Forecasters warned the storm could soak California with an inch of rain per hour Residents in southern California are bracing for flooding, thunderstorms, hail and even tornadoes from the worst in a week-long series of storms.
Governor Arnold Schwarzenegger has declared a state of emergency for a half a dozen communities in the state, some of which have already seen mud slides and flooded streets.
Hundreds of people have been evacuated in the suburbs of Los Angeles.
Forecasters said the storm could bring as much as 1in (2.5cm) of rain an hour.
“There’s going to be a six-hour time frame in the early morning when it’s really going to be dumping on us,” National Weather Service spokesman Bill Hoffer told the Associated Press news agency.
Forecasters say southern California will be the hardest hit area, although they expect the storm to move into Arizona, Utah and Nevada on Wednesday as well.
Some areas could expect up to 2in of rain per hour as thunderstorms move through the region, forecasters said.
Steady rain began coming down late on Tuesday and is expected to strengthen.
Heavy rains collapsed a hillside on a heavily user section of Interstate 10 early on Wednesday, covering three lanes near the city of Ponoma.
Homes in the mountains near Silverado Canyon in Orange County were blocked by boulders and mud as rescue workers helped residents seek shelter before the largest of the storms struck.
On Tuesday, officials ordered the evacuation of 232 homes that site beneath large hillsides in La Canada Flintridge and La Crescenta, in the suburbs of Los Angeles.
Flood warnings and emergency orders have also been put in place in Arizona, Nevada and Utah.
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