Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Sat
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Out of date — this forecast expired 6 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Heavy snowfall and wind-drifting are creating dangerous avalanche conditions. Large natural and human-triggered avalanches are likely. Very large avalanches that fail on weak snow buried 3-4 feet below the snow surface remain possible—especially on or below very steep, rocky, wind-affected slopes.

Avalanche problems

  • Storm Slabs

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

    This zone has received up to 2' of snow (0.9-1.2" SWE) since Tuesday night. Air temperature has continued to warm slightly as the snow piles up which aids in the formation of top-heavy slabs of snow on many aspects. These slabs are forming on a variety of weak snow surfaces including facets, crusts and facets, surface hoar, and wind-hammered ice. Observers in the Soldiers, the Wood River Valley, and the Sawtooths (photo) have found very sensitive slabs of new snow, failing on a layer of facets above a crust.  

    Expect slabs to be thicker, stiffer, and more dangerous in windier terrain. Areas of wind-drifted snow may be hard to identify due to low visibility and continuing snowfall. These slides have the potential to "step-down" causing avalanches to fail on more deeply buried weak layers near the ground (see Deep Persistent Slab problem).

    (1/27/2021) Cracking in freshly formed drifts along the Fishhook drainage in the Sawtooth Mountains. This photo is from early in the storm, conditions have worsened considerably since.

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    3

    Triggering a very large 3'+ thick slab avalanche is possible. These slides will fail on weak, faceted snow near the base of the snowpack. They are most likely in steep, rocky terrain at upper elevations where the snowpack is thinner. 

    Deep slab avalanche problems are challenging to deal with. Because the weak layer is "protected" by a thick slab of snow, you're not likely to see obvious signs of instability, like cracking and collapsing. Instead, the first sign of instability may be a wide, destructive avalanche. Tracks on a slope do not mean it's safe. These slides are often triggered after many riders have been on a slope.

    Smaller avalanches failing near the snow surface are likely. These slides can act as "triggers" increasing the odds that a deep slab avalanche may occur.  

    (1/18/2021) This very large avalanche occurred in our Banner Summit Zone. Based on the depth and appearance of the crown it probably failed on facets buried deep in the snowpack.

Avalanche Discussion

The brunt of the current storm system is having greater impacts to the south of the Banner Summit zone where AVALANCHE WARNINGS are in effect. Despite the lack of a WARNING around Banner Summit, dangerous avalanche conditions exist. Up to 2' of new snow has fallen since Tuesday, and an additional 3" of snow is expected before the storm exits this evening. 

Large, natural and human-triggered avalanches are likely, failing on the weak snow layers near the surface. These slides are most likely in steep, wind-affected terrain. As snow piles up and temperatures continue to warm, soft slab avalanches will become more likely, even on slopes that are relatively sheltered from the wind. Utilizing small, safe test slopes and digging frequent, small pits with your hands can be an effective way to monitor how new snow is bonding with the old snow surface. 

With all the new snow, don't forget about the monster lurking in the basement. Small avalanches within the storm snow can act as a trigger for our deep persistent slab problem. Today's a good day to build in some extra safety margins into your travel plans. 

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