Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Sun
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

Triggering an avalanche breaking 2 feet thick remains likely in many areas. Avoiding steep slopes—especially those exposed to the wind—will help you stay out of trouble. While unlikely, we can't rule out triggering a very large, 4-foot thick avalanche—especially on very steep, rocky, wind-affected slopes.

Avalanche problems

  • Wind Slabs

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

    The strong NW flow aloft is creating a multi-day wind event. The W to NW winds increased significantly yesterday, and large amounts of snow were blowing around at middle and upper elevations. From an avalanche danger perspective, the larger concern is the additional loading on the persistent weak layers described above. But the winds also transported the low-density new snow that fell yesterday, forming fresh drifts and wind slabs. These will be most common and dangerous along upper elevation ridgelines and in alpine terrain. Any wind slab you trigger could step down into the buried weak layers, creating a much larger persistent slab avalanche.

    (2/5/2021) Yesterday, strong NW winds were moving copious amounts of snow at upper elevations.

  • Persistent Slabs

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

    There are two persistent weak layers of concern in our snowpack:

    1. A thin layer of facets and surface hoar buried 2' deep. This layer formed during a week of cold, dry weather, and was quickly loaded by the multiple storms since late-January. Skiers Wednesday on Copper mountain near Banner Summit, likely remotely triggered an avalanche (photo above/right) on this layer from nearby lower-angle terrain. I found this layer still yield poor snowpack test scores on Thursday.
    2. Our well-known layer of December facets buried 4-5' deep. In contrast to our newer weak layer, this layer formed over a long period (3+ weeks) and was very slowly loaded. Because the slab over this weak snow is so thick and dense, it is difficult for us to impact the weak snow.  It would take a dose of bad luck to trigger a deep slab, but it could happen in very steep, rocky, wind-affected terrain where more "trigger points" exist.

    We are in the midst of a multi-day wind event that is producing strong W to NW winds. These winds are loading many slopes at middle and upper elevations, keeping these weak layers on edge. Avoiding slopes over 35 degrees and/or those that have been affected by the wind will greatly reduce your chances of triggering a persistent slab avalanche.

    (2/4/2021) This avalanche failed 2' deep on buried surface hoar. This weak layer has presented itself as surface hoar in some areas and small facets in others. NE, 8400', Copper Mtn.

Avalanche Discussion

To help make sense of the complex set of avalanche conditions we often experience in the backcountry, we divide things into "bins" that we call avalanche problems. Each of the nine avalanche problems has different characteristics, we look for them in different ways, and use different strategies to avoid them. Any time we try to simplify a complex situation, we're bound to encounter times when conditions don't fit into the neat little boxes. A common one for our area—and one we're experiencing right now—is the interplay between persistent slab and wind slab problems.

If someone triggers a slide today that was big enough to bury them, it would likely be a slope that was affected by the ongoing winds. So would the avalanche be a result of the wind slab problem? My guess is the avalanche will have failed on one of the weak layers described in Avalanche Problem #1, making it more accurately placed in the persistent slab problem bucket. So why even mention the wind slab problem? Two reasons: 1) There are places where you could likely trigger a pure wind slab avalanche large enough to be dangerous, and 2) Adding the wind slab problem to the list clues us into a major factor that is keeping conditions dangerous—recent strong winds.

Moving beyond semantics, the situation is fairly simple. Conditions today will be most dangerous where these two problems overlap. That is, where one of the weak layers exist (which is nearly everywhere) and where you see tell-tale evidence of recent wind-effect (stiffening of the snow surface, snow texture, pillowed and drifted areas).

Be sure to catch up on what happened last week in Ben's latest installment of our Weekend Update.

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