About 600 homes in La Canada Flintridge, and other communities will be under mandatory evacuation beginning at 9 a.m. People could be out of their homes for up to five days.
The communities northeast of Los Angeles are near mountain areas burned bare by a wildfire last year.
The third in a series of storms - and possibly the fiercest yet - could dump up to 1½ inches of rain per hour beginning in the late morning.
No flooding is reported there yet - but further west , a car-sized boulder dropped from a hillside onto the 101 freeway near Hollywood, blocking an offramp.
