A massive storm has unleashed high winds and torrential rains across California, threatening much of the state with flash flooding and mudslides.
One child has died following the storm, which has also caused power outages to tens of thousands.
The "atmospheric river" - an airborne current of dense moisture flowing from the ocean - was expected to persist, drenching much of California with heavy rains at a rate of 2.5cm per hour, the National Weather Service (NWS) said, with dangerous, gale-force gusts forecast throughout the day.
Overnight, a child younger than 2 years old was killed when a tree fell on his home in Occidental, a community of about 1,100 people 105km north of San Francisco, local media reported. Authorities did not provide more specific information.
Some 180,000 homes and businesses were without power early today, according to data by Poweroutage.us, after highwinds downed power lines and trees across the state overnight.
Atmospheric rivers will become larger - and possibly more destructive - because of warmer air and water temperatures from climate change, scientists say.
Governor Gavin Newsom declared a state of emergency yesterday, and state officials urged Californians to avoid travel during the storm.
Crews in San Francisco spent the night cleaning up debris from felled trees that blocked roadways. The city's fire department rescued a family trapped when a tree fell across their car.
Sonoma County residents living near the Russian River between Healdsburg and Jenner were advised by authorities to evacuate their homes, with the waterway forecast to crest at 10 meters late today before receding below flood stage. The area lies in the heart of the Sonoma Wine Country, a tourist magnet just north of San Francisco.
Evacuation warnings were also in place in coastal communities such as Santa Cruz. Officials shut down a 55-mile stretch of Route 1, a scenic highway that runs along the coast, due to flooding and debris.
Authorities warned the heavy downpours would likely unleash flash floods and mudslides, especially in areas where the ground remained saturated from a previous deluge that soaked northern California days earlier.
Fire-ravaged hill slopes stripped of vegetation were also particularly vulnerable to slides. In Santa Barbara County, homes in three areas where wildfires left burn scars behind were ordered to evacuate.
Nearly 100 flights were canceled at San Francisco International Airport yesterday, and an additional 15 had already been canceled today.
The storm was also bringing extremely heavy snow to the Sierra Nevada Mountains - more than 7.6cm per hour over 1,500m, the NWS said - making travel in those areas dangerous or impossible.
The rain was beginning to taper this morning, said Bob Oravec, a NWS meteorologist, though it was still inundating much of the state.
But forecasters predict more storms will move through the state in the coming days, with ten additional inches of rain likely in the next week across northern and central California, Oravec said.
The storms come after near-record rainfall was deposited 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 of rain on 31 December, 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 said.
"This storm alone without the previous storms would cause localised 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.
Mr Solum said while any rain was helpful in alleviating the drought - a long-term issue - these back-to-back storms could be destructive because there was 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."
