With the storm feeding on an atmospheric river of moisture stretching far out into the Pacific, precautionary evacuations of homes in some neighborhoods were requested due to the potential for mudslides and debris flows.
More than 300 arriving and departing flights were delayed or canceled at Los Angeles International Airport.
In the Sherman Oaks area of Los Angeles, a falling tree downed power lines and hit a car. A 55-year-old man was electrocuted and pronounced dead at a hospital, police and fire officials said.
Winds gusting to 70 mph or more lashed parts of the region. Heavy rains turned creeks and rivers into brown torrents and released slews of mud from hillsides burned barren by wildfires. Several stretches of freeways and highways were closed by flooding.
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"It's crazy," said Robin Johnson, an academic adviser at the University of California, Santa Barbara. "It's just pouring down rain. The wind is just going nuts."
In the desert town of Victorville, several cars were washed down a flooded street. A helicopter rescued one person from the roof of a car but another motorist was found dead in a submerged vehicle, San Bernardino County fire spokesman Eric Sherwin said.
Elsewhere in the county, a 20-mile stretch of State Route 138 in the West Cajon Valley was closed at the scene of a summer wildfire.
Two people in a car were rescued and four students on the bus were removed and taken to a school office, he said. Another road in the area was covered with 2 feet of mud.
In LA's Sun Valley, 10 cars were trapped in swift-moving water on a roadway and 15 people had to be rescued, the Fire Department reported.
Using ropes and inflatable boats, firefighters rescued seven people and two dogs from the Sepulveda basin, a recreation and flood-control area along the Los Angeles River.