- A flash flood is a rapid flooding of geomorphic low-lying areas: washes, rivers, dry lakes and basins. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, tropical storm, or meltwater from ice or snow flowing over ice sheets or snowfields.
- It may also occur after the collapse of a natural ice or debris dam, or a human structure such as a man-made dam.
- It is distinguished from a regular flood by a timescale of less than six hours.
- The temporary availability of water is often utilized by foliage with rapid germination and short growth cycle, and by specially adapted animal life.
- It is recommended that people get out of the area of a flash flood, rather than try to cross it because of its sudden nature and fast moving water. A vehicle provides little to no protection against being swept away.
- More than half of the fatalities attributed to flash floods are people swept away in vehicles when trying to cross flooded intersections. As little as 2 feet (0.61 m) of water is enough to carry away most SUV-sized vehicles.