The deadliest mudslides in more than 10 years hit communities along California's coast killing at least 15 people on Tuesday and causing many more to flee from their homes.
Mudslides, rocks and tonnes of debris hit several coastal cities after a series of wildfires burnt off protective vegetation late last year.
Downpours began before dawn on Tuesday after thousands of residents in Santa Barbara County north of Los Angeles were ordered to leave their homes, some of them for a second time since December.
But only 10 to 15 per cent complied with mandatory orders, said Amber Anderson, a spokeswoman for the Santa Barbara County Fire Department.
Emergency workers using search dogs and helicopters rescued dozens of people stranded in mud-coated rubble in the normally pristine area, sandwiched between the ocean and the Los Padres National Forest, about 110 miles north of Los Angeles.
The upmarket communities of Montecito and Carpenteria, just outside the city of Santa Barbara, were the hardest hit.
The mudslides toppled trees, demolished cars and covered blocks of residential neighbourhoods with a thick layer of mud, blocking Highway 101, a major road that runs parallel to America's west coast.
"The best way I can describe it is, it looked like a World War One battlefield," Santa Barbara County Sheriff Bill Brown said.
The toll could rise, with rescue workers still picking through dozens of damaged and demolished homes in the search for survivors, Mr Brown said. At one point on Tuesday, at least two dozen people were missing, but the sheriff said later in the day that it was not clear how many had been located.
About 300 people were stranded in a canyon. Local officials, using borrowed helicopters from the Coast Guard, were working to airlift them out, Mr Brown said.
The threat of mudslides had prompted the county to order 7,000 residents to leave their homes ahead of a rainstorm, and to urge 23,000 others to relocate voluntarily.
The mudslides swept through the mandatory evacuation zones and areas where people were urged to voluntarily leave, Mr Brown said.
The county set up an evacuation shelter at Santa Barbara City College, where some people showed up drenched in mud, and also provided a place for people to take their animals.
Last month's wildfires, the largest in California history, left the area vulnerable to mudslides. The fires burnt away grass and shrubs that hold the soil in place and also baked a waxy layer into the earth that prevents water from sinking deeply into the ground.
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Some local residents had to flee their homes due to the fires last month and again this week because of the rains.
Among them was Colin Funk, 42, who sat up watching mud and debris approaching his Montecito house overnight and fled on Tuesday morning with his wife and three young children as thigh-high mud approached the front doorway.
"We started looking around, and that's when we saw parts of roofs and there was a body against our next door neighbour's car," said Mr Funk, who works as a financial adviser.
"I feel lucky," he said. "Some people lost their lives in my neighbourhood."
Television personality Ellen DeGeneres, who is among celebrities such as Oprah Winfrey and Rob Lowe who own homes in the upmarket community of Montecito, posted a photo on Twitter of a roadway choked with mud and brown water.
"This is not a river," DeGeneres wrote on Twitter. "This is the 101 freeway in my neighbourhood right now. Montecito needs your love and support."
Oprah Winfrey, who also lives in Montecito and had left her home during the wildfires, had not posted about the mudslides as of Tuesday evening.
Some areas of Santa Barbara County early on Tuesday were pounded with more than a half-inch of rain in five minutes, a rate that far exceeds the normal flash flood threshold, officials said.
"Where are the frogs and locusts? We’re waiting for them," Dominic Shiach, a restaurateur from Montecito who relocated due to last month's fire, said by telephone. He lives just outside the latest evacuation zone.
