without visible rain. The National Weather Service distinguishes between a flash flood watch (conditions are favorable) and a flash flood warning (flooding is happening or about to happen), urging residents to move to higher ground and never attempt to cross flowing water. An enhanced version, the flash flood emergency, is declared for life-threatening situations, such as when emergency managers confirm a state of emergency, waters rise to rarely seen major levels, or a high-hazard dam fails catastrophically. A notable example occurred on August 27, 2017, when a flash flood emergency was issued as Hurricane Harvey brought catastrophic, record-breaking flooding to the Houston, Texas, area.