SAN FRANCISCO - A bomb cyclone smashed into California on Wednesday, bringing powerful winds and torrential rain that was expected to cause flooding in areas already saturated by consecutive storms.
Authorities issued ominous warnings of threats to life and property for a vast stretch of the most populous US state, focused around San Francisco and Sacramento.
Forecasters said gusts of up to 110km/h and intense rain could pummel the region, with the downpour continuing into Thursday.
Meteorologists at the National Weather Service said there could be "widespread flooding, roads washing out, hillsides collapsing, trees down, widespread power outages, immediate disruption to commerce, and the worst of all, likely loss of human life."
California Governor Gavin Newsom declared a state of emergency "to support response and recovery efforts," while San Francisco established an Emergency Operations Center.
Bars and restaurants were shuttered as the city girded for the storm, with local media showing flooded roads, and reports of long delays on public transport.
Dozens of flights in and out of the city were cancelled or delayed, while around 60,000 homes were without power, according to the PowerOutage.us website.
Thousands of sandbags were distributed to residents fearful of flooding.
"We're very worried about it," Deepak Srivastava told CBS in San Francisco.
"(I) just spent all day putting sandbags in front of the garage at every entering point and we're just crossing our fingers and hoping we won't have more damage."
City officials said they were working around the clock to try to help people prepare.
"We've been working very hard to source sandbags from wherever we can in northern California," said Rachel Gordon, of the city's public works department.
But she warned residents need to take the storm seriously.
"If you don't have to be out in San Francisco, please don't be out on the roads."
The intense rain was brought about by a combination of a bomb cyclone -- a sudden steep drop in air pressure -- and an atmospheric river, where moisture-laden air is drawn in from the oceans.
Up to 10cm of rain were expected in the Bay Area, with double that amount over nearby hills.
The Sierra Nevada mountain range was forecast to get up to 1.2m of snow.