An unusually strong storm system called an atmospheric river was dousing Southern California on Friday and was expected to continue dumping heavy rain over the region for days, as forecasters warned of rockslides and debris flows especially in the Los Angeles areas that were ravaged by wildfires earlier this year.

"Clearly we have a lot of lessons to learn from January 7th, but we are ready we are being aggressive we are pre-deploying," Los Angeles Mayor Karen Bass said, while speaking at a news briefing.

"Obviously the neighborhoods most affected are the Palisades, but there's a lot of other neighbourhoods too where there have been mudslides and making sure that they have the things that they need to have," Mayor Bass added.

More than four inches of rain fell over coastal Santa Barbara County early Friday as the storm moved south toward Los Angeles, according to the National Weather Service. Forecasters also warned of damaging winds and possibly even a tornado could form as they urged people to stay indoors.

The long plume of tropical moisture that formed over the Pacific Ocean began drenching the San Francisco Bay Area on Wednesday and was expected to unleash widespread rain over Southern California through Sunday with the heaviest amounts on Friday and Saturday nights. More than a foot of snow was predicted for parts of the Sierra Nevada.

Evacuation warnings were in effect for parts of Ventura County and areas that burned in and around Los Angeles in January. Evacuation orders, which are mandatory, were issued for specific high-risk properties in the Palisades and Eaton fire burn areas from Friday evening to Sunday morning. Law enforcement were going door-to-door in those areas to urge people to leave, the county said.