The storm was already lashing the Bay Area, where over 50,000 San Franciscans lost power Thursday morning. As it moves south Thursday night, it's expected to to bring thunderstorms and "intense, heavy-hitting rain," said NBC4 meteorologist Crystal Egger.
"It is going to be a soaking storm; it's going to pack a big punch," she said.
Mandatory evacuations were ordered for some residents of San Jacinto, a week after a storm triggered mud flows in a nearby road.
Winds up to to 70 miles per hour are possible in the mountains and flash flooding may occur near burn areas, according to the National Weather Service.
In Glendora, near the Colby Burn area, an orange alert was in effect.
Glendora officials urged residents to evacuate, and imposed parking restrictions on roads near the burn zone. Residents were required to remove vehicles, trash bins and other obstructions from the roadways.
Residents living close to burn areas should pay attention to news reports in case flooding or other announcements are made, be prepared for an evacuation with important items already packed and keep enough water and food for three days, Los Angeles County Fire officials said.
Flash flood watches were issued in Orange County's mountains and foothills, especially the Silverado Canyon.
High surf advisories are in effect for the coasts. In Long Beach, city crews were fortifying sand berms and testing pump stations, according to a press release.
Empty sandbags and sand bins are available to residents starting at noon Thursday, the release said.
Heavy rain is expected Thursday night, with cloud cover forecast to build through the day.
"A lot of us are going to be sleeping when the big stuff hits," Egger said.
The City News Service contributed to this report.