Bulletin d’avalanche

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Danger d’avalanche

Wed
Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 6 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Recent snowfall and wind loading are stressing layers of weak snow buried 6-14" deep. Triggering an avalanche is likely on upper elevation slopes exposed to the wind. Even on slopes sheltered from the wind, human triggered avalanches are possible. Avalanches may be triggered from below and to the sides of avalanche starting zones. Decrease your risk by avoiding steep, wind-affected slopes.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    2

    A very weak snowpack exists in this zone, thanks to a stack of weak, sugary, faceted snow that is buried 6-14" deep. In the past few days this weak snow has been stressed by additional snowfall and by the consolidating slab on top of it. On Sunday, local professionals reported triggering slab avalanches that failed on this weak snow in the mountains just north of this zone (see photo at right). Some of these were large enough to bury a person. You are most likely to trigger one of these slabs where they are more than 1 foot thick. You will find these thicker slabs in areas that have received more snow and those that have been wind loaded.

    Persistent slab problems are difficult beasts to deal with. As the name implies, they tend to stick around for a while. These types of avalanches may be triggered from below and to the sides of avalanche starting zones. Cautious route-finding and conservative decision-making continue to be essential today. 

    This type of avalanche problem can be difficult to manage. The Sawtooths and Western Smokys gave us a preview of how this weak layer will respond to a loading event last week when a widespread avalanche cycle occurred with just 4-8" of new snow. The slab in this zone is now 8-14" thick and human triggered avalanches involving this snow are likely. These slides are more likely in areas where the wind has built thicker slabs, but they are possible in sheltered terrain as well. Avalanches may be triggered from below and to the sides of avalanche starting zones. Cautious route-finding and conservative decision-making are essential today. 

    (12/20/2020) This avalanche was remotely triggered by local professionals on Sunday. It failed on the layer of weak snow that exists across our forecast zones. Based on the appearance of the crown, it appears that wind had loaded this slope.

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1

    A fresh batch of wind slabs has built overnight and will continue to grow in size and distribution today. While these slabs are likely to be relatively small and isolated, but they may be large enough to knock you off your feet or sled. In addition, these slabs are adding stress to the weak snow described above. An avalanche that starts as a small wind slab has the potential to step down to deeper weak layers buried in the snowpack, resulting in a large and dangerous avalanche.

Avalanche Discussion

As avalanche forecasters we spend a lot of time thinking about uncertainty and confidence. Thinking about these things helps us to produce forecasts that are both accurate and useful for you as a reader (at least, that's the idea). On days when we have a lot of information about the snowpack and the weather is simple, we can have high confidence in how the snow will behave. When the weather is changing rapidly and we find ourselves dealing with snowpacks that are unusual, our uncertainty can be high. Today is the later. 

There is no reason to hide the fact that we have a lot of unknowns and a lot of unknowables in today's snowpack. The widespread layer of weak snow buried in our snowpack is quite unusual, a once in a decade or two sort of thing. The warm, wet weather that we just experienced is also quite unusual for this time of year; Stanley set the record for highest temperature seen on the solstice, breaking a record that was set over 50 years ago. Because of the combination of these things I have a lot of uncertainty about how our snowpacks will behave. The first step in dealing with uncertainty is to acknowledge its existence.

After I issue my forecast this morning I will take off my forecaster's hat and put on my backcountry traveler's hat. When I walk out the front door I will expect to see things that will surprise me. We only see what we are looking for, so I'd advise you to have your eyes open to everything. You can manage the uncertainty by increasing your safety margins today. 

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