Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Fri
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

A snowmobiler remotely triggered a 1-2 foot thick slide on Tuesday near Cape Horn Summit. Similar avalanches are possible again today—particularly on steep, recently wind-drifted slopes. While unlikely, we can't rule out triggering an unsurvivable, 4-5 foot thick avalanche—especially in very steep, rocky, wind-affected terrain. 

Avalanche problems

  • Wind Slabs

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

    Tuesday, a rider triggered a slide near Cape Horn Summit (photo to right/below, observation). This avalanche is the product of two important factors: a wind-loaded slope, and a buried weak layer. Today, avalanches are much more likely in recently wind-loaded terrain than in areas more sheltered from the wind. So far these small drifts have been observed on or very near ridgelines. 

    ADDITIONAL DISCUSSION:There are two weak layers we are still tracking:

    1. A layer of surface hoar and small facets buried 2-3 feet deep: This layer was the likely culprit behind the snowmobile-triggered avalanche on Tuesday. 
    2. Depth hoar (large-grained weak snow) buried in December: A hard, 4-5 foot plus slab lies above this weak layer. You would have to be unlucky to trigger a slide this deep, but this layer has kept us out of the terrain where it is most likely to exist. What type of terrain? Slopes that are very steep, rocky, and wind-affected. The most recent slide involving this weak layer failed on exactly such a slope (photo).

    Chris found both of these layers in the Cape Horn area on Saturday—he describes them in this video.

    (2/16/2021) A snowmobiler triggered this avalanche from the bottom of the slope on Tuesday afternoon near Cape Horn Summit.

Avalanche Discussion

In the past few weeks, snowpack characteristics have diverged significantly across our forecast area. In our northern zones, we are primarily dealing with "new snow" avalanche problems including fresh wind slabs created by new snow + wind as well as loose snow avalanches in very steep terrain. Less snow has fallen in our southern zones and an overall shallower snowpack has kept persistent weak layers or "old snow" avalanche problems at the top of our problem list. Here's a quick rundown to highlight some of these differences. 

Banner Summit and Sawtooth & Western Smoky Mountains:In many ways, the snowpack in these northern zones is simplifying. The rise and fall of avalanche danger here is primarily tied to storm activity. In the last week, a series of storms have deposited anywhere from 2-3.5' of snow (2-2.5" SWE). The ebb and flow of NW wind built wind slabs in exposed terrain. The Banner Summit area, with overall lower elevations and less steep alpine terrain, tends to keep its head out of the wind a little better than the Sawtooths. For this reason, the avalanche danger has remained elevated in the Sawtooths where both wind slab and loose snow avalanche problems are more prominent. 

Weak layers in these zones do exist. In fact, it's likely that the snowmobile-triggered slide near Cape Horn Summit on Tuesday involved a layer of weak facets and surface hoar that we've been tracking since late January. This avalanche (if indeed attributed to this weak layer) bucks a trend of improving stability in the past week. So, as tempted as we may be to toss this avalanche aside as an outlier, that's not how it works—more information is warranted. The primary weak layer of concern in our southern zones, and the one which produced extensive natural and human-triggered avalanche activity earlier this season is buried under a hard 4'+ slab of snow. Triggering an avalanche on a weak layer this deep is unlikely. 

Galena Summit & Eastern Mountains and Soldier & Wood River Valley Mountains:In our southern zones, the waters are much murkier. Here, we're dealing with a highly variable snowpack both in recent storm totals and overall snow depth. The one constant tying these areas together, and what makes them much different than our northern zones is the overall shallower snow depth. In areas where the snowpack less than about 4' deep, our likelihood of impacting weak facets near the ground increase. 

The rise and fall of avalanche danger in these zones is still very much influenced by the presence of weak facets near the ground. It may take less snow + wind to see a spike in avalanche activity and the threat of human-triggered slides can persist for several days after a storm. In these zones, the possibility for small avalanches or cornice breaks to "step-down" to these facets is very real. 

 

Regions covered