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 in the next few days.

"Severe, widespread flash flooding is expected," the National Weather Service said, adding that "lives and property are in great danger."

The first burst of heavy rain hit the region overnight Tuesday, and intensified Wednesday morning, when authorities issued a brief tornado warning -- later canceled -- for several communities in Los Angeles, the second-most populous city in the United States.

Los Angeles woke up to downed trees blocking streets, as well as minor flooding, and thousands of people without power due to downed lines caused by the fury of the Christmas storm.

The coastal areas of Pacific Palisades and Malibu are under particular alert, as they are still recovering from the devastating wildfires of January. Due to the damage from the fires, the terrain is susceptible to mudslides with the continued rainfall.

Shelters opened their doors across the region as some residents in southern California received evacuation orders.

Several major roads were closed due to flash flood warnings in effect across large areas of the region.

National Weather Service meteorologist Ariel Cohen told reporters in Los Angeles that from late Wednesday through Friday, "many areas will likely be experiencing significant flooding, along with rockslides and mudslides, especially through the high terrain and canyon roadways."

Some southern California communities could see up to 12 inches (30.5 centimeters) of rainfall, Cohen said on Tuesday.

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, the major mountain range along the eastern border of California, more than a foot of snow had already fallen this week, with up to five feet of snow forecast before the storm is through.

The storm was also expected to bring wind gusts of up to 55 mph (88 kph) across the state.

"The combination of increasingly saturated soil and the strong winds will bring the potential for widespread downed trees and power lines," weather officials warned. "Lives and property are in great danger."