The first atmospheric river storm of season is forecast to hit Northern California on Sunday and Monday. And it’s not a small one — a 4 on the scale of 1 to 5.
“There’s a wet storm coming,” said Marty Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego. “It should be largely beneficial. And it’s going to be fairly long lasting — over 48 hours.”
But most Bay Area residents probably won’t need their umbrellas.
The storm, powering in from the North Pacific, will mostly hit the far northern part of the state, bringing 2 to 3 inches of rain in Eureka and other communities near the Oregon border, forecasters said Friday.
Farther south, there will be significantly less rain. About half an inch is expected late Sunday and Monday in Santa Rosa, Cloverdale and other parts of the North Bay, according to the National Weather Service.
San Francisco may get one-tenth of an inch, with much of the rest of the Bay area forecast for less farther south.
“It looks like any really measurable rain is going to be north of the Golden Gate,” said Jan Null, a meteorologist with Golden Gate Weather Services in San Mateo County. “There will be drizzle in Half Moon Bay and some other parts of the Bay Area.”
It’s been four months since the Bay Area received any significant rain. May 3 was the last time, when .54 inches fell in San Francisco.
The storm will shift Northern California’s weather from a summer pattern into more of a fall and winter feel.
The rain will bring major benefits, experts say. It is expected to help firefighters who are battling several lightning-started wildfires in rural forests near the Oregon border — including the 96,000-acre Six Rivers Complex Fire, which is burning in Del Norte County and was 79% contained Friday, and the 30,000-acre SRF Complex Fire, which is only 7% contained, and burning in Humboldt and Del Norte counties.
Those fires have been responsible for much of the smoke that has drifted in the Bay Area this week.
On Friday, the National Weather Service issued a flash flood watch for both Humboldt and Del Norte counties Sunday afternoon through Monday evening due to the incoming storm.
All that rain will slow the fires substantially, but also will douse their smoke and blow much of it west and north, experts say.
“It may not fully extinguish these fires,” said Daniel Swain, a climate scientist at UCLA. “A lot of the fires burning up there right now are in heavy heavy timber. But it really will dramatically reduce fire activity and if it’s followed up by another rain event that might really be the end of first season up there.”
For the Bay Area and other locations downwind, “it’s going to dramatically improve air quality,” Swain added.
Atmospheric river storms are the biggest “rivers” on Earth. Moisture-rich storms that often originate near Hawaii, they flow through the sky up to 2 miles above the ocean and carry twice the volume of water per second as the Amazon River and 25 times the volume of the Mississippi where it flows into the ocean.
When high-pressure ridges off the coast block atmospheric rivers from California, diverting them to Canada or the Pacific Northwest, California can 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.
When the ridges are gone, as they are now, the storms can move from the ocean into California, delivering a moisture-laden punch. They are key to the state’s water supply. In a typical year, California receives about a dozen such storms, which account for roughly 50% of its precipitation.
This past winter, the state received 31 atmospheric river storms, said Ralph, of UC-San Diego.
“They really broke the landscape drought and reservoir drought in most of California,” he said.
They also delivered the largest snowpack to the Sierra Nevada in 40 years. All that snow, and high levels of moisture in soils and trees, are a key reason that California has had a very mild fire season this summer.
As of Monday, 257,405 acres had burned in 2023 in California on lands overseen by Cal Fire, the state’s leading firefighting agency, and the U.S. Forest Service. That is just 22% of the five-year average over the same dates.
“It’s rather nice to be in a window where there where there aren’t ongoing concurrent fire crises in multiple parts of the state,” Swain said.
California’s water outlook for next year is very positive also.
On Friday, the two largest reservoirs in the state, Shasta near Redding, and Oroville, in Butte County, were 74% and 76% full — roughly 130% of their historical average for this date. San Luis, between Gilroy and Hollister, was 82% full — double its historic average. And the largest reservoir in Southern California, Diamond Valley, in Riverside County, was also 82% full.
With the winter rainy season about two months away from beginning in earnest, water managers are more concerned at this point with too much, rather than too little rain.
El Niño conditions are shaping up in the Pacific Ocean. They form when winds shift, causing a changing in the upwelling of cool waters, which heats the temperature of the water’s surface.
Although El Niño years are not a guarantee of wet winter conditions, they do increase the likelihood, particularly in Southern California. Several of California’s wettest winters, like 1982-83 and 1997-98, have occurred during strong El Niño events.
After the drought-busting storms of this spring, water managers and dam operators deliberately let some water out of reservoirs to free up space to catch more water and reduce the risk of downstream flooding — space they will need if this winter is another wet one.
