‘Extreme’ Atmospheric River Triggers Avalanche Warnings

“A slow-moving low pressure over the North Pacific will cause an atmospheric river (a narrow ribbon of enhanced moisture transport) to move slowly through the PacNW,” the National Weather Service (NWS) office in Portland said. . tweeted. “This will produce periods of moderate to heavy rain to start the work week.”

Flood watches are in place for more than five million people in Idaho, Oregon and Washington, including Seattle.

Heavy snowfall also triggered winter weather advisories in parts of Washington, Idaho and Montana and avalanche warnings in the Cascades.

The avalanches triggered today could be large enough to bury or kill you, depending on the Northwest Avalanche Center. “It will not be the day to try to tiptoe around danger.”
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High risk of avalanches

“the extreme” Level 4 of 5 the atmospheric river brings heavy snow to the Cascades where an additional 4 to 6 inches are forecast through Monday morning.
At the atmospheric river scale, an “extreme” event is characterized by being “primarily dangerous” but also “beneficial” in terms of the amount of rainfall expected, according to the Center for Western Water and Weather Extremes (CW3E).

This snow is expected to gradually turn into heavy rain on the high mountains during the morning as temperatures begin to warm. The rain could fall as high as 6,000 to 8,000 feet.

“Wet” avalanches usually occur when warm air, sun or rain causes water to escape in the snowpack, which lowers the strength of the snow.

And with warming temperatures and excessive rainfall in place today at higher elevations, the avalanche threat is “high” today.

The Washington Cascades, from the Canadian border south to the Columbia River, are under avalanche warning until 6 p.m. PST Monday.

“Avalanche warnings have been issued for nearly all of our forecast areas,” the Northwest Avalanche Center said. said in a tweet. “Avoid traveling in or under avalanche terrain.”
Snoqualmie Pass and Col Stevens were closed in both directions Monday morning with no estimated reopening time, according to the Washington Department of Transportation.
“Widespread avalanches large enough to bury and kill you are expected at Snoqualmie Pass on Monday,” the avalanche center said. noted.

They also warned that avalanches could become very large and travel very long distances, down to valley bottoms with “wet and destructive debris”.

Avalanches are not the only threat with this system. Flooding could persist for much of the week.

Heavy rain and melting snow cause flooding

As heavy rain falls over the freshly fallen snow and the current snowpack, melting is expected to occur which will worsen flood conditions.

“The increased runoff from some [snow] melting will act as an addition to rainfall rates, which could result in higher than normal runoff and flooding conditions,” the Weather Prediction Center said. noted.
Several rivers across Washington are expected to reach moderate to major flooding this week.
“With so much rain and increasing snow levels, rivers in the area will rise, with flooding forecast,” the NWS office in Seattle said. noted. “Current forecasts suggest that most rivers in the area off the Cascades potentially reach minor flood stage Monday evening through Tuesday, with moderate flooding possible on Skokomish, Snoqualmie, Nisqually and Cowlitz.”
A Level 3 on 4″moderate“The risk of excessive rainfall is in place Monday in parts of the Coastal Mountains of Washington and Oregon and the southern Cascades of Washington. A Level 2 threat encompasses the remaining Cascades and coastal areas of Washington and northern from Oregon.

The heaviest rain is expected Monday through Tuesday morning, with 24-hour rainfall totals of 3 to 6 inches forecast along parts of the Washington and Oregon coasts. Higher isolated precipitation amounts are possible over parts of the Cascades during the same period.

The episode of heavy rain comes from a relatively dry beginning of February. As of Sunday morning, Seattle-Tacoma Airport had received just over 1 inch of rain this month, which is a deficit of over 2.5 inches for the month of February.

While the atmospheric river event is expected to fade by Wednesday, rain showers and snow showers are expected to remain in place, albeit much lighter, for the rest of the week until warmer weather returns. quiet this weekend.

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