A barrage of storms known as atmospheric rivers is expected to soak the West Coast over the next eight to 10 days, raising the risk of flooding, power outages and holiday travel disruptions leading up to Christmas in a region that has already experienced significant weather activity this season.
The bulk of the rain and snow is likely to fall between British Columbia and Northern California, with Washington and Oregon poised to have some of the wettest, sloppiest weather.
Several storms, including one that swept into the Pacific Northwest on Tuesday night, are stacked up in the forecast, said Marty Ralph, the director of the Center for Western Weather and Water Extremes at the University of California, San Diego.
“These storms are back-to-back-to-back-to-back,” said Mr. Ralph, who leads a team that studies atmospheric rivers. “This is a situation that can saturate the landscape, and then if one of the later atmospheric rivers turns out to be big and stalls, that’s where you can end up with a flood risk — substantial.”
Together, the storms could bring up to 10 to 20 inches of rain over the next week to some of the wettest locations, such as the Olympic Peninsula in Washington. While there will be some breaks, forecasters said, it’s too soon to tell if one of them will fall on Christmas or if the holiday, too, will be soggy.
“The forecast is changing every day,” said Logan Howard, a meteorologist with the National Weather Service’s Seattle office. “We have a clear picture of the current atmospheric river and then the next one, but it’s less clear what the next ones are going to look like.”
Atmospheric rivers are rivers in the sky.
Atmospheric rivers are bands of water vapor that form over the Pacific Ocean and dump rain and snow when they reach land. They usually tap into warmer air and bring snow to the highest elevations, above 7,000 feet. They’re known for often slowing down and unleashing excessive amounts of rain that can cause flooding over localized areas.
The season for atmospheric rivers generally runs from September to October in Alaska, November to December in Oregon and Washington, December through February in Northern California and January through March in Southern California. “It’s the cooling of the pole through the winter that shifts the storm track south over time,” Mr. Ralph said of the change in the winter weather pattern that typically gradually pushes the storms south.
Mr. Ralph estimates that a dozen atmospheric rivers have affected the Pacific Northwest and California since Oct. 1. In November, a strong storm drenched the Pacific Northwest and California, bringing heavy rain to the region just north of San Francisco, where some places recorded more than 20 inches. During another atmospheric river event in California last week, a tornado — a rarity in the state — touched down in Scotts Valley, a town tucked in the Santa Cruz Mountains, and injured three people.
While atmospheric rivers are typical at this time of year, Mr. Ralph said, it’s unusual to have so many back-to-back.
The Pacific Northwest is especially at risk of flooding.
The first in the series of atmospheric rivers took aim at Oregon and Washington on Tuesday, and rain and wind continued to pummel the region on Wednesday. The system was expected to bring the heaviest rain to the coastal mountains of Washington and northwestern Oregon, with heavy snow at higher elevations in the Cascade Range.
The Olympic Peninsula, the arm of land that spreads across Puget Sound from Seattle, was predicted to see the heaviest deluge in this storm, with up to five inches of rain possible atop Mount Olympus, said Scott Kleebauer, a meteorologist with the Weather Prediction Center. Northwest Oregon, including the town of Astoria, is also likely to see some rain, with Portland maybe getting “a little bit” and Northern California “getting spared from this first storm,” Mr. Kleebauer said.
Two more weak-to-moderate systems are poised to bring a one-two punch to western Washington, western Oregon and Northern California over the weekend. The first is expected to arrive late Friday night into Saturday, and the second on Sunday. “It looks to be more of an impact farther south, especially in coastal areas of Northern California and southwestern Oregon,” Mr. Kleebauer said.
The Center for Western Weather and Water Extremes said that atmospheric river activity, especially over the Pacific Northwest, was “likely to continue into next week,” putting rivers in western Washington and western Oregon at risk of flooding. The Climate Prediction Center is warning of heavy precipitation, including snow, over western Washington, western Oregon and Northern California on ” rel=”noopener noreferrer” target=”_blank”>Dec. 26 and 27.
Next Monday to Tuesday, a stronger system could bring a 30 to 40 percent chance of “24-hour rainfall totals of four inches or more on Christmas Eve” in the mountains of the North Bay, the Weather Service’s Bay Area office said. Another system early next week is likely to hit a larger part of California, including the central and potentially southern parts of the state.
Much of the West Coast is likely to see a soggy holiday, Mr. Ralph said, but “there’s a lot of uncertainty about the details.”
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