That wildfire, known as the Station Fire, was the county's biggest ever, killing two firefighters, destroying 89 homes, and ravaging more than 250 square miles of Angeles National Forest.
The storm also sent mud and rocks sweeping down on roadways. Parts of a 12-mile stretch of the Angeles Crest Highway just north of Los Angeles were buried by mud and rock, prompting authorities to close the road while crews cleared the scene, said county fire Capt. Frank Reynoso. No injuries were reported.
Seventy of the stranded vehicles had been freed Saturday night, but another 20 would be forced to remain overnight, authorities said.
About 50 of the stranded motorists gathered at Newcomb's Ranch Restaurant off the highway.
"Everybody was just looking to get down off the mountain," restaurant manager Mike Noxin told the Los Angeles Times.
Several small slides were reported on the highway between La Canada Flintridge and Mount Wilson, and the road was to remain closed indefinitely, the California Highway Patrol said.
Debris flows can occur because the ground in recently burned areas has little ability to absorb rain, which instead instantly runs off, carrying ash, mud, boulders and vegetation.
Rain was expected to fall at rates up to three-quarters of an inch per hour through early Sunday.
Near the Hollywood Bowl in Los Angeles, a family decided to evacuate a home in a neighborhood where rain threatened to bring mud and debris down the hillside. A city building inspector put a yellow tag on the house, advising residents of potential structural problems.
Flooding shut down a three-mile section of the Pacific Coast Highway in Huntington Beach, authorities said. The stretch along Bolsa Chica State Beach was expected to remain closed until Sunday.
About 16,000 utility customers throughout Southern California were sporadically without electricity, mostly because of downed branches crashing onto power lines.
Topanga Canyon Boulevard in Los Angeles also was closed Saturday after mud and rocks slid down a hillside into lanes. Several cars had their tires flattened by sharp rocks along Topanga Canyon Boulevard. No injuries were reported.
Snow accumulations between 8 and 16 inches were expected by Sunday morning above 6,000 feet in the Southern California mountains. Winds were blowing at 25 to 35 mph.
The National Weather Service issued a winter storm watch through Sunday night for western Plumas County and the west slope of the Sierra Nevada in the northern part of the state and the mountains of Ventura and Los Angeles counties in the south.
"It looks like we'll be getting a lot of precipitation," said George Cline, a National Weather Service forecaster in Sacramento.
He expected the storm to bring an inch of rain to the Central Valley, with wind gusts up to 60 mph. Gusts up to 90 mph were expected on mountain ridges.
The California Department of Transportation was requiring chains for travel on all major highways over the Sierra. Chains were also required for Interstate 80 through most of Nevada.
Forecasters were warning of whiteout conditions, with snow expected to fall up to two inches an hour at the peak of the storm. They said the new snow would also bring extreme avalanche danger to the alpine backcountry.
The storm was expected to leave more than a foot of new snow at lower elevations and two feet in higher areas in the northern Sierra. Snow was falling at elevations of 6,000 feet Saturday and was expected to dip to 4,500 by Sunday morning.
Cline said most soils in the northern part of the state aren't saturated this early in the winter, minimizing the danger of mudslides even in burned areas.
"We don't anticipate anything really unusual from it yet," Cline said. "We're keeping an eye on the burn scar areas. Things have been so dry, it's still soaking into the ground."
Saturday's storm is the second in a series of warmer, wetter storms blowing in from the mid-Pacific Ocean. Another similar story is expected to move in by midweek, Cline said.
