A sudden, intense flooding event caused by heavy rain in a short period is called a flash flood.
Flash floods develop rapidly when rainfall overwhelms the ground, drainage systems, streams, or low-lying channels. They can occur in cities, deserts, mountains, and areas far downstream from where the rain fell. Besides thunderstorms, causes include tropical cyclones, rapid snowmelt, and the collapse of natural or human-made dams.
The defining feature is speed. Flash floods are commonly distinguished from slower flooding by an onset of less than six hours between the triggering rainfall or event and the flooding. Water can surge through normally dry washes and narrow valleys, carrying mud, rocks, vehicles, and debris with little warning.
“Urban flood,” “coastal flood,” and “river flood” describe settings or types of flooding, not necessarily the rapid timing that makes an event a flash flood. A flash flood can happen in an urban area or a river basin. Because moving water is deceptively powerful, the U.S. National Weather Service uses the warning: “Turn Around, Don’t Drown.”