A flash flood is a rapid rise of water along a stream or in a low-lying urban area, the National Weather Service said. Flash flooding can result from slow-moving thunderstorms, from numerous thunderstorms that develop repeatedly over the same area, or from heavy rains associated with tropical cyclones.

The area has been especially vulnerable to flooding since last summer when wildfires raced across dry forest and destroyed some 1,400 homes and structures.

Radar shows the rain is coming down at a rate of a half-inch to 1.5 inches per hour, according to the National Weather Service.