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.”