Bollettino valanghe

West Slopes South

Northwest Avalanche Center
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In sintesi

A dangerous storm is predicted to hit the West South on Saturday night, bringing very wet and windy weather, particularly on the volcanoes. The snowpack is going to need a minute to adjust to this new load on Sunday. So dial it back and avoid travel in areas where avalanches can run or stop. 

Problemi valanghivi

  • Wet Slabs

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

    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 snow crystals and release large natural avalanches. The timing of this process is always difficult to pin down, which can lead to surprising avalanches. 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) Wet Avalanches are difficult to predict. This Wet Loose avalanche was naturally triggered on Friday night at Snoqualmie Pass. Expect to see plenty of this around the West South on Sunday morning.

  • Storm Slabs

    • Alta quota
    • 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 in the West South. However, we continue to be concerned about this layer. The additional load from this storm could be all that is needed to tip the scales and start producing avalanches. Sunday isn’t the day to hang out in avalanche terrain. Use small slopes and hand pits to gather information about which storm layers fail and how they may react to your weight.

    (12/16/2020) Skin track test showing cracking in the surface snow. This can indicate storm slabs are present.

Avalanche Discussion

Saturday night should be a significant rain event for the West South, focused on the volcanoes as is often the case, with Paradise forecasted to receive 2.5-3.5” of water. We expect that a natural avalanche cycle will occur Saturday night across the zone. If this rain event were happening during the daylight hours, we would be forecasting High avalanche danger at all elevation bands. Since the peak instability should be happening overnight and no further significant loading is expected 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. 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. You should not be surprised to see a natural avalanche on Sunday. If you find that there are natural avalanches happening all around you, realize that the danger is heightened and you should avoid all avalanche terrain. On the flip side, if you find yourself in an area of the zone which did not receive significant precipitation, you could find that the avalanche danger is locally lower. Regardless, the warmup and rain should create heavy snow and challenging travel conditions in addition to the avalanche danger.

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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