The first atmospheric river storm of the season is forecast to slam into Northern California starting Wednesday, a powerful system expected to bring the rainiest weather to the Bay Area in nearly nine months.

Rain will continue throughout the week, forecasters said Monday, peaking with the highest amounts on Friday, and likely bringing wet conditions across 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 little drive-by fronts in the past month with a tenth or two-tenths of an inch of rain. This will be the first one with significant accumulations. The storm door opens Wednesday.”

The bull’s-eye of the system 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. By Sunday, up to 9 inches could fall in the Russian River watershed there — or 20% of the annual rainfall for that area in less than a week, he said.

“A few big storms make or break the water year,” Ralph said. “This one is looking like it is setting up to be a big kickstart for the winter.”

Ralph and his colleagues developed a scale that ranks atmospheric river storms between 1 to 5, with 5 being the strongest. This storm is expected to be a 3 or a 4, he said, from the Bay Area to the Oregon border.

By Sunday, rainfall totals could exceed 10 inches by along the coast in places like Eureka and Mendocino County, according to the National Weather Service, which issued a flood watch for that area through Thursday. In the North Bay, 6 to 9 inches are forecast.

Amounts will drop off, but still be significant south of the Golden Gate. San Francisco should see 2 to 4 inches by Sunday, with 2 to 3 inches in the East Bay and the Peninsula, and 2 inches in the South Bay. At higher elevations, such as the Santa Cruz Mountains and Big Sur, 3 to 4 inches are possible by Sunday, the National Weather Service said.

“It’s definitely it’s time to get the umbrellas out,” said Dial Hoang, a meteorologist with the National Weather Service in Monterey. “And if you can, clean out gutters and storms drains before Wednesday.”

Some localized flooding is expected he said, and creeks will rise, but authorities do not expect flooding on major rivers. Up to 2 feet of new snow is also forecast at the highest elevation in the Sierra Nevada, but this storm is generally expected to be warmer than many.

How long has it been since the Bay Area received a real winter storm? On Friday, San Francisco is forecast to get 1.1 inches of rain. The last time it received that much was nearly 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 in an area north of Hawaii.

Atmospheric river storms are the biggest “rivers” on Earth. Moisture-rich storms that often start in the tropics, they flow through the sky up to 2 miles above the ocean and carry 25 times the volume of the Mississippi where it flows into the ocean.

When high-pressure ridges off the coast block atmospheric rivers from hitting California, diverting them to Canada or the Pacific Northwest, the state can enter droughts. 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 barrel in from the ocean to California, delivering a moisture-laden punch. They are critical for the state’s water supply.

In a typical year, California receives about a dozen big atmospheric river storms, which account for roughly 50% of its precipitation, Ralph said.

Does the upcoming rain mean California will dodge drought conditions next summer? Nobody knows. There is no correlation between the amount of rain California receives in October and November, and how much ultimately is delivered by April when the winter season ends.

“I’m cautious about a big storm early in the season indicating a chance of a continued wet season,” Ralph said. “There’s no guarantee. But at least if you start off wet you are in a better position.”

Null agreed. The Bay Area receives roughly 75% of its annual rainfall in just four months: December, January, February and March, he noted. Those most important months are yet to come.

Still, after a huge winter in 2022-23 where the Sierra snowpack was the biggest in 40 years, and another solid season this past winter of 2023-24, where the snowpack was at 113% of average on April 1, nearly all of California’s reservoirs remain at or above historical averages. On Monday, the main reservoirs were at 112% of historical capacity for this date, according to the state Department of Water Resources.

The rains also will douse a landscape that has been at risk of wildfire since June.

“This will be a fire season-ending storm for major portions of Northern California,” Ralph said.

Southern California is forecast to receive small amounts later this week, but less than 1 inch, meaning fire danger there will remain.

This storm is a slow-moving event that will be drawn in due to a rapidly dropping low-pressure system in the Pacific Northwest. When atmospheric pressure drops suddenly this way, meteorologists call it a “bomb cyclone.”

On Monday, the National Weather Service was using that term to describe the unfolding weather, and called it a “significant and strong atmospheric river” in the agency’s official Bay Area forecast.

The Bay Area is overdue. Since Oct. 1, most Bay Area cities are at only 20% to 30% of normal rainfall.

“We are running behind,” Null said. “But by this time week we could be close to normal.”

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