The weather event known as a flash flood can be especially dangerous because of its sudden formation and potential for widespread destruction. Television meteorologists routinely broadcast warnings about flash floods along with severe thunderstorm and tornado alerts. Cities located along rivers or beneath dams are especially vulnerable if the amount of water generated during a storm overwhelms protective barriers.
There are a number of factors, some natural and some man-made, that can determine whether a storm system will produce a flash flood or merely a significant amount of rain. One factor is the nature of the storm cell itself. A fast-moving squall line of thunderstorms, for example, may move through an area so quickly that the rainfall is absorbed by the ground or sewer systems and does not back up. A slow-moving independent storm system, on the other hand, may dump a great deal of rain in a small area and create the potential for flooding to develop quickly. A short, intense downburst may be preferable to a long, slow rain event.
Assuming there is a slow-moving storm system dropping a lot of rain on the area, the next factor in a flash flood is ground absorption. If the ground is frozen or covered with ice, the rainwater will not be absorbed naturally and will continue to seek lower ground. The same holds true for urban areas with significant amounts of concrete and asphalt. When the rain is blocked from getting into the ground, it will stream towards sewer systems and natural troughs. If those drainage systems or troughs become overwhelmed, it could lead to a flood.
Rivers and streams also have a natural saturation level, and if enough rain falls in a short period of time, the water will overflow into an area known as a flood plain. If a levee or dam should fail during a heavy storm, a destructive flash flood would be unleashed on anything and anyone living below it. A flood of this magnitude is often called a 100-year or 500-year flood, primarily because destruction at this level should only be expected to occur once every 100 or 500 years. This doesn't mean that a second flood of equal power won't occur the following year, however.
In short, a flash flood occurs whenever the amount of rainfall during a storm overwhelms the natural and man-made drainage and absorption systems and continues to surge through a low-lying area. This type of flooding can occur at any time, so meteorologists often advise people to remain off the streets for several hours following an intense, slow-moving storm. Run-off from storm systems further upstream may not reach an area for a few hours, which means the potential for flash flooding still exists in some places even after the rain ends.