LOS ANGELES — As much of the Eastern United States begins to assess the damage wrought by a punishing winter storm, parts of the West are bracing for another severe weather event: an “atmospheric river” that forecasters say is likely to bring days of heavy rain and snow.

The “deep and fast-moving” storm system — a channel of wind in the atmosphere that transports water vapor from the tropics — had already begun pummeling parts of northwest California and Oregon on Tuesday. The system was expected to continue through much of the week, delivering excessive rainfall that could cause flash flooding, mudslides and debris flows, forecasters said.

At times, the rate of rainfall could be up to an inch per hour, said William Churchill, a forecaster and meteorologist with the National Weather Service’s Weather Prediction Center in College Park, Md.

Though the West often experiences atmospheric storms, what made this one unusual, he added, was the expected strength and duration. “California in general can largely use this precipitation,” Mr. Churchill said. “Unfortunately, when too much occurs all at once, it does cause problems.”

The greatest risk, he added, was in previously burned areas along the coast, where rapid, prolonged rainfall could cause mudslides or debris flows.

As of Tuesday evening, about two to six inches of rain had fallen in the hardest hit areas, and the storm system was expected to sweep through the region between the Rocky Mountains and the Sierra Nevada, as well as the Central Plains. The system was forecast to deliver rain across the West Coast as well as parts of Central and Southern California through early Wednesday, forecasters said, adding that a few scattered flash floods were possible.

Close to five million people in the Seattle and Portland, Ore., metro areas were under high wind warnings on Tuesday, with sustained wind speeds reaching up to 30 miles per hour and gusts of up to 60 miles per hour, Mr. Churchill said. The Seattle-Tacoma International Airport had recorded gusts of more than 50 miles per hour on Tuesday, he added. “That’s the more damaging component,” Mr. Churchill said of the wind gusts.

In Portland, the heavy downpour flooded roads and rivers, while high winds felled trees and power lines, knocking out power. As of Tuesday evening, more than 135,000 customers were without power across Oregon, according to PowerOutage.us, which tracks power interruptions. More than 60,000 customers had also lost power in California and Washington State, according to the site.

The Bay Area was also pummeled early Tuesday by the storm, with rain flooding roadways across the region. In San Ramon, Calif., about 35 miles east of San Francisco, the wild weather caused the collapse of a roof at a Big 5 Sporting Goods store, local authorities said, adding that the surrounding stores had closed for roof inspections.

After a brief lull on Wednesday, the storm is expected to regain momentum, pounding a stretch from Central California to the Pacific Northwest with heavy rain and snow at higher elevations. Some regions already soaked on Tuesday could receive up to seven more inches of rain, according to Mr. Churchill, the Weather Service forecaster.

The area likely to be hardest hit, he added, was the port city of Eureka, Calif., and the surrounding region. Portland and Seattle are also expected to receive around two to three inches of additional rainfall, Mr. Churchill said, adding that parts of Southern California would see rain on Saturday and Sunday.

The “unsettled” weather is expected to linger through the weekend, according to the National Weather Service, bringing showers and thunderstorms to parts of eastern Texas and the lower Mississippi Valley.

The majority of the Central and Eastern United States however, will finally experience a reprieve from the bitterly cold holiday weekend temperatures, according to the Weather Service.

“After a bone chilling Christmas weekend,” the service said, “the final days of 2022 are forecast to be much more comfortable.”

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