The first rain in months caused mudslides when it hit ground that had been scorched by December’s huge wildfires.
After a wildfire, burned vegetation and charred soil create a water repellent layer which blocks water absorption. Together with the loss of vegetation, this leads to an increased risk of mudslides and floods.
About 300 people are reportedly trapped in Romero Canyon neighbourhood east of Santa Barbara, with rescue efforts due to resume at daybreak.
Rescue workers in southern California are searching for survivors after mudslides and flooding in which at least 13 people have died.
More than 30 miles (48km) of the main coastal road have been closed and police said the scene “looked like a World War One battlefield”.
More than 50 people have been rescued already but many places are still inaccessible. Several roads are closed, including the major Highway 101.
Some 163 people have been taken to hospital. Twenty had “storm-related injuries” and four were critically hurt.
The death toll is expected to rise.
woman from a collapsed house in Montecito
“Recent burn areas will be especially vulnerable where dangerous mud and debris flows are possible,” said the National Weather Service in a statement.
In some places mud was waist-deep, officials said.
Thousands had to leave their homes, many for the second time in two months. The emergency services declared an exclusion zone, saying anyone moving around the area would be in the way of rescuers and would be subject to arrest.
Those who have stayed in the area have been warned to boil their tap water before drinking it.
Heavy rain run-off caused a mudflow in the community of Montecito, where some homes were knocked off their foundations, said Santa Barbara County Fire Department spokesman Mike Eliason.
Boulders the size of small cars were rolling down hillsides and blocking roads, reports the BBC’s Los Angeles correspondent James Cook.
County Fire Captain Dave Zaniboni said that five people were found dead on Tuesday in Montecito, possibly as a result of the storm.
The upmarket neighbourhood includes homes owned by celebrities such as actor Rob Lowe, chat show host Ellen DeGeneres and broadcaster Oprah Winfrey.
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