Atmospheric rivers dumped heavy rains on large parts of California, causing floods and mudslides and leaving three dead as of Christmas night, according to local officials.
The storms - which were expected to continue through Friday - brought 11 inches (27 cms) of rainfall to some parts of Los Angeles County, prompted evacuations, and shut down major roads.
Emergency responders have had to perform several rescues, including people stuck in vehicles as flood waters rise. California's Governor Gavin Newsom declared a state of emergency on Wednesday in Los Angeles and other southern California counties.
About 100,000 people in the state were without power as of Thursday evening.
The US Weather Prediction Center said on Thursday that Numerous flash flooding events are possible.
In addition, many streams may flood, potentially affecting larger rivers.
A 64-year-old man from San Diego, California, was killed on Wednesday morning by a fallen tree, the police department told US media.
Another person, a 74-year-old, died from flood waters over the weekend as police tried to rescue him from inside a vehicle in Redding, California, the town's mayor told local news.
On Monday, a woman in her 70s died after she was knocked off a rock by a large wave and swept into the ocean at MacKerricher State Park in Mendocino County, the sheriff's office said in a statement.
Evacuation warnings were in effect for some residents of San Bernardino County in southern California, and flash food warnings were issued for those in the San Francisco Bay area on Thursday morning.
Wind speeds in the Bay Area topped 100 mph (161kmh) at one observatory near San Jose, the San Francisco Chronicle reported.
In Altadena, near Los Angeles, residents experienced a mudslide from flooding in a burn scar area that was less able to absorb water due to the wildfires that scorched the neighborhood in January of this year.
Los Angeles Mayor Karen Bass has declared an emergency from the storms and warned residents to be cautious on roads during a busy holiday travel time.
The storms came from multiple atmospheric rivers that carried large plumes of moisture from the tropics during one of the busiest travel weeks of the year.
