Bollettino valanghe

West Slopes South

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 will be found within the West South zone on Monday. Travel in avalanche terrain on or near the volcanoes is not recommended. If you are closer to the crest, cautious route-finding and conservative decision-making will be essential.

Problemi valanghivi

  • Wet Slabs

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

    The current snowpack above the 12/9 crust will continue to be of concern for Wet Slab avalanches on Monday in areas where a widespread avalanche cycle has not yet occurred. With continued loading Sunday night and Monday, water could percolate through the snowpack and pool on this crust, weakening the bonds between snow crystals to release large natural avalanches. The timing of wet avalanches is very difficult to predict and they can act in surprising ways, so minimize your exposure by choosing safe travel routes, staying off slopes over 35 degrees, and being aware of terrain above you at all times. 

    (12/20/2020) This was one of many D2 natural avalanches that ran on Saturday night in the Crystal backcountry. Slabs were 40-80 cm deep and similar human triggered avalanches have been reported in the area via our public observations platform.

  • Storm Slabs

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

    Above the rain/snow line, 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. Expect to find multiple interstorm layers capable of producing a failure within the snow above the 12/9 crust. Use small slopes and hand pits to gather information about which storm layers have the potential to fail and how they may react to your weight. 

    (12/20/2020) Shooting cracks were observed on Sunday in the White Pass area showing that instability still exists in the upper snowpack and there is slab potential.

Avalanche Discussion

Saturday night produced a widespread avalanche cycle in the Crystal Mountain backcountry, and reports are coming in of natural avalanches from the Paradise and White Pass areas as well. At this point, we still have some uncertainty around which parts of the zone have reached their tipping point and are played out, and which areas are still waiting to produce large natural avalanches. Moving forward, the West South will continue its pattern of wet and windy conditions on Sunday night and Monday. The volcanoes are set to receive 3” of precipitation while areas closer to the crest receive less, as is often the case. The difference between this storm and Saturday night is that you could find that peak instability is during the day on Monday as temperatures warm and precipitation switches from snow to rain, producing a natural avalanche cycle. With the amount of input the volcanoes are set to receive, we can’t ignore that load and not forecast High avalanche danger, although it is not without uncertainty. As we mentioned yesterday, 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. Wet loose avalanches should begin first as snow surfaces become wet and weak with the onset of rain. 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 below the rain/snow line well after the rain has stopped. 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. Even though we are focusing a lot of messaging on the volcanoes, do not be surprised to see natural avalanches on Sunday closer to the crest. If you find that there are natural avalanches happening all around you, then you should avoid all avalanche terrain. 

Prior to Saturday night’s rain event, the snowpack around the West South contained 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). So far, we have not confirmed any avalanches on these weak grains in the West South, although some reports out of Crystal on Sunday confirmed weak snow grains as the culprit of slabs breaking 40-80cm deep. We are not out of the woods yet as there are plenty of slopes that haven’t slid which we expect should at some point. The additional load from this storm could be all that is needed to tip the scales and produce larger, more devastating avalanches.

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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