Each year, more deaths occur due to flooding than from any other thunderstorm-related hazard, resulting in more than 140 fatalities each year.
Most flash floods are caused by slow-moving thunderstorms, thunderstorms that redevelop over the same area, or heavy rains from tropical storms and hurricanes. These floods can develop within minutes or hours depending on the intensity and duration of the rain, the topography, soil conditions, and ground cover.
A flash flood is a rapid rise of water along a stream or low-lying urban area. Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Rapidly rising water can reach heights of 30 feet or more. Flash flood-producing rains also can trigger catastrophic mudslides.
The Sioux Falls office of the National Weather Service covers our area of northwest Iowa. Todd Heitkamp, their Warning Coordination Meteorologist says flash flooding does indeed occur in our area.
Flash Flood Watch:
Issued by the National Weather Service to indicate current or developing hydrological conditions that are favorable for flash flooding in and close to the watch area. The occurrence of flooding is neither certain nor imminent. Those in the watch area should be alert for flooding.
Flash Flood Warning:
National Weather Service meteorologists have determined that flash flooding is occurring or imminent. Those in the warning area should take the necessary precautions at once.
Flash Flood Emergency:
A Flash Flood Emergency is issued by the National Weather Service. It is not a new warning, but is used to highlight a severe threat to human life and catastrophic damage from a flash flood which is imminent or ongoing.