Rain from an atmospheric river was falling in parts of California early Monday, as a new storm began to test the state’s preparedness weeks after a deadly deluge caused power outages and destructive mudslides.

Flood watches were in effect for millions of people, mostly in California, through Wednesday as the possibility of thunderstorms, strong winds and rains continued to threaten its central and southern coasts.

“So soils are pretty sensitive,” said Brian Hurley, a senior meteorologist at the National Weather Service, alluding to the substantial rainfall the state has endured in recent weeks. “So that’s why you see a lot of the flood watches out right now.”

The state’s emergency services office said on Sunday that it had deployed emergency crews, including helicopters and swift water rescue teams, in 14 counties ahead of the expected impact of the atmospheric river.

The National Weather Service office in Los Angeles said that it expected rainfall totals of two to five inches for much of Southern California, with a higher range of four to eight inches expected in the mountains and their foothills.

Urban flooding and mudslides were possible in Santa Barbara, Ventura and Los Angeles counties during periods of heavy rainfall, the National Weather Service said, adding that there was a high chance of thunderstorms.

Officials in Santa Barbara County over the weekend issued an evacuation warning through Wednesday for certain communities.

Communities in Los Angeles County, where comparatively less rain was expected, still faced the risk of mudslides and debris on roads because that area “took the brunt of the last storm,” the Weather Service said.

Karen Bass, the mayor of Los Angeles, implored residents over the weekend to prepare.

Homeowners and workers in the city’s hilly neighborhoods spent Sunday preparing sandbags and laying plastic tarps over muddy hillsides that still bore the scars of the last storm.

Some residents, including Staci Broussard, 58, took care to reinforce their properties soon after that storm ended. Ms. Broussard’s home in Baldwin Hills Estates, a neighborhood overlooking South Los Angeles, was damaged by the previous atmospheric river to rip through the city.

The slope behind Ms. Broussard’s home crumbled, knocking down a portion of her backyard iron fence, bringing mud and vegetation down the hill from her neighbor’s home on a hill above.

Ms. Broussard and her neighbor staked down tarps over the hillside to prevent more mud from sliding down.

“As you can see, we have tarps all over because this is happening all over this neighborhood, unfortunately,” she said on Sunday .

The Weather Service in Los Angeles warned boaters of hazardous sea conditions, suggesting that they remain in port. The storm also threatened to erode the coast and damage the structures there, officials said.

Farther north, forecasters said that thunderstorms, gusty winds and lightning were possible in the Bay Area on Monday afternoon. The San Francisco Peninsula, which includes the city of San Francisco, was expected to receive up to two and a half inches of rain. The Santa Cruz Mountains and the Big Sur Coast may receive three to five and three to six inches, respectively.

Much of the Sacramento Valley was under a wind advisory through Tuesday morning. The Weather Service in Sacramento said that severe thunderstorms accompanied by “brief tornadoes” were possible in the area on Monday afternoon.

Vik Jolly contributed reporting from California.