Avalanche forecast

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Avalanche danger

Wed
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
2 · Moderate
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Out of date — this forecast expired 7 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

New snow and wind are combining to increase the avalanche hazard and human-triggered avalanches are likely. Avalanches may fail in unpredictable ways and may be triggered from below and to the sides of steeper slopes. Exercise cautious route-finding and conservative decision making to reduce your risk.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    By the end of the day, this zone will have picked up 5-10” of new snow. This snow is falling on a thin snowpack with several prominent weak layers, most notably a layer of surface hoar that is buried 1-2’ deep. Today, you’ll be most likely to trigger a persistent slab avalanche where the combination of new and wind-drifted snow is stressing the weak layers underneath. However, thanks to the fresh load of snow, triggering one of these slides in sheltered terrain is certainly possible today as well.

    On Sunday, before the new snow fell, Sun Valley Heli Ski staff found the “perfect” combination of wind loading and a weak layer when they triggered a slab avalanche large enough to bury a person (photo at right). The likelihood of triggering a similar slide today has increased, thanks to the new snow. Persistent slab avalanches can be triggered remotely, from above, to the sides, or below a slope. They fail in unpredictable ways, breaking wider than you might expect and pulling back into lower angle terrain connected to steeper slopes. Read more about this avalanche and what it means in the forecast discussion section below.

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1–2

    Fresh snow and steady winds are a great way to build sensitive wind slabs. These slabs will be thickest and most reactive in exposed upper elevation terrain. By the end of the day, favorable locations in this zone will have picked up 8-10” of snow, accompanied by light to moderate winds. Winds are hovering right near the border of perfect transport speeds. If they blow a little less than forecast they will struggle to move much snow, but if they blow a little harder they will be perfect for building slabs. Be on the lookout for and avoid dune-like drifts of snow. These drifts may be sensitive to triggering in their own right, but they will also be adding additional load to the weak layers described above.

Avalanche Discussion

Yesterday, snowfall returned to our area, a welcome sight. Unfortunately, this new snow is falling on a snowpack with widespread weak layers. The past is the key to the present, so let's take a quick look back on what happened the last time it snowed. Ten days ago, a series of storms set off a widespread natural avalanche cycle. Most of the avalanches observed failed on a layer of feathery crystals (surface hoar) that are buried 1-2’ deep. In addition to these natural avalanches we recorded over a dozen human-triggered avalanches in the time after this storm.

In the following days, obvious signs of instability slowly decreased, but continued cracking and collapsing in the snowpack and unstable snowpack test scores have kept us worried about this surface hoar layer. It was clear that this wasn’t going to be a one trick pony. On Sunday, Sun Valley Heli Ski staff verified this concern when they triggered a large avalanche while assessing snowpack stability. This avalanche failed on surface hoar on an E-NE facing slope at 8,600’. Although the angle of the path that failed was in the low 30 degree range, the slab pulled back into terrain above a rollover that was only 26 degrees. Recent winds had loaded this slope, increasing the stress on the weak layers in the snowpack.

The ongoing storm is once again adding a significant amount of stress to the snowpack, pushing it back towards its breaking point. As you are traveling in the backcountry today, expect to see an increase in obvious signs of instability, particularly in areas where the wind is building slabs. The fact that human-triggered avalanches were still occuring before this storm (10 days out from the last snowfall), should be an indication of how weak our snowpack is and how well it will deal with the new load today.

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