Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Fri
Alpine
4 · High
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

Recent snow and strong winds are combining to create very dangerous avalanche conditions. Human-triggered avalanches are very likely–especially in recently wind-loaded terrain. Recognize you could trigger avalanches remotely from gentle or flat terrain above, to the side, or from below steeper slopes.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2–3

    The Banner Summit Zone picked up around a foot of new snow (0.7-0.9” SWE) in the last 48 hours. The weight of this new snow is adding stress to a variety of weak layers in the snowpack. This stress is amplified in areas where dense, thick, wind slabs formed during recent strong wind. 

    Our most concerning persistent weak layers sit near the middle and base of the snowpack. An avalanche failing on one of these layers has the potential to break hundreds of feet wide, spanning multiple terrain features. 

    There’s a fair bit of uncertainty and variability in how these layers will react to the recent load. Slides may be triggered from a distance and have the potential to pull back into low-angle terrain near the top of a slope. Deal with these unknowns by building wide safety margins into your travel plans.

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Very Likely
    Size
    2

    Yesterday’s strong to extreme W and NW wind raked the forecast area building fresh wind slabs. These slabs will be thicker and more abundant as you climb in elevation. Recent wind-loading may also be found in windier locations at low elevations, such as the mouths of main drainages.

    Extreme wind can cause unusual loading patterns. Expect not only to find fresh wind slabs along exposed ridges (see photo) and in cross-loaded gullies but also lower on slopes. Cracks shooting from your boards or sled are an obvious sign that you’ve found a sensitive slab. Smaller wind slab avalanches have the potential to step down to more deeply buried weak layers (as discussed above) resulting in a very large and destructive avalanche. 

Avalanche Discussion

Yesterday, natural slides were reported near Banner Summit, in the White Clouds, near Galena Summit, in Baker Creek, and in the Soldiers. The bulk of the reported avalanche activity failed at the interface beneath our recent storm snow but some observations point toward activity involving one of our more deeply buried weak layers. 

Our wind slab problem is the most obvious concern today, so let’s start there. These slabs formed lee to W-NW wind and were deposited on an old snow interface that proved to be sensitive. Soft slabs failed in relatively sheltered areas, and a slide near Galena Summit showed surprisingly wide propagation.

Ben and I produced shooting cracks and large collapses on nearly every wind-loaded feature we touched as we climbed an exposed ridge in the White Clouds. In this location, the thin snowpack of facets and crusts showed no intention of quickly bonding to the overlying slab.

We don’t quite have a good handle yet on how the primary weak layers we’ve been tracking since the start of the season (surface hoar from early December and depth hoar from September and October) dealt with the added load. Some observations from Baker Creek suggest activity on these layers, but I’m curious to see how the regions with the largest snow totals–particularly the Headwaters of the Salmon River and the western Smokys–fared. These locations may have received enough snow load, even in the absence of wind, to cause natural avalanches on deeper weak layers.

With either problem, the common thread driving your decision-making today should be recent wind-loading patterns. Both problems are more likely where a fresh slab sits at the surface and the resulting slides will be larger and more dangerous in areas that received more recent snowfall. Today is a good day to take a big step back and stick to gentler terrain as a weak snowpack adjusts to a significant load.

Regions covered