Some 163 people have been taken to hospital. Twenty had “storm-related injuries” and four were critically hurt, the BBC reported on Wednesday.
A group of around 300 people were reportedly trapped in Romero Canyon, east of Santa Barbara. A police officer said the scene “looked like a World War One battlefield”.
Emergency services said a number of people are unaccounted for and they expect the death toll to rise. Thousands have fled the deluge and over 50 were rescued.
Heavy rain run-off caused mudflow in the community of Montecito, where some homes were knocked from their foundations, the BBC quoted Santa Barbara County Fire Department spokesman Mike Eliason as saying.
Boulders the size of small cars were rolling down hillsides and blocking roads, the BBC report said.
The upmarket neighbourhood includes homes owned by celebrities like actor Rob Lowe and chat show host Ellen DeGeneres. Oprah Winfrey also has a property in Montecito that is reportedly worth nearly $90 million.
After the wildfire, burned vegetation and charred soil created a water repellent layer that blocked water absorption and increased risk of mudslides and floods.
About 30,000 residents were under evacuation orders.
This comes after a record-setting year of $306 billion of weather and climate-related disaster costs in the US, its National Oceanic and Atmospheric Administration said.