Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

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Out of date — this forecast expired 6 years ago.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

Highlights

Human-triggered avalanches breaking 1-2 feet below the snow surface are likely on steep slopes. 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

  • Persistent Slabs

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

    Last week's storm brought 1.5-2' of new snow to this zone. This snow fell on a variety of surfaces, including crusts, facets, and surface hoar. On Friday, Chris found a layer of surface hoar in several locations and reported it to be reactive in his stability tests (video at right/above). Before the storm, I found this layer near the southern margin of this zone. Observers on Saturday reported easy shears beneath the 2' of storm snow, felt several collapses of the snowpack, and received propagating results in their snowpack tests.

    It will take some time to determine how these layers of weak snow are distributed and how long they will remain reactive. Conditions remain dangerous today—keep your slope angles in check, and pay attention to the consequences of the slopes you plan to play on.

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Unlikely
    Size
    3

    A thick layer of weak, faceted snow is buried 4 feet below the surface in this zone beneath a dense slab of snow. Chris looked at this layer during the storm on Friday and found that it was showing little signs of healing, a disconcerting sign. The new snow that fell last week is adding new stress to this layer.

    It would likely take a dose of bad luck to trigger a deep persistent slab avalanche, but the flip side is that you'd need to be extremely lucky to survive it. Triggering a deep slab is more likely in very steep, rocky, and wind-affected terrain where shallow "trigger points" exist. Trigger points are spots where the slab is thinner and where your weight can more easily impact the weak layer.  You can reduce the chances of triggering one of these beasts by sticking to slopes less than 35 degrees with a more uniformly deep snowpack.  

    (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

STABILITY AND COMPLEXITY

We have spent a significant amount of time and energy this season looking at and talking about the weak snow near the base of our snowpack. Before this storm our snowpack was weak and unstable, but relatively simple: weak snow near the ground with a variably thick slab on top. Ugly, but simple. However, in a matter of just a few days, our snowpack has gone from being relatively simple to being pretty complex. I know, just when you thought it couldn't get any worse out there. Ugly and complex, nobody's favorite combination.

It will take time to sort out which of the new weak layers in the snowpack are most unstable and how long they might be sticking around for. Some of the new weak layers may heal themselves over the course of days while others may be hanging with us for much longer. It takes time for us (and for you) to determine this, and even with days of fieldwork, there is a lot of uncertainty around these things. As we work to resolve some of this uncertainty we will be traveling cautiously and conservatively in the mountains. Alert to the terrain that we are in and the terrain that we are exposed to from above. I strongly urge you to do the same, it is a complex world out there.

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