LOS ANGELES | The first of two back-to-back atmospheric rivers drenched California on Thursday, flooding roads and toppling trees while triggering statewide storm preparations and calls for people to get ready for powerful downpours, heavy snow and damaging winds.
Heavy rain and gusty winds that began hitting the San Francisco Bay Area on Wednesday moved south and arrived in Los Angeles in time to snarl the Thursday morning commute.
The river of rain “will be taking aim at mostly Southern California” on Thursday, weather service forecaster Bob Oravec said.
The Los Angeles and San Diego areas will be in the bullseye for heavy rain on Thursday, “especially for some of the higher elevations where they tend to get most of the rainfall — or the heaviest rainfall — with these atmospheric river events,” MR. Oravec said.
The weather service issued a flood watch into Friday morning for the Bay Area and the Central Coast because of possible flooding of rivers, streams, some roads and areas scarred by previous wildfires.
Forecasters also said the Central Coast could see waves up to 18 feet high on Thursday and Friday.
Service on San Francisco’s cable cars was halted as a safety precaution, and Pacifica, a coastal city in San Mateo County, saw more than an inch of rain in a single hour.
Widespread coastal flooding was reported Wednesday in Humboldt County, said the weather service office in Eureka. Scattered power outages were reported.
In the far south, all of San Diego County was under a flood watch Thursday. Forecasters said some areas could see 2 inches of rain, with up to 3 inches in the mountains and winds gusty to 40 mph or more.
The storm came a week after heavy rain caused flooding that inundated homes and overturned cars in the county.
The “Pineapple Express” — called that because its long plume of moisture stretched back across the Pacific to near Hawaii — will be followed by an even more powerful storm on Sunday, forecasters said.
The California Governor’s Office of Emergency Services activated its operations center and positioned personnel and equipment in areas most at risk from the weather.
Brian Ferguson, Cal OES deputy director of crisis communications, characterized the situation as “a significant threat to the safety of Californians” with concerns for impact over 10 to 14 days from the Oregon line to San Diego and from the coast up into the mountains.
“This really is a broad sweep of California that’s going to see threats over the coming week,” Mr. Ferguson said.
An atmospheric river is an area where “the moisture is more confined and the winds are stronger and pushing these higher moisture values on shore,” Mr. Oravec said.
“They typically occur ahead of cold fronts across the Pacific,” he said. “And when they interact with the west coast topography, you often get some very heavy rain both along the coastal ranges and also inland through the Sierras.”
Last winter, California was battered by numerous drought-busting atmospheric rivers that unleashed extensive flooding, big waves that hammered shoreline communities and extraordinary snowfall that crushed buildings. More than 20 people died.
The second storm in the series has the potential to be much stronger, said Daniel Swain a climate scientist at the University of California, Los Angeles.
Models suggest it could intensify as it approaches the coast of California, a process called bombogenesis in which a spinning low-pressure system rapidly deepens, Mr. Swain said in an online briefing Tuesday. The process is popularly called a bomb cyclone.
That scenario would create the potential for a major windstorm for the San Francisco Bay Area and other parts of Northern California as well as heavy but brief rain, Mr. Swain said.
The new storms come halfway through a winter very different than a year ago.
Despite storms like a Jan. 22 deluge that spawned damaging flash floods in San Diego, the overall trend has been drier. The Sierra Nevada snowpack that normally supplies about 30% of California’s water is only about half of its average to date, state officials said Tuesday.
A winter storm warning was in effect through Friday morning for nearly a 300-mile stretch of the Sierra from north of Lake Tahoe to south of Yosemite National Park, said the weather service office in Reno, Nevada. Snow could fall at rates up to 2 inches per hour in some areas, with winds gusting up to 100 mph, forecasters said.
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