Bollettino valanghe

Snoqualmie Pass

Northwest Avalanche Center
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Pericolo valanghe

Tue
Alta quota
4 · Forte
Limite del bosco
4 · Forte
Sotto il limite
3 · Marcato
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In sintesi

Very dangerous avalanche conditions are expected on Monday as new snow loads a weak snowpack. Recent avalanches, persistent grains in the snowpack, heavy snowfall, increasing winds, and warming temperatures all indicate unstable conditions. Let nature do its thing and stay out of all avalanche terrain.

Problemi valanghivi

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Very Likely
    Dimensione
    2

    Storm Slabs have been reactive the past two days, breaking naturally or by human trigger on weak snow at the 2/11 interface. As we prepare for more snow on Sunday night and Monday, slightly warmer temperatures should deliver heavier snow than we have received recently. We expect both human triggered and natural avalanches to be widespread on Monday, with larger slabs as you ascend in elevation due to the wind’s influence. If the 2/11 interface continues to be active, avalanches could propagate far distances and wrap around terrain features or be remotely triggered. Many of the slabs that were triggered so far this weekend failed easily and propagated the width of the feature. Any triggered avalanche could entrain much of the recent storm snow and gauge down toward deeper layers in the snowpack, growing large enough to kill you. You should avoid all terrain where avalanches can run or stop on Monday.

    (2/13/2021) This slab was remotely triggered by Hyak Ski Patrol on Saturday and is suspected to have broken on the 2/11 interface. [Photo: Erik Olson]

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    3

    Another major loading event will further test our buried persistent deep layers. NWAC professionals on Sunday were still getting planar failures on the Mid-January crust with deep tap tests, showing that weak snow still exists. These avalanches are difficult to predict but we have had action on these layers as recently as this past Thursday.  You may have been traveling around the zone recently and not seen any signs of instability on these deeper layers, but that doesn’t mean that they shouldn’t still be a factor in your decision making process. The lack of direct feedback may be leading you to believe that these layers have started to move toward the low-likelihood, high consequence category. We’re not there quite yet, but even if that’s true, adding a significant load plus a widespread avalanche cycle above them seems like a logical time for them to awaken. Monday is not a day to tempt fate. Any avalanche occurring on these layers would most likely be fatal for anyone caught in their path.

    (2/10/2021) This recent natural deep slab avalanche was observed from the Denny Trees area on Wednesday. It may have been triggered by a piece of cornice from above. Low Mountain, NE, 5000' HS-N-D2.5/R2-O, Photo: Jonathon Spitzer

Avalanche Discussion

The snowpack around Snoqualmie Pass has been complex recently, with overlapping avalanche problems, deeply buried weak layers, unusual wind transport, and lots of new snow. On Monday, you can distill it down to simpler terms. We have seen a lot of recent avalanches at the pass, which means that the snowpack is unstable. We are now expecting a large amount of new snow which we believe will come in much heavier than the snow we currently have near the surface. Heavy snow over weak snow is a good recipe for avalanches. Recent avalanches plus a good setup for new avalanches is the meat of the forecast and should be enough for you to know that you don’t want to be near avalanche terrain on Monday. 

If you want to nuance it, we also have a known weak layer (2/11) that has been consistently producing surprising avalanches for both professionals and recreationalists this weekend. Avalanches on Monday will be larger and could also break in surprising ways on this layer, naturally or by human trigger. The 2/11 interface was found to be 45 cm deep on Sunday by NWAC forecasters and contained weak snow grains, which is why these slides are breaking easily and propagating far distances. Throw in a few other sensitive density changes in the upper snowpack, and you have plenty of failure points for avalanches.

Then there are those buried persistent weak layers. The 2/1 crust was measured 102cm down while the Mid-January crust was measured to be 130cm from the surface on Sunday. Weak snow still exists around these layers and failures were still occurring in snowpack tests. As stated above, with so much other activity going on, those layers will be tested and we will have to wait and see what happens. We don’t have enough confidence to say that they will not be activated but we are confident in saying that we wouldn’t want to be anywhere near a large avalanche path on Monday due to the possibility of these layers failing.

You also need to recognize that so much recent snow can be a hazard on its own. Learn more about snow immersion hazard here.

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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