Heavy rains and high winds slammed northern California leaving more than 220,000 people without power.
The storm brought rainfall of more than an inch an hour in San Francisco and winds gusts of 140mph in the Sierra Nevada mountains.
Flooding has already closed two major motorways in the area, delayed public transport, cancelled 240 flights and shut ferry services.
The rain is much needed in the drought-hit state but mudslides are a concern.
Power cuts were widespread, from the suburban area south of San Francisco to Humboldt, near the Oregon border.
National Weather Service forecaster Diana Henderson said: “It’s a two-pronged punch – it’s wind and rain.
“Once the ground gets saturated and the winds are howling, there’s a bigger chance of trees going down on power lines.”
There were multiple vehicle accidents but no series injuries.
Rain and floods also led to rare weather-related school closures for students in San Francisco, Oakland, Berkeley, and Santa Cruz County.
Some 240 flights at San Francisco’s airport were cancelled and delays averaged two hours, said a spokesman.
In Santa Cruz, a young child was trapped after an 80ft (24m) tree fell on his arm and shoulder,
Rescuers with chain saws cut it apart and the student was taken to hospital in a good condition.
In the small town of Healdsburg, cars were stalled in heavily flooded streets.
However, surfers welcomed forecast of waves as high as 15ft and unseasonably warm temperatures near San Francisco Bay and ski resorts in the northern Sierra Nevada were hoping for more than two feet of snow.
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