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 experiencing 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 vapour from the tropics – had already begun pummelling parts of northwest California and Oregon on Tuesday, and it killed five people in three car crashes all involving felled trees, Oregon authorities said.

The system was expected to continue through 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 (2.54cm) per hour, said Mr William Churchill, a forecaster and meteorologist with the National Weather Service’s Weather Prediction Centre in College Park, Maryland.

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. 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 Wednesday afternoon, about 3-6 inches of rain had fallen in the hardest hit areas, Mr Churchill said.

The storm system was expected to sweep through the region between the Rocky Mountains and the Sierra Nevada, as well as the central Plains. Areas across the West Coast as well as parts of Central and Southern California saw rain through early Wednesday.

Another system was expected on Thursday, resulting in more rain to the Pacific Northwest southward to central California, forecasters said, adding that this “unsettled weather pattern” would likely continue into the weekend.

Close to 5 million people in the Seattle and Portland, Oregon, metro areas were under high wind warnings on Tuesday, with sustained wind speeds reaching up to 30mph (48kmh) and gusts of up to 60mph, Mr Churchill said.

The Seattle-Tacoma International Airport had recorded gusts of more than 50mph 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 Wednesday afternoon, some 50,000 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.

Oregon State Police said the severe weather had caused at least three fatal accidents on highways.

The Bay Area was also pummelled early Tuesday by the storm, with rain flooding roadways across the region. In San Ramon, California, 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.

In other parts of the Bay Area, roads were closed after high winds knocked down trees and power lines, local authorities said. A lane on a major interstate highway was also closed because of flooding, causing delays.

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 7 more inches of rain, Mr Churchill said.

The area likely to be hardest hit, he added, was the port city of Eureka, California, and the surrounding region. Portland and Seattle are also expected to receive around 2-3 inches of additional rainfall, Mr Churchill said, adding that parts of Southern California would see rain on Saturday and Sunday.

As the weather system lingers through the weekend, according to the National Weather Service, it is expected to bring showers and thunderstorms to parts of eastern Texas and the lower Mississippi Valley.

The majority of the Central and Eastern US, 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 final days of 2022 are forecast to be much more comfortable.” NYTIMES