Driven by an atmospheric river known as "the Pineapple Express," which moves heavy moisture from the tropical climes of Hawaii to the US West Coast, the storm was expected to deliver months' worth of rain over a few days.

Early Thursday, the National Weather Service (NWS) warned about the risk of excessive rainfall over parts of southern California, including in Los Angeles, the second-most populous city in the United States.

The NWS warned of a "broad plume of moisture" producing heavy rain in California on Christmas Day, adding there was a "moderate risk" of excessive rainfall over southern parts of the state.

"Numerous flash flooding events are possible. In addition, many streams may flood, potentially affecting larger rivers. The flooding may include debris flows in or near recently burned areas," the bulletin added, referring to areas affected by wildfires.

The rain was forecast to continue on Friday, the service said.

State authorities declared a state of emergency in several counties, including Los Angeles.

- Rescue work -

Heavy rain lashed southern California on Wednesday, spawning floods and debris flows. Some communities had already received 10 inches (25.4 centimeters) of rain in the first storm, forecasters said.

The Los Angeles Times reported that at least three people had died in storm-related incidents, including a San Diego man who was killed by a falling tree.

On Wednesday, the Los Angeles County Department of Public Works warned Angelenos to stay well away from waterways such as flood control channels, rivers, and streams -- even after the rain stopped.

"Water from upstream communities can move quickly through channels with little warning. What may look shallow or calm can suddenly become fast-moving and dangerous," the agency warned in a post on X.

In San Bernardino County, adjacent to Los Angeles, authorities told AFP they were working to divert the flow of floodwater on Thursday.

Muddy water streamed through the town of Wrightwood a day earlier, trapping people in their homes, said Christopher Prater, spokesperson for the county fire department.

"Fire department personnel were out there rescuing people, assisting them from their houses, getting them to safety, also while effecting rescues from people that were stranded in their vehicles," he said, with work going on into the night.

- Fire-affected communities -

Fire-burn scar zones, which are less able to absorb water due to vegetation being stripped from them, were under special alert -- including the coastal areas of Pacific Palisades and Malibu, both still recovering from devastating wildfires in January.

In northern California, a dangerous storm was tracked developing early Thursday morning in the San Francisco Bay Area, with an emergency alert for flash flooding issued in the area, the San Francisco Chronicle reported.

The National Weather Service was also predicting that a winter storm could bring heavy snowfall to the Sierra Madre Mountains.

Ariel Cohen, an NWS meteorologist, had warned that from Wednesday afternoon through Friday, "many areas will likely experience significant flooding, along with rockslides and mudslides, especially in the higher elevations."

"If you were planning to travel on the roads during Christmas, please reconsider your plans," he added.