Areas around Los Angeles saw more than five inches of rain, which is being caused by a flow of atmospheric moisture known as 'Pineapple Express'. The severe storm is the wettest to strike California this winter and is set to peak today. It has sparked flood watches from Arizona to Washington.
The main parts affected by the storm are areas which were gutted by the recent wildfires in 2018.
Residents of Lake Elsinore were ordered to leave the area over risks the scorched hillsides could turn into dangerous rivers of mud and debris.
Officials fear these could carry away cars and homes.
Bob Oravec, meteorologist with the NWS's Weather Prediction Centre in College Park, Maryland, said: "It's going to be heavy and fast.
"Debris flows and mudslides are a risk in any area scorched by the wildfires.
Northern Californian was one of the worst areas hit, where rain driven by winds of up to 75 mph pounded parts of Sonoma County's wine country.
Cars are reported to have ploughed through standing water on San Francisco streets and water reached the wing mirrors of an abandoned car on a flooded street.
The so-called 'Pineapple Express' weather system is a flow of moisture that heads east from waters near Hawaii to soak the US West Coast.
The NWS said it expected more than eight feet of snow in some areas of the Sierra Nevada mountains.
Cory Mueller, a meteorologist at NWS, warned people to steer away from visiting the area.