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.
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.
"(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 four inches (10 centimeters) 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 four feet (1.2 meters) of snow.
- 'Several storms' -
The downpours come on top of a series of storms that have brought near-record rainfall over recent weeks.
On New Year's Eve, parts of northern California were lashed by a storm that caused landslides and power outages, as levees were breached and roadways were flooded.
At least one person is known to have died after being trapped in a submerged car.
San Francisco recorded almost 5.5 inches (14 centimeters) of rain on December 31, the city's second wettest day in recorded history.
The waterlogging caused by those previous storms would exacerbate the danger of this one, meteorologist Matt Solum told AFP.
"This storm alone without the previous storms would cause localized flooding concerns and rock slides and mudslide concerns," he said.
"But with the recent wet conditions, a lot of the rainfall that's already fallen has already saturated the ground so any additional rainfall is going to run off instead of soak into the ground."
While it is difficult to draw a straight line to this storm from human-caused climate change, scientists say a warmer planet brings more unstable weather, with more ferocious storms as well as longer, hotter dry periods.
The western United States is in the grip of a decades-long drought, with below-average precipitation leaving river and reservoir levels worryingly low.
Solum said while any rain is helpful in alleviating the drought -- a long-term issue -- these back-to-back storms can be destructive because there is nowhere for the water to go.
"It's just the compounding impact of all the storms is what's going to be the most impactful," he said.
"Typically, we don't see this many series of storms with this much heavy rain."
And there is more to come.
"It's definitely going to continue," he said.
"We are expecting another one over the weekend. And then another several storms potentially for next week. And even possibly the following week as well."