Heavy rains in the mountains of Oregon and Washington are expected to cause flooding starting on Monday, forecasters said.
The Pacific Northwest is bracing for what meteorologists call an “atmospheric river,” a stream of water vapor that is set to arrive from the Pacific Ocean on Sunday night and bring heavy rains and flooding through Wednesday.
The brunt of the storm system 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.
It was too early on Sunday afternoon to know which rivers might have the most flooding, so the Weather Service had not yet posted any flood warnings, Mr. Guy said.
Parts of Oregon and Western Washington were under a winter weather advisory as heavy rain was expected from Sunday night into Tuesday morning, according to the Weather Service in Seattle. Lighter rainfall is in the forecast through Wednesday, with possible flooding beginning late on Monday or on Tuesday.
Gusty winds and “distant energetic ocean waves” were expected along the coast, according to the Weather Service in Portland, Ore.
“Y’all thought winter was over didn’t you?” the Weather Service in Seattle said on Twitter on Sunday morning. “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 one 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.
