Bulletin d’avalanche

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Danger d’avalanche

Tue
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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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, low elevation rain, and wind are combining to increase the avalanche hazard. Triggering an avalanche 12-18 inches thick is likely on slopes exposed to the wind. On slopes sheltered from the wind, human triggered avalanches are possible, thanks to layers of very weak snow in the snowpack. Decrease your risk by avoiding steep, wind-affected slopes.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    A very weak snowpack developed across our area during the extended drought that came to a close last week. Immediately following this storm, observers reported many signs of instability, including widespread cracking, collapsing, and small human-triggered avalanches. The slab of snow resting on this weak, sugary snow has gradually grown in the past days and observers continue to report obvious signs of instability. In the past 24 hours, additional snowfall and rapidly warming temperatures have pushed the "fast-forward" button on this process. Yesterday, in the mountains north of this zone, multiple local professionals reported triggering slab avalanches that failed on this weak snow. Some of these were large enough to bury a person.

    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. 

    (12/17/2020) Small avalanche that ran on the faceted old snow surface in the Soldier Mountains. In the past days, this slab has grown in thickness and density, thanks to additional snowfall and warming temperatures.

Avalanche Discussion

For weeks we have been talking about the incredibly weak snowpack that has developed in our region. We’ve explained that just because the snow is weak doesn’t necessarily mean it is unstable. A pile of sand doesn’t become unstable until we try to build our house on it. We started construction on the house last week and it is starting to get large, particularly in our northern areas.

The missing ingredient in the avalanche equation so far has been the slab. That changed this week. Much of our area has seen snowfall on each of the last six days. You can look at our Storm Table to get a sense how much your destination of choice has picked up. As the snow has fallen it has settled into a denser slab. It is bonding well to itself, but not to the weak snow underneath. Near freezing temperatures in the past 24 hours have helped accelerate this process, producing human-triggered avalanches even in the absence of significant new snow. 

The snowpack changes every day, but sometimes this change takes place much more rapidly than others. Times of rapid change are when it is easiest to get ourselves in trouble. Today is a good day to take a few steps back and think about the way you are traveling in the mountains. The hazards you are facing today are very different than they have been up to this point.

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