The high-wind second storm, starting Friday, could drop as much as a half a foot of rain in some areas within 24 hours, the National Weather Service said.
Despite the drenching, the storm is unlikely to put much of a dent in region's severe drought, the weather service said.
Strong winds from the first storm knocked out electricity for more than 6,000 customers from the southern San Francisco Bay Area north into Sacramento, a United Press International check with Pacific Gas & Electric Co. early Thursday indicated.
Airlines flying into and out of San Francisco International Airport canceled nearly 140 flights Wednesday and delayed nearly 450, FlightAware.com said.
Few Thursday flights were canceled or delayed as of early in the morning.
In the Lake Tahoe area, the first storm was dropping an expected 4 to 8 inches at lake level and a foot in the higher passes, with wind gusts of 60 to 70 mph along ridge lines, weather service forecaster Brian Brong told the San Francisco Chronicle.
Near Los Angeles, officials in Azusa, at the entrance to the San Gabriel Canyon, said residents of some areas would have to evacuate by noon Thursday ahead of expected major landslides.
The storm was expected to drop about a half-inch of rain in downtown Los Angeles and 1 to 2 inches in foothill and mountain areas.
But the second storm, which could bring thunder and lightning, was expected to drop up to 3 inches along the Southern California coast and valley areas and 3 to 6 inches in the mountains, the weather service said.
This could create life-threatening conditions from mudslides in fire-scarred hillside communities in the San Gabriel and Antelope valleys, said officials cited by the Los Angeles Times.
The second storm could cause winds gusts of more than 60 mph in mountain areas and waves from 10 to 15 feet at Southern California beaches.
The system is expected to move out of the region Saturday night.
Even if Los Angeles and San Francisco get 2 inches of rain from the two storms, Los Angeles would still be only 30 percent to 40 percent of normal for this time of year and San Francisco would be only about 50 percent of normal.
The drought has wrought havoc on the state's $45 billion agriculture sector, which supplies much of the country's fruit, nuts, vegetables, wine and dairy products.