Bollettino valanghe

Selkirk Mountains

Idaho Panhandle Avalanche Center
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Pericolo valanghe

Thu
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 8 months ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Warming temperatures and light rain will add stress to the slab sitting above multiple problematic weaker snow layers and crusts, increasing your chances to trigger a large slide. Wetting rain may cause dry surface snow to quickly loose cohesion and sluff, which could provoke a failure in the complex array of buried weak layers. Give yourself a wide margin of safety by choosing lower-angled slopes not directly exposed to steeper overhead hazards.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    Multiple faceted rain crust exists in the upper snowpack below an overlying slab that ranges one to two feet thick. We have not received reports of human or machine-triggered slides since these weaker layers got buried, but they have continued to perform in tests throughout our forecast zones and produce clean failures with easy to moderate initiation. Then on Tuesday, we received the first report of the lower crust producing propagating failures in Extended Column Tests - likely a function of the slab above settling and becoming more cohesive.

    The introduction of warmer air temperatures and light rain Wednesday will add extra stress to buried weak snow layers and help nudge them closer to the edge. Rain totals may not be enough to provoke many natural failures, but could enhance slab stiffening to where these layers are more prime to fail from the extra nudge of a rider or machine. With adjacent slopes and terrain features well-connected and bearing the same concerning structure, triggering an avalanche in one or both of these faceted crust has the potential to break wider than you expect and above you on the slope.

    Manage your terrain choices and give yourself a wide berth from steeper, wind-drifted slopes to help reduce your avalanche risk. Persistent weak layers often provide little warning signs before failing, upping the chances of getting caught off guard in a large, dangerous slide. Travel with a cautious mindset, continually evaluate conditions, and be ready to peel back on your travels if signs of unstable snow or fresh avalanches present themselves.

    Stability test results on Tuesday 12/23 yeilded propagating results 1 to 2 feet below the surface on a layer of facets above the December 15th interface (AR Crust). This site was located near ridgetop at about 6200 feet.

  • Loose Wet

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–1.5

    Light rain wetting the cold, dry snow surface could introduce wet snow hazards. This will likely be in the form of rollerballs, pinwheels, or loose wet sluffs. While generally harmless compared to slab avalanches, wet snow sluffs tumbling downhill could be all that’s needed to provoke a deeper failure in the complex array of faceted crust below. Monitor how wet the surface gets and stick to lower-angled slopes if conditions turn gloppy. Wet snow slides will likely originate first around cliff bands and on slopes over 35 degrees. Avoid lingering below steeper terrain or in depressions as wet slides can entrain additional snow volume making a bigger pile at the bottom.

    This image from December 9th highlights similar conditions that may arise today. Warm temperatures at middle elevations and the potential for light rain will work to destabilize the dry surface snow. Rollerballs, pinwheels, and Wet Loose avalanches will be the first sign of instability, but have the potential to add stress to buried weak layers and initiate larger slab avalanches.

    Photo: Mikey Church

Avalanche Discussion

Don’t dismiss the moderate danger rating as a green light to huck without concern today. The moderate danger rating can be very tricky to understand, and forecast, because it covers a broad range of conditions, to include large human triggered avalanches in isolated areas are possible. The extreme weather events of the past couple weeks have left us with a complex array of crusts and weak layers in the upper snowpack. Although the exact depth and sensitivity vary, the basic setup is widespread: multiple crusts with faceted snow above, below or both. As the overlaying slab gains strength and weight the potential for a wide breaking human triggered avalanche increases too. The biggest challenge with navigating a Persistent Weak Layer problem is managing their inherent uncertainty. PWL’s often provide little or no feedback until something finally happens. If that thing that happens is a person dropping into a bowl… well, that’s a bad thing. There are two acceptable ways to manage this problem type. 1) Spend time digging into the snowpack to perform stability tests, such as the Extended Column Test, that help inform your decisions on whether a slope is safe or not. Or, 2) Avoid avalanche terrain. One of these options is simpler and more full-proof than the other. Even with training and experience assessing slope stability in the field can be tricky and involves a degree of uncertainty that can catch people off guard.

Moderate danger rating also includes small avalanches in specific areas. Today we are concerned about the new snow from the past couple days being wetted for the first time. If rain totals exceed those forecasted, or are prolonged at the near tree line elevation band, the possibility of small Loose Wet avalanches cannot be dismissed. A skier or rider triggered small wet sluff could step down to provoke the above-mentioned PWL’s.

 If you are looking closely, you will notice that we have two different forecasts differentiating the northern zones and the Silver Vally/ Northern Bitterroot. This is due mainly to the continued lack of snowpack around the compass rose at our lowest elevation band in the SV/NB zone. In the Selkirks and West Cabs the situation is a little more nuanced. While snowpack on southerly aspects may remain too shallow to support winter recreation, those on the north aspect will find a deeper snowpack that is possible to travel on. While the overall depth is much shallower than elevation bands above, thus limiting the size of potential avalanches, the same widespread PWL’s effecting the upper elevation bands are present all the way down the mountain side. Green shading means Low danger, rather than no avalanche danger.

Recent incremental increases in the snowpack have added nice snow on top, especially in our northern zones. Take advantage if you can. Our next chance of snow accumulation is forecast for Friday into Saturday, followed by a period of stable weather. This is starting to get ridiculous. It’s time to do the snow dance, burn skis, appease Ullr…what ever it takes. In the meantime, we hope everyone has a safe and happy holiday season.

Regioni coperte