Bulletin d’avalanche

Soldier & Wood River Valley Mtns

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

Points saillants

Your odds of triggering a large avalanche are greatest on slopes where you find wind-drifted snow. Avalanches are possible, but far less likely, in steep terrain sheltered from the wind. You may be able to trigger avalanches remotely—from gentler terrain below, above, and to the sides of steep slopes.

Problèmes avalancheux

  • Persistent Slabs

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

    Since Tuesday, two skier-triggered avalanches have been reported near Galena Pass (observation, photo). Snowmobilers in Smiley Ck triggered a thin but wide avalanche from flat terrain near the base of the slope (photo). Each of these slides released on weak snow (sugary facets, surface hoar, and/or crusts) buried by recent storms about 1-1.5' deep. 

    Below the storm snow, a myriad of weak layers exist. Wednesday, I observed sled-triggered cracks breaking on weak layers buried 2.5' deep (photo). Skiers north of Ketchum and in the Pioneers felt very large rumbling collapses, likely involving weak layers in the middle and near the base of the snowpack.

    Assume that most slopes harbor one or more of these weak layers. You're most likely to trigger avalanches on slopes where the wind has formed drifts or slabs or stiffened the snow at the surface (example). Wind-exposed bowls and slopes near ridgelines, prominent gullies, and chutes are most likely to produce large avalanches.

    Persistent slab avalanches are tricky. These avalanches may break wider than expected or higher on the slope above you. You may be able to trigger avalanches remotely—from gentler terrain below, above, and to the sides of steep slopes.  You can significantly reduce your risk by steering clear of steep, wind-affected terrain. 

    ADDITIONAL DISCUSSION: While triggering slides on faceted snow near the base of the snowpack is unlikely, it can't be ruled out in steep, rocky, wind-affected alpine terrain. Slopes like these have variable snow depths, and thinner snowpack areas allow our weight to more easily impact deeply buried weak layers.

    (1/12/2023) A small, likely skier-triggered avalanche near Galena Pass. Two sets of ski tracks were observed near the crown. The avalanche released on a thin layer of facets sitting atop a crust. NE aspect at 8,800'.

Avalanche Discussion

LIKELIHOOD AND CONSEQUENCE

Last night, a student in a local Level 1 avalanche course asked me, "how do you choose where to go on your field days?" I replied, "well, I assume you look for good stability and soft snow to ski and ride and I generally look for the opposite." When I head to the field, I try to fill gaps in our knowledge. That can mean heading to a far-flung corner of the zone or simply setting out to answer a nagging question to better understand what's happening in the snowpack. Often, we're looking for where conditions are at their worst. This means going to the windiest, snowiest portions of the zone during a storm, or seeking out the touchiest weak layers and ugliest snowpack structure. Establishing an upper bound can help us all avoid surprises.

Today's avalanche conditions are complex. A few weeks ago, in a conversation with Ben, he said "I feel like the weather does two things here, create weak layers or bury them." That has certainly been the case this winter. Keeping track of these layers, their distribution, and their sensitivity is a full-time job. The complexity and spatial variability make slope scale stability assessment a nearly impossible task. 

I find it helpful to consider our persistent slab problem in terms of likelihood and consequence. You're most likely to trigger a persistent slab avalanche on weak layers buried under the recent storm snow. This layer is 1-1.5' down in sheltered areas, but may be buried much deeper in wind-loaded terrain. As mentioned above, each of the recent human-triggered avalanches involved this layer. Weak layers near the surface are easier to impact, but if we avoid wind-loaded terrain, any triggered slide is likely to be small. Weak layers near the middle and base of the snowpack are older, less sensitive, and harder to impact. The obvious downside however is that any avalanches involving these layers will be large and destructive. 

So where are conditions at their worst? Your odds of triggering a large avalanche are greatest on steep slopes where you find wind-drifted snow. This is the case no matter which weak layer you have under your skis or sled. Very large avalanches are most likely where overall snowpack depths are thinner and older weak layers are easier to impact. Continued reports of deep rumbling collapses continue to trickle in. Many of these occurred in the Galena Summit & Eastern Mtns zone. Given the complex, overlapping nature of weak layers (both old and new) the prudent choice is to steer clear of wind-loaded terrain and carefully consider the consequences of triggering a slide before committing to a steep slope.

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