Avalanche forecast

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Sawtooth Avalanche Center
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Avalanche danger

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Out of date — this forecast expired 7 years ago.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

Highlights

Human-triggered avalanches are possible today. You’re most likely to trigger a slide where the wind has drifted snow, adding weight and compacting the snow at the surface. Avalanches may fail in unpredictable ways and may be triggered from below and to the sides of steeper slopes.

Avalanche problems

  • Persistent Slabs

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

    By the end of the day this zone will have picked up 2-4” of fresh snow. A strong snowpack without weak layers would have no trouble dealing with this sort of load, but we don’t have one of those this year. A few prominent weak layers are present in our snowpack, most notably a layer of surface hoar 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.

    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). 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.

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 as a result of the 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.

A few inches of new snow are helping to keep the snowpack on its toes today, particularly where the wind is building fresh slabs. As you are traveling in the backcountry, you may be seeing fewer obvious signs of instability than you did a week ago. Perhaps you are seeing other folks skiing and riding in avalanche terrain without consequences. None of these things mean that the snowpack is now stable. You may be a bit less likely to trigger an avalanche today than you were last week, but the consequences of triggering one will be exactly the same. Keep this in mind as you travel in the backcountry today.

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