By Haley Brink, CNN meteorologist
Updated: Tue, 01 Mar 2022 01:41:14 GMT
An "extreme" atmospheric river is hitting the Pacific Northwest, bringing the threat of avalanches and flooding that will persist for days.
"A slow moving low pressure spinning over the north Pacific will result in an atmospheric river (a narrow ribbon of enhanced moisture transport) slowly shifting across the PacNW," the National Weather Service (NWS) office in Portland tweeted. "This will produce periods of moderate to heavy rain to begin the workweek."
Flood watches and warnings were in place Monday for over five million people across Idaho, Oregon and Washington, including Seattle. As of Monday afternoon, Seattle had received more than three and a half inches of rain in just 48 hours.
Over two inches of that has fallen on Monday alone, making the day the fourth-wettest February day on record (and rain was still falling).
Heavy snow has prompted winter weather advisories across portions of Washington, Idaho and Montana and avalanche warnings across the Cascades.
Avalanches triggered through Monday afternoon could be large enough to bury or kill you, according to the Northwest Avalanche Center. "This won't be the day to try to tiptoe around the hazard."
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High risk for avalanches
The "extreme" Level 4 out of 5 atmospheric river is bringing heavy snow to the Cascades where an additional 2 to 4 inches is forecast through midweek.
On the atmospheric river scale, an "extreme" event is characterized by "mostly hazardous" but also "beneficial" in terms of the amount of rain expected, according to the Center for Western Water and Weather Extremes (CW3E).
"Wet" avalanches typically occur when warm air, sun or rain cause water to drain into the snow pack, in turn decreasing the strength of the snow.
And with warming temperatures and excessive rain in place across higher elevations, the threat of avalanches was "high" on Monday.
The Washington Cascades, from the Canadian border south to the Columbia River, are under avalanche warnings through 6 p.m. PST.
"Avalanche warnings have been issued for almost all of our forecasting zones," the Northwest Avalanche Center said in a tweet. "Avoid travel in or below avalanche terrain."
Snoqualmie Pass and Stevens Pass were closed in both directions Monday morning. Snoqualmie was expected to open later in the day and Stevens on Tuesday, according to Washington's Department of Transportation.
"Widespread avalanches large enough to bury and kill you are expected at Snoqualmie Pass on Monday," the avalanche center said.
They warned avalanches could become very large and run for very long distances, all the way down to valley bottoms with "wet and destructive debris."
Avalanches are not the only threat with this system. Flooding could linger for much of the week.
Heavy rain and snow melt lead to flooding
As heavy rain falls across freshly fallen snow and current snow pack, melting is expected to happen, which will worsen flood conditions.
"Increased runoff from some [snow] melt will act as an addition to the rain-rates bringing potential for higher than normal runoff and flooding conditions," the Weather Prediction Center said.
Several rivers across Washington are forecast to reach moderate to major flood stage this week.
"With this much rain and rising snow levels, area rivers will rise, with some forecast to flood," the NWS office in Seattle said. "The current forecast suggests potentially most area rivers off the Cascades reach minor flood stage Monday night into Tuesday, with moderate river flooding possible on the Skokomish, Snoqualmie, Nisqually and Cowlitz."
A Level 3 out of 4 "moderate" risk of excessive rain was in place across portions of Washington and Oregon's coastal mountains and the southern Washington Cascades. A Level 2 threat encompasses the remaining Cascades and coastal areas of Washington and northern Oregon.
The heaviest rain is expected through Tuesday morning, with 24-hour rainfall totals of 3 to 6 inches forecast along portions of the Washington and Oregon coasts. Isolated higher rainfall amounts are possible across portions of the Cascades during the same time span.
The heavy rain event is coming off of a relatively dry start to February. As of Sunday morning, the Seattle-Tacoma airport had only received just over 1 inch of rain this month, which is more than a 2.5-inch deficit for the month of February. By Monday afternoon, that deficit had been made up and then some as the city essentially received its entire February average in a 48-hour period.
While the atmospheric river event is forecast to wane by Wednesday, rain and snow showers are forecast to remain in place, albeit much lighter, through the remainder of the week, until quieter weather returns this weekend.