Flash floods are exactly what the name suggests: floods that happen in a flash.
Flash floods generally develop within 6 hours of the immediate cause, which could include heavy rain, ice or debris jams, and levee or dam failure.
Flash floods exhibit a rapid rise of water over low-lying areas. In some cases, flooding may even occur well away from where heavy rain initially fell.
There are many reasons that flash floods occur, but one of the most common is the result of copious amounts of rainfall from thunderstorms that cause flash flooding.
This can also occur when slow-moving or multiple thunderstorms move over the same area.
These sudden downpours can rapidly change the water levels in a stream or creek and turn small waterways into violent, raging rivers. Urban areas are especially prone to flash floods due to the large amounts of concrete and asphalt surfaces that do not allow water to penetrate into the soil easily.
Steep, hilly, or mountainous terrain produces rapid runoff and quick stream response, since the water will travel downhill at greater speeds into rivers and over land.
Rocky terrain can exacerbate the development of flash floods and raging waters since rocks and clay soils do not allow as much water to infiltrate the ground.
Steep, narrow valleys generate rapidly flowing waters that can quickly rise to considerable depth. For instance, a mountain creek that is usually only 6 inches deep can swell to a 10-foot depth in less than one hour.
(SOURCE: NOAA)