Heavy overnight rains in Northern California left two individuals dead inside a submerged car on Thursday as a storm rolled through the state.

California authorities on Thursday urged residents of several mountain and canyon communities situated in the southern portion of the state to voluntarily leave their homes due to the threat of mud and debris flows.

California Experiences Heavy Rain, Flooding

According to U.S. News, San Mateo County Sheriff's Detective Javier Acosta said the firefighters doing the rescue operations in Millbrae, just south of San Francisco, were able to rescue a driver who climbed on top of his car at a flooded underpass.

However, they were not able to reach the two individuals in another car. Acosta told the Mercury News that "overwhelming rain" rose rapidly in the underpass when the firefighters responded to a call for help shortly before 6 a.m.

In order for California authorities to recover the bodies, it took them several hours to drain the area. An investigation is ongoing regarding the deaths.

Moreover, an evacuation warning was issued in the Sierra Nevada for about 150 homes downstream of Twain Harte Lake Dam after cracks were found in the granite that adjoins the man-made portion of the 36-foot-high structure.

According to Tuolomne County sheriff's Sgt. Nicco Sandelin, authorities in the area have started to release some water, but the dam did not seem in any immediate danger.

READ NEXT: Magnitude 6.2 Earthquake Hits Northern California Coast: USGS

Storm Threatens Southern Portion of California

The precautions for California's southern portion came as precipitation that had been mostly falling in Northern California this week spread across the state.

The National Weather Service (NWS) has issued an advisory warning of potential heavy rains in Southern California late Thursday. There was also an advisory for minor flooding of low-lying areas and roadways around San Francisco Bay counties.

Forecasters said the storm, which rolled in from the north Thursday morning, would grow in intensity into the night and hover over the region the following day.  An avalanche warning was already released for eastern Sierra Nevada backcountry areas in Mono and Inyo counties.

State authorities have also required tire chains on vehicles traveling to some major routes through the Sierra. Flooding has already closed a section of U.S. 101 in Santa Barbara County and a stretch of coastal Highway 1 in San Luis Obispo County.

Meanwhile, a flood watch was also issued for the southeast and east areas of Los Angeles by Thursday evening due to possible heavy overnight rain.

Orange County officials have already issued evacuation warnings for three canyons near a wildfire burn scar where rain unleashed muddy torrents last week. San Bernardino County authorities also issued warnings for a half-dozen mountain areas.

Periods of rain and snow were predicted for California on Christmas until early next week. Forecasters said another storm was expected to roll in again on Saturday. They warned holiday travelers to be ready as snow levels in the north could drop to 1,000 feet or lower by Sunday.

The Sacramento weather office said that foothill locations that do not normally receive snow should prepare for winter, especially from Sunday morning until Tuesday morning.

READ MORE: California Mother Who Allegedly Told Daughter to Punch Opponent in Basketball Game Charged With 2 Misdemeanors

This article is owned by Latin Post.

Written by: Jess Smith

WATCH: Winter Storm: Northern California Has Rain, Snow in Christmas Week Forecast - From ABC 10