Gov. Arnold Schwarzenegger ordered state agencies to prepare to aid local organizations in case of disaster.
"The state stands ready to help local governments protect lives and property," he said.
A low-pressure area about 700 miles off the coast was heading northeast and could bring an inch of rain through Thanksgiving and up to 4 inches in the mountains, said Stan Wasowski, a forecaster with the National Weather Service in San Diego.
While some scattered showers were reported in the San Diego area, the brunt of the storm was not expected to hit until Tuesday night, he said.
Flash flood watches were posted through Wednesday evening for areas where grass and brush that normally anchors the soil, helping to prevent mudslides, burned away. Some places could get a half-inch of rain in an hour, the Weather Service said.
A series of wildfires stoked by Santa Ana winds damaged or destroyed about 1,000 homes this month in Los Angeles, Orange, Riverside, Santa Barbara and San Bernardino counties. Those burn areas equaled a total of about 65 square miles.
In addition, October wildfires burned dozens more homes and scorched a total of more than 35 square miles. Other areas remain scarred from fires in recent years.
Northwest of Los Angeles in Santa Barbara County, residents living below burn zones were warned to be prepared to leave. In Orange County, voluntary evacuations were in effect for three burned areas of Yorba Linda, a city of about 65,000 southeast of Los Angeles.
"Residents are encouraged to remove themselves from these areas until the current rain event has passed," a statement said.
More than 135 members of the California Conservation Corps were sent to canyons in Yorba Linda to place sandbags and clean out culverts and spillways to handle runoff.
Some homeowners paid a company to spray a gluey substance to protect a bare area containing rye seeds that are expected to sprout in a few weeks.
"That's kind of the whole purpose of it, is to keep the soil in place until something can be germinated and get some vegetation going again," Jeff Weaver, president of Corona-based All-Preferred Hydroseed Inc., told KTLA-TV.
A call also went out for volunteers to fill sandbags in Sierra Madre, a Los Angeles suburb at the foot of the steep San Gabriel Mountains, where a wildfire threatened homes in April.