Bollettino valanghe

Snoqualmie Pass

Northwest Avalanche Center
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Pericolo valanghe

Mon
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

A dangerous storm is set to bring wet and windy conditions to Snoqualmie Pass on Saturday night. Give the snowpack time to adjust to the new load on Sunday. Avoid travel on or below slopes 35 degrees and steeper, and steer clear of large avalanche path run-out zones.

Problemi valanghivi

  • Wet Slabs

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

    The current snowpack contains up to 3 feet (a meter) of snow sitting on top of some weak snow crystals (12/13) which are just above a crust (12/9). Wet avalanches are notoriously difficult to predict since they are not well understood, but avalanche danger often peaks during and shortly after periods of heavy precipitation, which in this case is Saturday night. Wet loose avalanches should begin first as snow surfaces become wet and weak. These avalanches are capable of entraining lots of snow and running far distances, even on low angled slopes. They can even trigger slabs as they descend. Wet Slabs often don’t occur until water is able to percolate into the snowpack and destabilize a weak layer. This can happen hours after the onset of precipitation, meaning the snowpack should still be capable of producing wet avalanches on Sunday below the rain/snow line. 

    The current snowpack above the 12/9 crust checks off many boxes for a Wet Slab problem and water could percolate through the snowpack and pool on this crust. In this scenario, the water could weaken the bonds between the 12/13 snow crystals and release large natural avalanches. The timing of this process is always difficult to pin down, and therefore these avalanches are very difficult to predict and act in surprising ways. Luckily, you can minimize your exposure by choosing safe travel routes, staying off slopes over 35 degrees, and being aware of terrain above you at all times. Being caught in a Wet Slab avalanche would have devastating consequences.

    (12/19/2020) Small (D1) Wet Loose avalanches were observed throughout the Alpental Valley on Saturday morning. Expect more wet avalanche problems on Sunday.

  • Storm Slabs

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    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2

    If you happen to make it through the wet and nasty to elevations above the rain/snow line on Sunday, you should find significant new snowfall and dangerous Storm Slabs. The new snowfall combined with strong winds will thicken existing slabs sitting above the 12/9 crust. Avalanches could break within the new/wind loaded snow or involve all of the recent snowfall from this past week by breaking on weak snow grains a few inches above the 12/9 interface. So far, we have not confirmed any avalanches on these weak grains at Snoqualmie Pass. However, several avalanches occurred on this layer at nearby Stevens Pass. The additional load from this storm could be all that is needed to tip the scales in our zone. Sunday isn’t the day to hang out in places where an avalanche may run or stop. Use small slopes and hand pits to gather information about which storm layers fail and how they may react to your weight. 

    (12/18/2020) There is roughly a meter of snow over the 12/9 crust at middle and upper elevations at Snoqualmie Pass., and that depth could grow Saturday night. Avalanches would be large enough to bury and kill you if this slab is released.

Avalanche Discussion

Saturday night should be a significant rain event for Snoqualmie Pass and we expect a natural avalanche cycle because of it. Even though there was a new rain crust found on the surface on Saturday, the rain should make quick work of breaking this crust down and entering the snowpack. If this event were happening during the daylight hours, we would be forecasting High avalanche danger at most elevation bands. Since we are lucking out, relatively speaking, by the peak instability happening overnight and no further significant loading on Sunday, we are avoiding rating the danger as High. The avalanche danger should be highest in the morning, with cooling temperatures and tapering precipitation helping the snowpack to adjust to its new load throughout the day. Make no mistake, just because the rain has stopped and the temperature is dropping doesn’t mean that avalanches are done. You should not be surprised to see a natural avalanche on Sunday. As you ascend in elevation above the rain/snow line, realize that your avalanche problem will be changing from wet to dry slabs and you will need to adjust your mindset. If you find that there are natural avalanches happening all around you, the danger may is heightened and you should avoid all avalanche terrain. 

The warmup and rain will create heavy snow and challenging travel conditions in addition to the avalanche danger. Anticipate open creeks, thin coverage, and plenty of obstacles to navigate. 

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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