Rescue workers are now searching for survivors.
More than 30 miles of the main coastal road have been closed and police said the scene “looked like a World War One battlefield”.
A group of 300 people are reportedly trapped in Romero Canyon neighborhood east of Santa Barbara, with rescue efforts due to resume at daybreak.
The death toll is expected to rise.
Up to now, more than 50 people have been rescued, but many places are still inaccessible. Several roads are closed, including the major Highway 101.
Some 163 people have been taken to hospital while other 20 had “storm-related injuries” and four were critically hurt.
The first rain in months caused mudslides when it hit ground that had been scorched by December’s huge wildfires.
According to specialists, 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.
The Federal Emergency Management Agency (FEMA) says the risk of flooding stays “significantly higher” for up to five years after a wildfire.
The National Weather Service said in a statement: “Recent burn areas will be especially vulnerable where dangerous mud and debris flows are possible.”
According to officials, in some places mud was waist-deep.
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.
Santa Barbara County Fire Department spokesman Mike Eliason said that heavy rain run-off caused a mudflow in the community of Montecito, where some homes were knocked off their foundations.
On January 9, County Fire Captain Dave Zaniboni said that five people were found dead in Montecito, possibly as a result of the storm.
The upmarket neighborhood includes homes owned by celebrities such as Rob Lowe, Ellen DeGeneres and Oprah Winfrey.
The US Coast Guard has sent “multiple airships to support rescue operations” and warned the public not to fly drones, otherwise the flights would be grounded.
According to the US National Oceanic and Atmospheric Administration, this comes after a record-setting year of $306 billion of weather and climate-related disaster costs in the US, with 2017 the third warmest year on record.
California has suffered severe drought in five out of six of the past years.
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