A flash flood warning is given by the National Weather Service when flash flood conditions are being observed or are likely to occur. A flash flood is the rapid pooling of water, usually in areas such as basins, dry rivers and creek beds, or other areas where water will flow to a low point. The distinction between a flash flood and a traditional flood is that the duration of flooding is expected to be less than six hours.
Flash floods may be caused by a number of different phenomena, but all of them have in common the appearance of more water than the geologic features of a given area are able to divert or absorb. In many instances, flash flooding is caused by major storms such as hurricanes or severe thunderstorms which have the ability to drop more rain than an area can absorb.
Urban areas with water retention ponds and well-designed storm sewers are less likely to see flash flooding, because these architectural features are designed to provide a storage capacity for heavy rainfall. Yet even in a well designed area, sustained heavy rain can overwhelm the storm water collection systems and this might result in floods. One factor affecting the likelihood of flash floods is whether the soil is already saturated with water, and whether the soil type in the area has a high capacity to absorb. In addition, many systems are weakened by the accumulation of leaves, sand, and other debris in the water runoff system.
Isolated areas are also susceptible to flash flooding, through often for different reasons. Desert areas might receive an occasional heavy thunderstorm, and this unusual presence of large amounts of water coupled with the absence of vegetation and other absorptive materials results in fast moving flash floods. The situation is often made worse in desert areas by the presence of clay type soils that are less absorptive. Heavy rain in high desert areas of arid land and mountains can be a deadly combination.
Flash flooding need not be caused by rainfall. A break in a natural or man-made water restraining feature, such as a dam, can cause temporary flooding sufficient to raise a flash flood warning. This situation is distinct from the problem of levees or dykes being topped by rising waters, because in most such situations the flooding would be expected to last longer than six hours.
Flash flooding has been known to be caused by snow melt and even by the rapid melting of a glacier when a nearby volcano erupted.
Flash flooding is believed to cause more deaths than any other recurring natural disaster in any year, including tornados, hurricanes, and lightning strikes. Many of those deaths occur when drivers attempt to enter a flooded area of a road. The National Weather Service advises that drivers should never enter flooded areas in cars. It can take as little as two feet of water to sweep a car away, and it is almost impossible to tell the depth of flooded areas of a road.
If driving in areas where flash floods are possible, a battery operated weather radio is a must, as well as normal car emergency kit supplies.