Avalanche forecast

Snoqualmie Pass

Northwest Avalanche Center
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Avalanche danger

Sat
Alpine
4 · High
Treeline
4 · High
Below
4 · High
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Highlights

 Update: High danger and very dangerous conditions expected at all elevations. Expect very stormy conditions Friday, making it difficult to identify avalanche terrain and manage your risk. Natural avalanches will occur, especially at mid and upper elevations. Choose simple terrain, where you can easily avoid traveling into or underneath slopes greater than 35 degrees. 

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    2

    Update: Changed avalanche problem to reflect overnight observations and precip intensities. At mid and lower elevations expect storm slabs. At mid and upper elevations you may find the problem presents more as a wind slab.

    Friday appears very windy at the Pass. This should easily build large new slabs on the lee sides of any open feature even below treeline. As snow continues to fall and the wind continues to blow, we expect many of these slopes to become overburdened and fail. This will result in large natural avalanches, especially near and above treeline. While it’s easy to identify and avoid the large wind-loaded bowls near the ridgeline, during wind events like this you’ll also want to watch out for more insidious pieces of terrain further down the slope. You may encounter wind slabs on the sides of gullies, subtle mid-slope features, the base of cliffs, and even in treed terrain. With strong and swirling winds, dial it back and stay off of all slopes greater than 35 degrees and limit your exposure to areas where avalanches can run and stop at mid and upper elevations. 

    While the wind seems to define this storm, you may still see soft storm slabs in other areas. On Thursday thin slabs failed on a layer of graupel allowing them to propagate easily. If avalanches fail on this same layer Friday, they may trigger remotely or break widely. Even though you’re out of the wind doesn’t mean you’re out of danger. Avoid all slopes greater than 35 degrees as this storm plays out.

    (2/25/2021) This small human triggered storm slab failed on a layer of graupel Thursday in the Alpental Valley. Similar observations were reported from other parts of the zone. NE, 4500' 6" deep. SS-ASc-D1-R2-I Photo: Ian Nicholson

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    3

    The recent atmospheric river Feb 21-22 significantly tested the deeply buried weak layers in our snowpack and produced several very large and destructive avalanches. While it’s easy to downplay this storm in comparison, we’re still expecting a significant loading event over a very short time frame. You could see isolated deep slab avalanches occur due to heavy precipitation or if another slide acts as the trigger. If an avalanche fails on these deeper layers it would likely be 8-10ft deep, entrain significant amounts of snow, break trees, and run to the valley bottom. Avoid traveling in the runout zones of very large avalanche paths Friday.

    (2/24/2021) Heli-bombing teams triggered a very large avalanche off Denny Mountain, above I-90 at Snoqualmie Pass. The destructive powder cloud of the avalanche in motion is well captured.

Avalanche Discussion

Coming on the heels of such an active weather period, this storm may feel mundane. However, it is not. You could find around 2 feet of new snow and very strong winds at the Pass on Friday. As a result, expect natural avalanches particularly during periods of heavy precipitation and intense blowing snow. Avalanches may fail within the storm snow or at its base (2/25). Recent observations found graupel on W-N-E aspects and a thin sun crust on E-S-W aspects that allowed slabs to fail easily. 

Deeper layers continue to linger in the snowpack. The most prominent persistent weak layer continues to be the mid-January facet-crust. Several very large slides occurred on these grains this week. Many broke mature timber and ran to valley bottoms. At this point, these grains sit 8-10 ft below the snow surface. While triggering this layer has become very difficult any resulting avalanche would likely be unsurvivable. 

While we tend to focus on the snow under our feet, don’t forget about the snow hanging over your head. Cornices grew very large and overhung during the recent storms. As they continue to grow, some of them will fail and may act as very large triggers for the slopes below.

During the ongoing Covid-19 pandemic, continue to follow the latest guidance from local, state, and federal health officials. Avoid unnecessary risks that could impact first responders and stress medical resources.  Wear a mask when appropriate, practice social distancing, and consider recreating closer to home.

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