flash flood

A sudden, violent flood caused by intense rainfall in a watershed, arriving with little or no warning — the water rising from ankle-deep to chest-deep in minutes. Flash floods are the deadliest weather-related hazard in the desert Southwest, where impermeable rock, sparse vegetation, and narrow canyons concentrate runoff into walls of water, mud, and debris. A flash flood can be triggered by rain falling miles away, in a storm you cannot see, in a canyon where the sky is clear overhead.