The atmospheric river flowing into the Bay Area will bring the first major storms of the winter rainy season
The first atmospheric river storm of the season is expected to hit Northern California starting Wednesday, a powerful system expected to bring the rainiest weather to the Bay Area in nearly nine months.
The rain will continue all week, forecasters said Monday, peaking with the highest amounts on Friday, and likely bringing wet conditions to the Bay Area for at least five days in a row until next Monday.
“This is the beginning of the rainy season,” said Jan Null, a meteorologist with Golden Gate Weather Services in Half Moon Bay. “We’ve had very few drive-by fronts this past month with one-tenth or two-tenths of an inch of rain. This will be the first with significant accumulations. The storm door will open on Wednesday.”
The system’s bull’s-eye could still waver but is expected to hit around Guerneville in Sonoma County, said Marty Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego. Up to 10 inches could fall in the Russian River basin there by Sunday — or 20% of the area’s annual precipitation in less than a week, he said.
“A few big storms make or break the water year,” Ralph said. “This one looks like it will be a big kickstart for the winter.”
Ralph and his colleagues developed a scale that ranks atmospheric river storms between 1 and 5, with 5 being the strongest. This storm is expected to be a magnitude 3 or 4, he said, from the Bay Area to the Oregon border.
Total rainfall could exceed 10 inches along the coast in places like Eureka and Mendocino County by Sunday, according to the National Weather Service, which issued a flood warning for that area through Thursday. 6 to 9 inches are forecast in the North Bay.
Quantities will decrease, but south of the Golden Gate they will still be significant. San Francisco should see 2 to 4 inches by Sunday, with 2 to 3 inches in the East Bay and Peninsula, and 2 inches in the South Bay. At higher elevations, such as the Santa Cruz Mountains and Big Sur, 3 to 4 inches is possible by Sunday, the National Weather Service said.
“It’s definitely time to bring out the umbrellas,” said Dial Hoang, a meteorologist with the National Weather Service in Monterey. “And if you can, clean out the gutters and storm drains before Wednesday.”
Some localized flooding is expected, he said, and creeks will rise, but authorities don’t expect flooding on major rivers. Up to 2 feet of new snow is also forecast at the highest point in the Sierra Nevada, but overall this storm is expected to be warmer than many others.
How long has it been since the Bay Area had a real winter storm? San Francisco is forecast to receive 1.1 inches of rain on Friday. The last time it received that much was almost nine months ago, on March 1, when 1.17 inches fell.
The source of the upcoming wet week is an atmospheric river storm originating from the Pacific Ocean in an area north of Hawaii.
Atmospheric river storms are the largest ‘rivers’ on Earth. Moisture-rich storms that often begin in the tropics flow through air up to 2 miles (3.2 km) above the ocean, carrying 25 times the volume of the Mississippi, where it flows into the ocean.
When high-pressure ridges off the coast prevent atmospheric rivers from reaching California and divert them toward Canada or the Pacific Northwest, the state could enter a drought. That happened repeatedly during California’s severe drought from 2012 to 2016 and again regularly during the most recent drought from 2020 to 2022.
But when such high-pressure ridges are gone, as they are now, “AR storms” can rush from the ocean toward California and pack a moisture-laden punch. They are critical to the state’s water supply.
In a normal year, California experiences about a dozen major atmospheric river storms, which are responsible for about 50% of its precipitation, Ralph said.
Does the coming rain mean California will avoid drought next summer? Nobody knows. There is no correlation between the amount of rain California receives in October and November and how much ultimately falls in April, when the winter season ends.
“I’m cautious about a major storm early in the season, which signals the potential for a prolonged wet season,” Ralph said. “There is no guarantee. But at least if you start wet you’re in a better position.”
Null agreed. The Bay Area receives about 75% of its annual precipitation in just four months: December, January, February and March, he noted. Those most important months are yet to come.
But after a huge winter in 2022-23, in which the Sierra snowpack was the largest in four decades, and another solid season last winter of 2023-24, in which snowpack was 113% of average on April 1, almost all California areas reservoirs remain at or above historical averages. Major reservoirs reached 112% of historic capacity on this date Monday, according to the state Department of Water Resources.
The rain will also extinguish a landscape that has been at risk of wildfires since June.
“This will be a fire season-ending storm for much of Northern California,” Ralph said.
Southern California is expected to receive small amounts later this week, but less than an inch, meaning fire danger will remain.
This storm is a slow-moving event that will strengthen as a result of a rapidly descending low-pressure system in the Pacific Northwest. When atmospheric pressure suddenly drops in this way, meteorologists call it a “bomb cyclone.”
On Monday, the National Weather Service used that term to describe the developing weather, calling it a “significant and strong atmospheric river” in the agency’s official Bay Area forecast.
The Bay Area is late. Since October 1, most Bay Area cities have received only 20% to 30% of normal rainfall.
“We’re behind,” Null said. “But we could be close to normal by this week.”
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