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Atmospheric rivers migrate to California, bringing Bay Area rain – San Francisco Chronicle

Atmospheric rivers migrate to California, bringing Bay Area rain. An unusual September storm drenches streets in Chinatown in San Francisco. The rainstorm marked a dramatic shift of events for many residents after a record heat wave and grueling wildfire season.
Back-to-back atmospheric rivers are expected to bring 3 to 6 inches of precipitation to parts of the Pacific Northwest through the middle of next week. But is the storm system going to make it all the way to California?
Not exactly, Newsroom Meteorologist Gerry Díaz said, but the remnants of the atmospheric river means the Bay Area will likely see rain next week.
Díaz compared moisture from an atmospheric river to gasoline.
“You’re filling up your car at a gas station – that gas is the atmospheric river and the car engine is the storm itself,” Diaz said. “Eureka, Redding … the storm will have plenty of fuel to work with up there, but as it starts moving toward the Bay Area the atmospheric river will have less and less fuel.”
Rain that reaches San Francisco will be due to a more familiar type of weather system.
“It’s a cold front,” said Jan Null, a meteorologist with Golden Gate Weather Services and an adjunct professor with San Jose State University.
The amount of precipitation is expected to be far less in the Bay Area than in the Pacific Northwest.
“Good wetting rain but nothing anyone would call a storm, let alone an atmospheric river,” Null said.
Atmospheric rivers are essentially rivers in the sky, carrying water vapor rather than liquid. They transport moisture from the tropics toward land, including the western United States. As they rise over mountains, water vapor condenses into droplets that fall as rain or snow.
In December 2021 a historic atmospheric river brought intense rainfall to the Bay Area.
The atmospheric rivers set to hit the Pacific Northwest are right on schedule. Late summer and early fall are around the time that atmospheric rivers typically make their way to the region, said Alexander Gershunov, a research meteorologist with the Scripps Institution of Oceanography at UC San Diego. Over the following months, atmospheric rivers make landfall further south.
“By December, it’s the peak of the season in California,” Gershunov said. In December 2021, an atmospheric river drenched the Bay Area and Central Coast, flooding streets and triggering mudslides.
It’s still unclear what the precise impacts from the front will be in the Bay Area next week, because weather models don’t agree on the path that the system will take.
“We need to see exactly how this low-pressure system will dig into California,” Díaz said.
A more northern route could bring the atmospheric pressure to Northern California, north of the Sacramento Valley. A more southerly trajectory could bring beneficial rains to the Bay Area and even as far south as the San Gabriel Mountains in Southern California.
At the moment, models are leaning toward wet weather in the Bay Area, with a 50 to 60 percent chance for rain during the Tuesday to Wednesday timeframe, Díaz said. That could mean anything from half an inch to upwards of an inch in parts of the North Bay and East Bay. The chance of rain decreases moving down the state. The possibility of storm remnants reaching Southern California is only around 15 percent.
Any precipitation would be welcome in California. Though totals from this cold front won’t make a dent in the rainfall deficit that’s racked up over the past three years, with a fourth year looming.
The rain is good news for California’s fire season, though. Dampened vegetation means reduced risk for future fires. But it could also produce dangerous conditions in previously burned areas.
“That will raise concerns for burn scars,” Díaz said. “The risk of landslides and mudslides will be low, but it won’t be zero.”
San Francisco Chronicle Weather Science Editor Hannah Hagemann contributed to this story.
Jack Lee (he/him) is a San Francisco Chronicle staff writer. Email:
Jack Lee joined The San Francisco Chronicle’s Weather Science team in 2022 as a data reporter.
He has written for a variety of science journalism outlets, covering everything from COVID-19 to songbirds to extreme weather events. Most recently, he has been writing about cancer prevention and early detection for the National Cancer Institute.
Before coming to science writing and journalism, Lee earned a Ph.D. in molecular biology at Princeton University and then worked as a data engineer for several years in the Bay Area. He obtained a master’s in science communication from UC Santa Cruz in 2020.


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