About 150,000 people were left struggling for power as the strongest storm to lash the US West Coast in several years blew in snow, heavy rain and ferocious winds.
Winds gusting up to 230 kilometers (140 miles) per hour and the biggest snowfalls for six years were reported, although it was expected to do little to compensate for a historic drought now ravaging California for a third year.
The storm, which began to batter the north of the state late Wednesday, was expected to last through Friday, causing severe flooding in coastal areas and mudslides in higher regions.
Some 240 flights were canceled at San Francisco International Airport, local newspaper reports said, spelling travel misery.
The inclement weather sweeping into the region was being carried on a current with the unlikely name "Pineapple Express" -- an intense stream of moisture stretching from Hawaii to the US West Coast.
"It's clearly the strongest storm to impact the West Coast in the last three years," Todd Morris, a spokesman for the National Weather Service, told AFP, adding that the highest winds were expected in the mountains.
Businesses shielded shop windows with wooden or cardboard coverings, while others protected their property with makeshift sandbags.
Tens of thousands were temporarily left without power, although most had it restored by late afternoon, according to the PG&E utility company.
In the San Francisco Bay area, ferry crossings were suspended, while two stations on the BART public transport system were temporarily closed.
The number of commuters using the BART system was down by about 40 percent, said spokesman Jim Allison.
"A lot of people are clearly staying home," he said. "With the light ridership, combined with, I think, people's patience, it turned out pretty well," he told the LA Times newspaper.
Blizzard warnings were issued for mountain areas, the first such alerts since 2008, with up to a meter (three feet) of snow expected, said Morris.
- Threatening LA -
Blowing in from the northern Pacific, the unusually strong storm was expected to move south, reaching Los Angeles and San Diego from Thursday evening.
In the suburbs of Los Angeles parking restrictions were imposed in areas vulnerable to mudslides, although the storm had not reached LA by late afternoon.
Schools were closed in San Francisco, Oakland and Marin County and elsewhere in northern California, while local media reported that San Francisco's scenic Great Highway had been closed as a precaution.
The National Weather Service said a flashflood warning would remain in effect all day Thursday, and cautioned motorists that "driving conditions may be very poor."
Flashflood watches and warnings were in place all along the western United States from the Pacific northwest to San Diego at the Golden State's southernmost tip.
The heavy precipitation comes with California already saturated by several weeks of punishing rains.
Epic rainfall was also expected to be felt in Oregon and Washington states.
US weather experts said the last time the region was hit by such heavy rainfall was in October 2009.
Experts said that, while rain was welcome to compensate for California's historic drought, it would take a lot more to have a real impact.
"The current storms will not alleviate the drought, unfortunately," Stephanie Pincetl, an environmental studies professor at the University of California Los Angeles (UCLA), told AFP.
"We will need several years of unusually high rainfall."