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 has warned of numerous flash flooding events and potential impacts on larger rivers. The fatalities included a 64-year-old man from San Diego killed by a falling tree, another victim aged 74 who drowned in Redding, and a woman in her 70s swept into the ocean by a wave at MacKerricher State Park.

Evacuation warnings were in effect for some residents of San Bernardino County, and flash flood warnings were issued for the San Francisco Bay area. Wind speeds exceeded 100 mph in some areas.

Meanwhile, Los Angeles Mayor Karen Bass has declared an emergency, urging residents to exercise caution during their holiday travels as multiple storms carry substantial moisture from the tropics during this busy time of year.