LOS ANGELES >> Heavy overnight rains in Northern California left two people dead in a submerged car as authorities on Thursday urged residents of several Southern California mountain and canyon communities to voluntarily leave their homes because of possible mud and debris flows.

Firefighters in Millbrae, just south of San Francisco, were able to rescue a driver who had climbed atop his vehicle at a flooded underpass. But they were not able to reach people in another car, said San Mateo County Sheriff’s Detective Javier Acosta.

Acosta told the Mercury News in San Jose there was “overwhelming rain” rising rapidly in the underpass when firefighters responded to a call for help shortly before 6 a.m. It took several hours to drain the area, which is when authorities discovered the bodies. An investigation is underway.

In the Sierra Nevada, an evacuation warning was issued for about 150 homes downstream of Twain Harte Lake Dam after cracks were found in granite that adjoins the manmade part of the 36-foot-high (11-meter) structure. Authorities began releasing some water, but the dam didn’t seem in any immediate danger, Tuolomne County sheriff’s Sgt. Nicco Sandelin said.

The precautions for Southern California came as precipitation that had mostly been falling in Northern California this week spread throughout the state. Potentially heavy rain was expected late Thursday in the southern part of the state.

Firefighters used a litter basket to rescue a man stranded on a bridge pillar above the flowing Los Angeles River.

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The National Weather Service issued an advisory for minor flooding of roadways and low-lying areas in counties around San Francisco Bay and an avalanche warning was posted for eastern Sierra Nevada backcountry areas in Mono and Inyo counties.

Tire chains were required on several major routes through the Sierra, and flooding closed a stretch of coastal Highway 1 in San Luis Obispo County and a section of U.S. 101 in Santa Barbara County.

Forecasters issued a flood watch for areas east and southeast of Los Angeles starting Thursday evening because of possible heavy overnight rain fed by an atmospheric river, a long plume of moisture from the Pacific Ocean.

Evacuation warnings were issued by Orange County authorities for three canyons near a wildfire burn scar where rain last week unleashed muddy torrents. Warnings were also issued by San Bernardino County authorities for a half-dozen mountain areas.

Periods of rain and snow were predicted for California through Christmas and into next week. Snow levels in the north could drop to 1,000 feet (300 meters) or lower by Sunday, forecasters said, warning holiday travelers to be ready.

“Foothill locations that do not normally receive snow should prepare for winter conditions, especially from Sunday morning through Tuesday morning,” the Sacramento weather office said.