The National Weather Service issued a flood advisory for Ventura County mountains on Sunday.
Brief heavy downpours from an "unseasonably moist and unstable air mass" could also cause flash flooding in other mountain communities and the Antelope and Cuyama valleys, according to NWS. That excludes the Santa Monica Mountains, forecasters said.
More than 1 inch of rain per hour could arrive in showers and thunderstorms, which are mainly expected in the mountains and interior areas from Sunday afternoon through Monday. Coastal communities in Santa Barbara and Ventura counties could also receive some precipitation.
Parts of Southern California could see wind gusts of up to 50 mph, hail up to 1/2 inch in diameter, and cloud-to-ground lightning, according to NWS.
"If you come across swift moving water crossing the roadway, make the safe choice and find an alternate route," forecasters said.