Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

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Out of date — this forecast expired 2 years ago.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

Highlights

Triggering an avalanche large enough to bury a person is likely, especially in areas where wind has stiffened the snow surface or built thick drifts. Pay attention to the terrain above you. Avalanches may be triggered remotely—from flat terrain below, above, and to the sides of steep slopes.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1.5–2

    Triggering a large, 1-3 foot thick Persistent Slab is likely. These slabs are most likely to be triggered in wind-loaded locations where the slab is stiffer and thicker.

    These slabs can be found on most aspects and elevations. Weak snow below the surface is getting progressively stressed by additional snow loading during this ongoing storm. Feeling for softer snow under stiffer snow is the best way to identify this problem.

    Cracking, collapsing, and whumpfing are bullseye information for unstable conditions. Avoiding wind-loaded terrain, managing slope angles, and identifying the avalanche terrain you’re connected to (above, below, and to the side of) are all tools to reduce your chances of triggering an avalanche. Persistent slabs can release remotely (be triggered from a distance) and break wider than expected.

    (12/27/2024) Large natural avalanche in Castle Creek that broke an estimated 4 feet thick on 12/26. Photo: Sun Valley Guides

  • Wind Slabs

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

    Large wind slabs are likely to be triggered as you climb into more exposed upper elevations. Wind speeds today will continue to move snow and grow yesterday’s sensitive slabs. Triggering a wind slab has the potential to step down into deeper weak layers resulting in a larger avalanche as described in Problem #1 below.

    These slabs are most likely found on and near ridgelines or steep open terrain features. Looking for obvious signs of wind loading, such as cornice growth and rippled or pillow-like surface textures, and avoiding those slopes can reduce your chances of triggering a wind slab.

Avalanche Discussion

All we wanted for Christmas was snow, and this atmospheric river keeps delivering. Storm totals since Christmas are 5-25 inches of new snow throughout the forecast area, with an additional 2-9 inches of snow forecast to fall today. This new snow is light and easily transported by wind. Increased wind speeds yesterday afternoon, overnight, and expected for today will continue to build sensitive wind slabs over various buried weak layers in exposed terrain and upper elevations.

All this snow continues to stress widespread persistent weak layers throughout the region. We’re all wondering how close to the “tipping point” we are. The answer is, unfortunately, not straightforward.

One factor to consider is how much weight we're adding and how fast. In areas with a deeper snowpack, such as Banner and the Western Smokys, another inch or two of SWE only adds 10-20% of additional load to our most concerning weak layers. In contrast, areas with thinner snowpacks before this storm, such as the southern and eastern ranges, an inch or two of SWE adds 30-50% SWE load atop those same layers. These thin snowpack areas, which had little hazard over the previous weeks, have both the weakest snow and the greatest relative loading during this storm. The take-home is that there are less safe places to play as the storm continues.

With the uncertainty around the tipping point of loading for the current persistent slabs, I like to revert to a simple strategy for simplifying the complexity: “When in doubt, mellow out.”

Regions covered