Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Mon
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
1 · Low
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Out of date — this forecast expired 7 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Triggering an avalanche is most likely on upper elevation slopes with wind-drifted snow, but it's also possible on sheltered slopes due to weak layers buried 2-3 feet deep. Since New Years, the snowpack has been kept on edge by new and wind-transported snow. Choose conservative terrain, and avoid steep slopes with signs of windblown snow.

Avalanche problems

  • Persistent Slabs

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

    The snowpack throughout our area has well-defined layers of surface hoar and depth hoar in areas with a deeper snowpack, and a mix of facets and crusts where there is a shallower snowpack (less than about 2.5 feet). If someone were to trigger an avalanche today, it would most likely be in one of two different scenarios:

    1. Avalanches are most likely on upper elevation slopes with recent or past wind loading. These slopes have a dense, cohesive slab atop the persistent weak layers. In some cases, the New Year's storm created dense wind drifts atop shallow, weak snow in areas with a thin snowpack.
    2. Although less likely, it's still possible to trigger a slide on wind-sheltered slopes where the snowpack is deeper (greater than about 2.5 feet deep). Snowpack tests continue to indicate that the persistent weak layers could still produce an avalanche.

    In some ways, we're right back to where we were before the recent snowfall and wind—New Year same as the old year. We have widespread weak layers, but the signs of instability are decreasing and becoming less obvious. So that puts us back to the same travel advice: make conservative terrain choices, and avoid upper elevation slopes with signs of past or recent wind drifting. With the nature of our weak layers, many slides this year have been triggered from gentler terrain above, below, or to the sides of steep slopes.

  • Wind Slabs

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

    The New Year has already delivered two significant wind events, with the most recent occurring yesterday. Observers yesterday reported gusty, swirling winds and inconsistent loading patterns, but I'm relatively confident that you could find some fresh, touchy wind slabs today. These will be most common up high, but you may find a few isolated pockets at middle and lower elevations too. In the sort of terrain where you might encounter the wind slab problem, remember that it's a secondary concern to the persistent slab problem discussed above and that any wind loading is adding weight to buried weak layers.

Avalanche Discussion

For a while, it seemed so simple. In areas where there was enough snow to recreate, there were well-defined weak layers consisting of surface hoar buried in early December, and well-developed facets and depth hoar in areas that held the old October snow. But there is a simple rule to snowpack evolution: the longer a shallow snowpack sits around, the weaker and more variable it gets. When the snowpack is 1-3 feet deep, a few inches can mean big differences in the layering of the snowpack.

If we were forced to simplify our complex snowpack, we could split it into two general buckets:

  1. Areas with a "deeper" snowpack, say more than 2.5-3 feet. Depths like these can be found on shady slopes at mid to upper elevations in our northern mountains, and at upper elevations of our southern mountains. In these areas, the snowpack layering remains fairly well-defined. You have the December 7th surface hoar buried 2-3 feet deep, as well as well-developed facets and depth hoar near the ground.  These layers have been the primary players so far this year. Here are videos from the Western Smokys and Banner Summit showing that it remains possible to trigger a dangerously large avalanche in these deeper snowpacks.
  2. Areas with a "thinner" snowpack, less than about 2.5 feet. Prior to the New Year's storm, these places had a thin snowpack consisting mostly of weak, faceted snow with some intermingled crusts. A shallow snowpack is common in windy, alpine terrain, on sunny slopes, and in much of our southern mountains. In some places, the recent storm added thick layers of wind slab on top of this weak snow, effectively doubling the snowpack depth in places. These wind-loaded areas have displayed the most signs of instability, including cracking, collapsing, and triggered slides.

While obviously a simplification of the situation, it may help make sense of a highly variable snowpack. The common theme is that we have plenty of weak layers, and slopes that have new, recent, or even old wind drifted snow remain the most dangerous.

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