Today's storm is the latest of a series to dump rain on the region, which has been hard hit by six years of drought.
Officials warned of potential mud flows in areas affected by wildfires last year and heavy rains in the afternoon, when there will be a slight chance for thunderstorms. Forecasters say southern and eastern Los Angeles County will be hardest hit.
A major mudslide at Donner Summit on Sunday night closed Interstate 80 in the Sierra Nevada, cutting off the main transportation route between Reno and San Francisco.
California Highway Patrol Officer Peter Mann said the flow of mud and rock extended across the westbound lanes of the road for about the length of a football field and was 7 feet deep in places.
Downed power lines forced the closure of the eastbound lanes and complicated cleanup efforts, he said.
The road was expected to remain closed for hours, possibly until Monday morning. By then, it could be snowing thick enough to require chains for passage.
"The mess just keeps getting better," Mann said.