A flash flood is a sudden flood in a low-lying area, most commonly caused by periods of particularly heavy rainfall. They can also occur when a dam breaks or a river overruns its banks. Flash floods generally occur over a period of 6 hours or less.
Flash flooding can also occur as a result of an extended period of precipitation. When heavy rainfall over a long period saturates the ground, it reduces its capacity for water to run off or be absorbed. This can create a situation where much of an intense rainfall remains above ground and can cause a flash flood in a low-lying area.