Bulletin d’avalanche

Galena Summit & Eastern Mtns

Sawtooth Avalanche Center
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Danger d’avalanche

Sun
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 6 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Strong winds are keeping the snowpack dangerous, and triggering a large avalanche remains likely today. Avoiding steep slopes—especially those exposed to the wind—will help you stay out of trouble. Avalanches can be triggered remotely—from below, above, and to the sides of steeper terrain.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    2–3

    There are two persistent weak layers of concern in our snowpack:

    1. A thin layer of facets and sporadic surface hoar buried 1.5-2' deep. This layer formed during a week of cold, dry weather, and was quickly loaded by the late-January snowpocalypse. It appears to be responsible for a large portion of the recent avalanche activity in this zone. Recent natural avalanches—for example, the large slides in the Boulders and the head of Eagle Creek (photo above/right) that occurred midweek—as well as poor snowpack test scores, indicate that this layer remains an issue.
    2. Our well-known layer of December facets buried 3' deep or more. In contrast to our newer weak layer, this layer formed over a long period (3+ weeks) and was very slowly loaded. Now that it's deeply buried, it's behaving more like a deep slab problem. In other words, difficult to trigger with terrible consequences if you do.

    We are in the midst of a multi-day wind event that is producing strong W to NW winds. These winds are loading many slopes at middle and upper elevations, keeping these weak layers on edge. Avoiding slopes over 35 degrees and/or those that have been affected by the wind will greatly reduce your chances of triggering a persistent slab avalanche.

    (2/3/2021) This very large persistent slab avalanche is over 1000' wide. It released naturally on Feb 2nd near the head of the Eagle Creek drainage. SW aspect (S-SW-W) near 10,000'.

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    The strong NW flow aloft is creating a multi-day wind event. The W to NW winds increased significantly yesterday, and large amounts of snow were blowing around at middle and upper elevations. From an avalanche danger perspective, the larger concern is the additional loading on the persistent weak layers described above. But the winds also transported the low-density new snow that fell yesterday, forming fresh drifts and wind slabs. These will be most common and dangerous along upper elevation ridgelines and in alpine terrain. Any wind slab you trigger could step down into the buried weak layers, creating a much larger persistent slab avalanche.

    (2/4/2021) Wind began moving snow Thursday afternoon in exposed middle and upper elevation terrain. Low-density new snow and strong W and NW are continuing to build fresh slabs in exposed terrain.

Avalanche Discussion

To help make sense of the complex set of avalanche conditions we often experience in the backcountry, we divide things into "bins" that we call avalanche problems. Each of the nine avalanche problems has different characteristics, we look for them in different ways, and use different strategies to avoid them. Any time we try to simplify a complex situation, we're bound to encounter times when conditions don't fit into the neat little boxes. A common one for our area—and one we're experiencing right now—is the interplay between persistent slab and wind slab problems.

If someone triggers a slide today that was big enough to bury them, it would likely be a slope that was affected by the ongoing winds. So would the avalanche be a result of the wind slab problem? My guess is the avalanche will have failed on one of the weak layers described in Avalanche Problem #1, making it more accurately placed in the persistent slab problem bucket. So why even mention the wind slab problem? Two reasons: 1) There are places where you could likely trigger a pure wind slab avalanche large enough to be dangerous, and 2) Adding the wind slab problem to the list clues us into a major factor that is keeping conditions dangerous—recent strong winds.

Moving beyond semantics, the situation is fairly simple. Conditions today will be most dangerous where these two problems overlap. That is, where one of the weak layers exist (which is nearly everywhere) and where you see tell-tale evidence of recent wind-effect (stiffening of the snow surface, snow texture, pillowed and drifted areas).

Be sure to catch up on what happened last week in Ben's latest installment of our Weekend Update.

Régions couvertes