California Braces for Atmospheric River That Could Bring More Floods

The West Coast braced for more flooding on Sunday as heavy rains from an atmospheric river were forecast to spread over California starting on Sunday, in the latest series of storms to pound the state this month.

A milder storm moved over California’s northern and central coast on Saturday night, kicking off the period of rain for the nation’s most populous state. Forecasters said it was a precursor to a more powerful system on Sunday that was expected to bring the bulk of the precipitation.

“Sunday night and Monday alone, we’re looking at areas three to six inches of rain down the coast south of the Bay Area to Santa Barbara,” said Brian Hurley, a senior meteorologist at the National Weather Service.

More than 37 million people nationwide were under a flood watch on Sunday. Most were in California, where the watch was in effect through Wednesday, according to the Weather Service.

Atmospheric river is the name given to the narrow bands of moisture blown over the West Coast by winds in the Pacific. They are the cause of California’s heaviest rains and floods.

“The weather conditions will be drastically different from Sunday morning to Sunday night,” the Weather Service said on Sunday. “A strong storm will arrive today. Rain will begin around midday and will be heavy through the night. Moderate to major impacts from this storm will last into Monday with heavy rain, strong winds, high surf, thunderstorms and flooding potential.”

While the system was largely expected to be a rainmaker even in mountain communities, Mr. Hurley said, some areas above 6,800 feet, such as Mammoth Mountain in the Sierra Nevada, could get more than four feet of snow.

California’s Office of Emergency Services said on Saturday that it had deployed emergency workers, including two swift water rescue teams, in six counties ahead of the storm.

Bay Area cities, including San Francisco and San Jose, were expected to receive between one and two inches of rain, according to the Weather Service’s San Francisco Bay Area office.

The system could bring hail and thunderstorms and wind gusts of 30 to 45 miles per hour to the parts of the Bay Area and the Central Coast on Sunday, the Weather Service said.

Farther south, the risk of flooding was high in the coastal communities of Santa Barbara and Ventura Counties, northwest of Los Angeles.

The National Weather Service in Los Angeles said the region could expect two to five inches of rain, with up to eight inches in the mountains accompanied by damaging wind gusts from 40 to 60 m.p.h. at higher elevations and 20 to 40 m.p.h. elsewhere.

Officials in Santa Barbara County on Saturday issued an evacuation warning for some areas. The warning, which was in effect through Wednesday, implored residents to be ready to leave at a moment’s notice.

The City of Santa Barbara said it would offer free emergency parking in a downtown lot for residents in flood-prone areas.

Los Angeles was forecast to get an inch and three quarters to two inches of rain Monday night into Tuesday, Mr. Hurley said.

Though the rainfall rates in Los Angeles were not expected to reach the levels of the destructive storm that hit the city two weeks ago, Mr. Hurley said the loosened soil caused by that storm meant the area was not in the “best position” to absorb heavy rainfall.