Two avalanche warnings have been issued for parts of Washington and Oregon as heavier-than-usual rainfall and snowfall is expected to hit part of the West Coast through Monday.
One of the warnings, a Level 4 on a scale of 5, said there was a high avalanche danger for parts of North Cascades National Park by the Canadian border, extending south through Okanogan-Wenatchee National Forest and into parts of Gifford Pinchot National Forest, which is about 140 miles southeast of Seattle.
The warning was in effect until Monday evening and also covered part of Mount Hood National Forest, which is about 70 miles east of Portland, Ore.
A separate, Level 5 warning, indicating extreme danger, also covered smaller parts of the Okanogan-Wenatchee National Forest, about 80 miles west of Moses Lake, Wash. That warning, which said heavy snow, strong winds and warming temperatures could create avalanche conditions, was also in effect until Monday evening.
The warnings were issued by the Northwest Avalanche Center, which said at least 30 people in the United States had been killed in avalanches so far this season. That’s the highest number of fatalities since the 2015-16 season, according to the center.
That type of weather event — “a long river of moisture” that can hover over concentrated areas for a period of time — is expected to lead to very heavy rainfall or, in higher elevations, intense snowfall, said meteorologists at the Weather Service in Seattle.
The National Oceanic and Atmospheric Administration described such events as “rivers in the sky.” This one extends about 3,000 miles, from the coast of British Columbia to the coast of Hawaii, said Dustin Guy, a Weather Service meteorologist.
Though Seattle may see only about half an inch of rain, coastal areas and mountain regions can expect up to three inches, said another Weather Service meteorologist, Matthew Cullen. In high-elevation places, like the Cascade Mountains, one to two feet of snow may fall in elevations above 4,000 feet, he said.
A handful of atmospheric rivers have hit the area each season, and this one, so far, is expected to be moderate in terms of precipitation and duration, Mr. Cullen said. Still, it is coming to the region shortly after a particularly intense snowfall.
A total of about half an inch of snow fell in Seattle in December and January, he said. Then, nearly 12.5 inches of snow fell over a three-day period ending on Feb. 14.
“There was definitely several hours of just consistent snow on the order of about an inch an hour,” Mr. Cullen recalled.