‘Atmospheric River’ Begins Soaking Pacific Northwest

Scattered showers were falling across the Pacific Northwest early Monday as meteorologists expected an “atmospheric river” to bring heavy rain and flooding across the region through the middle of the week.

The brunt of the storm system, which was described as a stream of water vapor, was expected in the mountains of northern Oregon and Western Washington, where up to seven inches of rain might flood the rivers that flow off the mountain ranges, said Dustin Guy, a meteorologist at the National Weather Service in Seattle.

“Amounts like that combined with pretty high snow levels can push our rivers up significantly,” Mr. Guy said.

A flood warning was issued for the Skokomish River near Potlatch, Wash., with the river expected to exceed flood stage early Monday, the Weather Service said.

Drivers should not attempt to drive around barricades or attempt to cross flooded areas, meteorologists said.

Parts of Oregon and Western Washington were under a winter weather advisory as heavy rain was expected into Tuesday morning, according to the Weather Service in Seattle. Lighter rainfall was forecast through Wednesday, with possible flooding beginning late Monday or on Tuesday.

Gusty winds and “distant energetic ocean waves” were expected along the coast, according to the Weather Service in Portland, Ore. That Weather Service station also advised commuters to slow down and allow for extra time on Monday morning, when rain was expected to be heavy or moderate.

“Y’all thought winter was over didn’t you?” the Weather Service in Seattle said on Twitter on Sunday. “Well it’s not.”

It is “definitely not unheard-of” to have an atmospheric river in the West at this time of year, with the end of the wet season just weeks away, Mr. Guy said.

An atmospheric river is a trail of water vapor that moves through the sky much the way a river moves over land, according to the National Oceanic and Atmospheric Administration.

Think of it as “a long ribbon of very moist air being aimed right at us,” Mr. Guy said.

The atmospheric river this week will be at least the second to pummel the Western United States in the past several months.

In October, an atmospheric river converged with a bomb cyclone to unleash heavy rains in the San Francisco Bay Area. About 100,000 customers lost power as landslides and dangerous road conditions were reported in areas across Northern California.

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