The storm is the second so-called Pineapple Express weather system, or atmospheric river storm, to hit the most populous U.S. state in the past week. Statewide, 516,000 electric customers were without power, according to data from PowerOutage.us.

The next day base power prices in California's SP-15 hub rose as high as $63.35 per megawatt-hour (MWh) for the 18th hour on Monday, the highest in more than a week.

Pacific Gas and Electric Co (PG&E), a unit of PG&E Corp and the state's largest electric utility, logged about 500,000 customers without power as of 8:28 a.m. ET. The company had restored electricity to more than 565,000 customers, PG&E said on its website.

"Members of the public should use caution and remain vigilant for hazards including weakened trees, flooding and downed powerlines," PG&E said.

California Governor Gavin Newsom declared a state of emergency in eight counties with a combined population of more than 20 million people, and flash flood warnings were issued for parts of Los Angeles, Santa Barbara and San Luis Obispo counties.

The U.S. National Weather Service's Los Angeles office said the city saw its third wettest day for the month of February since 1877 on Sunday, highlighting high risk for life-threatening and damaging flooding through Tuesday afternoon.

(Reporting by Laila Kearney in New York, Harshit Verma and Brijesh Patel in Bengaluru; Editing by Angus MacSwan and Bill Berkrot)