LOS ANGELES – A bomb cyclone – used to describe a hurricane-force low-pressure system – slammed into Northern California, packing winds that toppled power lines, shattered windows in downtown San Francisco and caused a big rig to overturn on the Bay Bridge to Oakland.

Heavy rain and wind gusts as high as 129kmh lashed the region, upending travel and leading to at least one death in San Mateo County, south of San Francisco.

A flood advisory was in effect in the San Francisco Bay Area throughout Wednesday morning, the National Weather Service’s local office said on Twitter. 

The latest storm system was forecast to bring up to 10cm of rain to parts of southern and central California, and up to 1.2m of snow in elevations above 1,800m through Wednesday, according to the National Weather Service.

The low-pressure system would gradually weaken on Wednesday, the weather service said in a statement, adding that its outer edges would, however, ripple through the south-west and into the Rockies. “Dangerous to difficult travel is likely” there, with as much as 0.6m of additional snowfall expected in mountain areas.

San Francisco International Airport saw flight delays early on Wednesday.

About 137,000 homes and businesses across the state remained without electricity as at 3am local time, according to the website PowerOutage.us.

In the Bay Area, Stanford University cancelled final exams on Tuesday because of a widespread power outage, the school’s emergency information centre said. One of the main transmission lines that feeds the campus was impacted by the storm.

Also on Tuesday, a train between Oakland and Sacramento derailed after contact with a tree, shutting service just before the evening commute. 

“This was a violent, sudden windstorm,” said Mr Daniel Swain, a climate scientist at the University of California at Los Angeles. “We’re seeing impacts probably that are similar to a strong tropical storm or a weak hurricane.”

California has been hammered by a series of storms known as atmospheric rivers since late December. This has brought flooding rain and record snowfall across the Sierra Nevada and other mountains.

More than 20 people have died and billions of dollars in damage and losses have mounted from collapsed roads, inundated homes and power outages.

Santa Cruz County, about 120km south of San Francisco, on Tuesday again appeared to suffer the brunt of an atmospheric river.

Reports of downed electrical lines and car crashes swamped the county’s fire dispatch centre.

Going door-to-door

Meteorologists with the weather service in Hanford, California, were keeping an eye on water levels in rivers, creeks and streams, which they said were “extremely high”, adding that heavy rain below 1,200m could cause flooding.

Reports had said the heaviest rain was expected on Tuesday evening.

In the central part of the state, officials went door to door starting on Sunday to urge residents in portions of Tulare County to evacuate. Tulare County has been flooded in previous storms in 2023 and more rain was expected there this week.

“We’re going to every home that we anticipate would be impacted by the waters,” said Sheriff Mike Boudreaux of Tulare County at a news conference on Sunday.

It was not clear on Monday how many people had been encouraged to leave.

Last week, the Sheriff’s Office shared images of ranches and crops that were still flooded from recent storms.

The county said it was working to assess the damage, and that it was seeking federal reimbursement for those who lost crops.

Sheriff Boudreaux said he wanted to quell fears that dams in the area would not hold up to more bad weather, and that he had consulted with the United States Army Corps of Engineers about its capabilities ahead of the storm.

“The stability of those dams is solid,” he said.

“You will see lots of water coming through the spillways, but, rest assured, the dams are in good functioning position.”

Be prepared for the worst

The California Governor’s Office of Emergency Services said residents should be prepared to evacuate if needed. It also urged them to have emergency kits ready at home and vehicles full of fuel.

About a dozen swift-water rescue teams were being positioned across the state, especially in regions where flooding was a greater concern, the department said.

The state is trying to recover from a series of storms that have brought heavy rain and snow, causing flooding in portions of the state.

It is the second-snowiest season in the Central Sierras since researchers at the University of California, Berkeley, began keeping records in 1946. This season, 17m of snow have fallen there, the researchers said, compared with a record 20m in 1952. NYTIMES, BLOOMBERG