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

California Governor Gavin Newsom declared a state of emergency in several counties, including Los Angeles, to facilitate the rapid mobilization and allocation of resources if needed.

"Life-threatening flash flooding continues over Southern California today and tomorrow; heavy mountain snow and high winds also ongoing," the National Weather Service (NWS) said, adding that "lives and property are in great danger."

The first burst of heavy rain hit the region Tuesday night, leaving Los Angeles with fallen trees and some debris in the streets, as well as minor flooding that hampered traffic.

The rainfall intensified early Wednesday, leaving thousands of people in southern California without power due to downed lines caused by the fury of the Christmas storm. Across the state, some 101,000 homes and businesses had no electricity, according to tracking site Poweroutage.us.

Some communities could receive up to 10 inches (25.4 centimeters) of rain this week, forecasters say.

The coastal areas of Pacific Palisades and Malibu are under a special alert, as they are still recovering from the devastating wildfires in January. Due to the damage from the fires, the terrain is susceptible to mudslides from heavy rain.

The Red Cross opened shelters in several communities as some residents in southern California received immediate evacuation orders.

Authorities were responding to a series of accidents Wednesday, and several major roads were closed due to flash flood warnings, worsening already congested local traffic.

Ariel Cohen, an NWS meteorologist, 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.

- 'Doesn't feel safe' -

However, many Angelenos were still out and about Wednesday, buying last-minute gifts, grocery shopping, and even jogging.

For some though, Christmas travel was upended.

"We decided to stay home," said Jim Lewis, who opted against going to his cousins in nearby Pasadena and was doing last-minute shopping in the city instead.

"We've been receiving all these alerts, it doesn't feel safe, I don't feel like driving there at dark," he told AFP.

Larissa Peet, who was planning a party, said her celebration was still on.

"Nothing that we're doing differently. Just hanging out, eating, drinking and having a good time," she said.

In addition to the threat of flooding, meteorologists are forecasting wind gusts up to 80 miles per hour (nearly 130 kilometers per hour) in the mountains and deserts of Los Angeles County.

Some Californian communities are still reeling from thousands of wildfires that killed 31 people across the state during 2025, including residential neighborhoods of Los Angeles at the start of the year.

In the Sierra Nevada range along the eastern border of California, more than a foot (30 centimeters) of snow had already fallen this week, with up to five feet forecast before the storm is through.