Bulletin d’avalanche

Banner Summit

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 3 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

You can trigger avalanches involving dense, 2-3 foot thick slabs on many slopes. Slopes with weak snow at the base of the snowpack are much more dangerous than those without. Triggered avalanches may break wider than you might expect, and can be triggered from a distance away from the slope that avalanches. 

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1.5–2.5

    Weak snow from November forms the base of the snowpack on many slopes here. The slab over this weak snow has been building in December and is now 2-3 feet thick. Avalanches failing on this layer will easily be large enough to bury you. Slabs may break wider than you expect, wrapping around terrain features. Triggering slides remotely — from a distance away from the slope that avalanches  — is a real possibility. 

    This layer is at its worst on upper elevation slopes and middle elevation slopes that face away from the sun (W-NW-N-NE-E). Here, a pile of facets is resting on top of a stiff, icy, crust. This combination produced a number of avalanches during loading events and has not had time to gain significant strength. The slab over the weak layer has grown thick enough that it keeps us above the weak layer in places. However, if you are able to collapse the weak layer, the avalanche you create will not be small.

    Yesterday, I experienced 6-8 deep collapses while traveling at middle and upper elevations in this zone. This clear indicator of instability, combined with the unstable test scores I received on this layer in 3 different snowpits, was more than enough evidence to have me avoiding steep avalanche terrain where this layer exists. 

Avalanche Discussion

We've got a beautiful, sun-soaked powder day in store for us. Unfortunately, a weak layer near the base of the snowpack keeps our hackles raised. Early-season snowfall weakened significantly in November and was buried by storms in December. The main driver of avalanche activity thus far has been the presence and distribution of this weak snow. Its not all doom and gloom out there, but if you head into the backcountry be prepared to deal with a complex, variable snowpack. Read on. 

The Good: In November, warm weather and sunshine melted the snow off of all low elevation slopes and many middle elevation slopes that face the sun. When snowfall returned at the start of December the snow was falling on bare ground in these areas. Observations thus far indicate that this newer snow is bonding well to itself and to the ground. On these slopes, you'll have a much lower likelihood of triggering a large avalanche. The downside is that you'll also find a shallow snowpack with plenty of stumps and rocks and logs still exposed.

The Bad: On all upper elevation slopes and most middle elevation slopes that face away from the sun, you'll find our problematic early-season weak layer at the base of the snowpack. In the mountains on the eastern edge of our forecast area (Pioneers, Boulders, White Clouds), the slab on top of this weak snow is in the 1-1.5 foot thick range on sheltered slopes. In the mountains on the western edge of our area (Western Smokys, Sawtooths, Banner Summit area) this slab is 2-3 feet thick. Across our area, this layer is well within the realm of human-triggering. 

In many places, it will not be visually apparent whether the early-season weak snow exists or not. It is possible to walk from a slope without this weak snow to one with it and not be able to tell that you have. When you do, your chances of triggering a large avalanche increase dramatically. If you head out today, you'll find yourself enjoying a stunning winter day. You'll also find a complex snowpack with some very real avalanche problems. Before committing to avalanche terrain, take a deep breath of cold air and an honest account of your understanding of the snowpack. Be prepared to travel conservatively in areas with weak snow at the base of the snowpack, understanding that triggering avalanches large enough to bury you is uncomfortably likely. 

Watch Brooke's Weekend Update to get a sense for what's been happening in our snowpack.

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