Three days of heavy downpours that spawned flash flooding and mudslides across Southern California subsided on Friday, as residents of homes in the hard-hit mountain resort of Wrightwood began digging out mud and assessing damage.

The holiday storm drenched the greater Los Angeles basin with up to 6 inches of rain by Friday, with 12 inches or more measured in lower-elevation mountains east of the city, according to the National Weather Service.

The deluge, which began around Christmas Eve, was spawned by the region's latest atmospheric river storm, a vast airborne stream of dense moisture siphoned from the Pacific and carried inland.

The torrential rains were accompanied by strong, gusty winds that toppled trees and power lines across the region, causing power outages. Heavy snow fell in upper mountain areas.

Even before the storm hit, authorities were issuing evacuation warnings to neighbourhoods considered vulnerable to flash floods and debris flows, especially near hillsides previously ravaged by wildfires. Motorists were urged to avoid travel whenever possible.

Although rainfall was tapering off on Friday, a flood watch remained in effect for much of Southern California.

Homes swallowed in mud

In Wrightwood, a town of about 5,000 residents that bore the brunt of the storm in the San Gabriel Mountains northeast of Los Angeles, county safety inspectors began initial assessments of property losses.

Several dozen homes were heavily damaged by rivers of mud that poured through the town on Wednesday, and officials were on standby for additional debris flows that might occur, San Bernardino County Fire Department spokesman Ryan Beckers said.

"Evacuation warnings for Wrightwood are still in effect, and all the roads in the area are closed, except to residents," he said.

Misty Cheng, 49, an accountant who owns a vacation home in Wrightwood, said she learned the property was being swallowed by a mudslide from a neighbour who sent her video footage.

"My house is buried in over 5 feet of mud," said Cheng, speaking to Reuters by cellphone from her primary residence in nearby Upland, where she was staying when the slide occurred.

A stream of mud had forced its way into the house through a crushed wall of the attached garage, filling the living room. By the time she ventured back to the property herself to see the damage first-hand and salvage some belongings, the mud had hardened into a mound solid enough for her to stand on.

"I was able to get a truckload of personal items" out of the house, mostly from the second floor, which was left untouched, she said. Without flood insurance, Cheng said she started a GoFundMe page to raise money for repairs.

Aerial video footage posted online by the fire department showed clusters of homes and vehicles in the town caked in walls of mud as crews in front-loaders began clearing clogged roadways.

Beckers said emergency teams rescued a couple of dozen people who were trapped by high water and debris flows in their vehicles or homes over the holidays, but no deaths or serious injuries were reported in Wrightwood.

The Weather Service said Southern California was expected to dry out over the weekend, while across the country a major winter storm threatened to begin dumping record levels of snow over parts of New York state starting on Friday night.