The storm is the second Pineapple Express weather system, or atmospheric river storm, to hit the US state in the past week and arrived just as Los Angeles welcomed celebrities for the music industry's Grammy awards, where the red carpet was tented but other attendees were forced to slog through heavy rain in glitzy cocktail attire.
The severe conditions prompted the National Weather Service's (NWS) Bay Area office to issue a rare hurricane-force wind warning for Big Sur and nearby areas.
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 Santa Barbara and San Luis Obispo counties.
"This has the potential to be a historic storm, severe winds, thunderstorms, and even brief tornadoes," Los Angeles Mayor Karen Bass told a news conference.
The San Francisco Bay Area and Los Angeles county were not only getting drenched but the storm was expected to stall over some areas into Tuesday, creating severe risk of flooding mudslides.
"The Monday evening commute is going to be a complete disaster to say the least. In fact, it's going to be bad enough that I would recommend everybody stay home in LA if we possibly can," Daniel Swain, a climate scientist at the University of California Los Angeles, said in a livestream on Sunday.
The NWS recorded peak wind gusts of 129km/h in some places.
Nearly 750,000 homes and businesses lacked electricity on Sunday afternoon, according to PowerOutage.us.
"This is a DANGEROUS SYSTEM with major risks to life and property. Substantial flooding. Residents should heed any evacuation orders," the Los Angeles office of the NWS said on social media. "Stay off the roads, especially the freeways, this afternoon through at least Monday morning."
Near Los Angeles, the port city of Long Beach could get more rain this week than it does during an entire year, said Mayor Rex Richardson, who is expecting 13-18cm starting Sunday through Tuesday.
California's southern and central coasts are bracing for an inch of rain an hour and totals of7-15cm, the US National Weather Service said.
The Los Angeles and Santa Barbara areas were both at high risk for excessive rainfall on Sunday and Monday, with forecasters anticipating "near continuous rainfall" for the next 48 hours.
