Bollettino valanghe

Snake River Range

Bridger-Teton Avalanche Center
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Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 2 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Triggering a large avalanche remains likely on many steep slopes. You can find safer travel on gentler slopes. Carefully consider the terrain above and around you today, giving steep (>35°) areas a wider margin than usual. To reduce your risk, steer clear of obvious drifts and slopes where the wind has stiffened the snow surface. 

 

Problemi valanghivi

  • Wind Slabs

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

    In addition to rain at lower elevations yesterday, an additional 8+ inches of snow fell overnight with moderate to strong west/southwesterly winds. As a result, wind slabs have formed on the lee sides of ridges and other exposed terrain features. Warm temperatures yesterday have helped these wind slabs bond to underlying snow surfaces, but more snow and wind today will create a fresh batch that may be sensitive to your weight. Over the past several days, natural and triggered wind slab avalanches have been reported here. Several storm interfaces due to shifting temperatures exist in the upper portion of the snowpack; these layers are more challenging to identify and are less obvious than a prominent weak snow layer. The best way to avoid this problem is to avoid steep slopes or consequential terrain where you find these indications of recent wind loading. Watch for snow that is denser than snow found on wind-sheltered slopes and tell-tale clues such as surface texture and a pillowed, drifted appearance. 

    While wind slabs are the most likely type of avalanche to trigger, don’t rule out slabs unaffected by the wind. Prepare to be surprised and ensure your route choices allow for these surprises by allowing a wider buffer around terrain than usual.

    (12/29/2024)

    The photo was taken by a skier in the Snake River Canyon, illustrating how winded new snow in steep terrain remains sensitive to triggering. 

    Photo: Second-hand report to Bridger-Teton Avalanche Center

  • Persistent Slabs

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

    Weaker snow layers now buried 3-4+ feet deep have not gone away, but they've become less reactive to the weight of a skier or rider. Due to their depth, they've also become very difficult to impact with your weight. Bear these in mind and don't completely write them off—especially since the consequences of triggering a large persistent slab increase as more snow falls. I continue to be wary of very steep (>35°), north-facing alpine terrain that is more likely to have thin spots where your weight may reach the weak layers.

     One avalanche outside of the boundary at Grand Targhee Resort was triggered by a skier 2 days ago on this deeper weak layer. Fortunately, this person walked away unharmed. 

     

    (12/27/2024)

    Skier triggered avalanche outside of the resort boudnary at Grand Targhee.  NE aspect at 9,500 feet. Approximate slab depth 2.5 feet.

    Photo: Daniel Nagy

Avalanche Discussion

FAQ's regarding the current snowpack

Q: How do avalanches fail on storm layers...and why?A: Multi-day storms like the one we're experiencing produce a variety of different snow crystal types. Some of these types don't get along very well and can behave much like a persistent weak layer. Like surface hoar, they're slippery. When these crystals are buried, they can remain preserved for longer than we expect and cause avalanches to fail within the new snow or at an interface between the new/old snow, particularly if storms start cool and end warm. Fortunately, the snowfall last evening did just that; however, weaker snow remains preserved in the upper portion of the snowpack from snowfall over the past 72 hrs. Storm layers can be tricky, more challenging to identify, and capable of surprisingly wide propagation and remote triggering. 

Q: How do persistent weak layers like basal facets or buried surface hoar heal?A: One way is through mechanical compaction or compression. When a very thick, dense slab overlies a weak layer, the sheer weight of the slab begins to "squash" the weak layer. Like a group of introverts at a social gathering, at some point, they have no choice but to begin to connect. We're just starting to see this mechanism in some places where the primary weak layer from early December is showing signs of relenting under the weight of the 3+' thick slab. This layer remains shallowly buried and more developed in other areas (primarily the north half of the compass, rocky, or alpine terrain). As snow stacks up, these are the spots to keep on your radar. Continually probing the snow and looking beneath the surface to observe the snow you're riding is the only way to assess this problem and track it over time. 

Thank you to all who have contributed observations during this cycle. This allows everyone to gain a better and more complete understanding of conditions. You can submit observations and photos HERE or text 307-264-3050

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