SEATTLE – The Pacific Northwest is on alert for potential flooding and avalanches as a series of atmospheric river storms slam the region with heavy rain and mountain snow this week.
It was a wet and snowy start to the weekend across the region, with several inches of snow falling in the Cascades, including more than a foot at Mt. Rainier’s Paradise Ranger Station.
Interstate 90 through Snoqualmie Pass near Seattle was closed for about an hour on Friday after 30 semi-trucks spun out on snow-covered roads.
One person was killed in a crash in Stevens County in northeastern Washington after a driver lost control on a snow-covered road and hit a tree, according to the Washington State Patrol.
Troopers said more than two dozen crashes were reported in central and eastern Washington, including in the Tri-Cities area.
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The first atmospheric river storm pushed into the West Coast on Sunday, bringing heavy rain up and down Interstate 5 in Washington and Oregon.
“Think of a massive cloud of water vapor hovering along the ocean traveling to the West Coast,” FOX Weather Meteorologist Craig Herrera said. “It gets to the West Coast, and you have this huge mountain range here. You’ve got another one in the Cascades. But banking up against the mountains, it creates lift, squeezes out the moisture, and you just get round after round of rain.”
And, Herrera added, since the moisture is from a warmer source, the snow levels will go up in elevation and melt the snow that has already fallen.
“The major concern here is that you’re going to be seeing a lot of that flooding rain,” he said. “The lower-lying areas could receive an additional 3 to 8 inches of fresh rain on top of what has already fallen.”
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As the rain continues to fall, rivers and creeks will rise.
Flood Watches are now in effect for all mountain-fed rivers in western Washington and northwestern Oregon through Wednesday.
The Seattle and Portland, Oregon, metro areas could see an additional 3-5 inches of rain, leading to flooding potential on multiple rivers across the region and areas of urban flooding closer to the city centers.
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“You’ll also watch for debris flows, especially over burn-scar areas,” Herrera said.
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Snow is also expected to continue to pile up across the Cascades, with an additional 1-1.5 feet of new snow by Sunday night.
Winter weather alerts are in effect across the Intermountain West.
“We’ve got these winter weather advisories extending over the Intermountain West, and now you think about all that runoff,” Herrera said. “We’re talking Avalanche Warnings for the Cascades.”
The Northwest Avalanche Center in Seattle issued Avalanche Warnings for Stevens and Snoqualmie passes, the western slopes of the southern Washington Cascades and Mt. Hood in Oregon.
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“Heavy snow, rapidly warming temperatures, and a transition from snow to rain will lead to very dangerous avalanche conditions Saturday night and Sunday,” the National Weather Service said. “High avalanche danger exists at all elevations.”
The combination of the warmer air and rain will cause snow on the ground in the Cascades and Olympics to melt. And by the middle of the week, several rivers are expected to rise to flood stage, with moderate to major flooding possible.