Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

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Out of date — this forecast expired 7 years ago.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

Highlights

Avalanche conditions remain dangerous, especially at upper elevations. Buried weak layers have not yet adjusted to the new and wind-transported snow from the New Year's storm, and strong winds today may keep these weak layers on edge. Choose conservative terrain, and avoid steep slopes with signs of wind drifted snow.

Avalanche problems

  • Persistent Slabs

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

    The snowpack throughout our area contains a number of buried persistent weak layers. These consist of well-defined layers of surface hoar and depth hoar in areas with a deeper snowpack, and a hot mess of facets and crusts where there is a shallower snowpack (less than about 2.5 feet). In this zone, you can find a deeper snowpack on mid and upper elevation, wind-sheltered slopes facing E through N through W. In windy, alpine terrain, on sunny slopes, or at lower elevations, the snowpack remains thin and weak. 

    These weak layers need more time to adjust to the new and wind-transported snow provided by the New Year's storm. The slopes that remain the most dangerous—and the ones you will want to avoid—are those that received extra loading in the form of windblown snow. Strong winds and a few inches of new snow today could add even more load to our weak snowpack. Even on wind-sheltered slopes with a deeper snowpack, snowpack tests on Banner Summit yesterday indicate it remains possible to trigger an avalanche 2-3 feet thick.

    Remember that many slides this year have been remotely-triggered. This means that you could initiate a slide from gentler terrain above, below, or to the sides of steep slopes.

  • Wind Slabs

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

    Just when the wind slabs that formed over New Years were showing signs of improvement, the next round of wind rolls in. Today, strong SW to NW winds are forecast along with a few inches of new snow. These may create a fresh batch of wind slabs along upper elevation ridgelines. These wind slabs could be touchy on their own, but will also make it more likely to trigger a slide breaking into the more deeply buried weak layers described above—double jeopardy if you will. 

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.

Regions covered