Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Sat
Alpin
2 · Modéré
Limite forestière
2 · Modéré
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

Last night's new snow and wind likely built a round of small, fresh drifts and sensitive cornices. These hazards can be dangerous on their own, but may also act as triggers for larger avalanches that fail on weak snow buried 3' deep. You can reduce your risk by steering clear of steep slopes with drifts or stiff snow near the surface. While unlikely, we can't rule out triggering an unsurvivable, 4-5 foot thick avalanche—especially in very steep, rocky, wind-affected terrain. 

Problèmes avalancheux

  • Wind Slabs

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

    Light to moderate wind and continued snowfall is expected over the next few days. Small, sensitive slabs and weak cornices will form in terrain exposed to the wind. These hazards are most likely in upper elevation terrain near and just below prominent ridgelines but may also exist in more subtly wind-loaded terrain (photo). 

    You can greatly reduce your risk of triggering a slide by sticking to terrain that's more sheltered from the wind. Keep in mind that even small slides can have big consequences if they drag you through trees or bury you in gullies or creek beds. 

    ADDITIONAL DISCUSSION:There are two persistent weak layers worth tracking. The first is a layer of facets and surface hoar buried about 3' deep.  This layer is the likely culprit behind the snowmobile-triggered avalanches near Cape Horn Summit on Tuesday (photo to right/below, observation). The other is a weak layer of depth hoar near the ground. You would have to be unlucky to trigger a slide this deep, but this layer has kept us out of the terrain where it is most likely to exist—namely very steep, rocky, and wind-affected terrain at upper elevation.

    (2/16/2021) A snowmobiler triggered this avalanche from the bottom of the slope on Tuesday afternoon near Cape Horn Summit.

Avalanche Discussion

In the past few weeks, snowpack characteristics have diverged significantly across our forecast area. In our northern zones, we are primarily dealing with "new snow" avalanche problems including fresh wind slabs created by new snow + wind (photo, photo, photo) as well as small slabs failing on mid-storm interfaces and loose snow avalanches in very steep terrain. Less snow has fallen in our southern zones and an overall shallower snowpack has kept persistent weak layers or "old snow" avalanche problems at the top of our problem list (video). 

In many ways, the snowpack around Banner Summit and in the Sawtooths simplifying. The rise and fall of avalanche danger here is primarily tied to storm activity. In the last week, a series of storms have deposited anywhere from 2.5-3.5' of snow (2.2-2.75" SWE). The ebb and flow of NW wind built wind slabs in exposed terrain. The Banner Summit area, with overall lower elevations and less steep alpine terrain, tends to keep its head out of the wind a little better than the Sawtooths. For this reason, the avalanche danger has remained elevated in the Sawtooths where both wind slab and loose snow avalanche problems are more prominent. 

Weak layers in these zones do exist. In fact, it's likely that the snowmobile-triggered slides near Cape Horn Summit on Tuesday involved a layer of weak facets and surface hoar that we've been tracking since late January. These avalanches (if indeed attributed to this weak layer) stand in glaring opposition of a trend of improving stability in the past week. So, as tempted as we may be to toss them aside as outliers, that's not how it works—more information is warranted. The primary weak layer of concern in our southern zones, and the one which produced extensive natural and human-triggered avalanche activity earlier this season is buried under a hard 4'+ slab of snow. Triggering an avalanche on a weak layer this deep is unlikely.

Régions couvertes