Boletín de aludes

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Peligro de aludes

Wed
Alta montaña
3 · Notable
Límite del bosque
2 · Moderado
Bajo el límite
1 · Débil
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Desactualizado: este boletín caducó 5 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

Triggering an avalanche is likely in wind-affected terrain, where more snow has fallen and fresh drifts are adding weight to buried weak layers. Look for evidence of wind such as drifts, stiffened surfaces, or cornices that indicate the wind has been at work. Wind slabs may step down to deeper weak layers, producing very large and destructive avalanches. 

The Avalanche Danger will continue to increase after sunset today as another band of snowfall impacts the region. 

Problemas de aludes

  • Wind Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Likely
    Tamaño
    1–2

    Last night's storm brought strong, gusty winds and 4-8" of snowfall to this Zone. The combination of these factors has built a set of slabs that will grow in size and distribution today as the wind continues to transport snow. As they grow they will become more sensitive to your weight and will be easier to trigger. Feel for stiffened snow surfaces and look for textures like ripples and dunes to help you identify wind-affected areas.

    Observations from this Zone point to the existence of weak layers in the upper 1-2' of the snowpack (before the storm) as well as a weak snow surface (that was just buried by the storm). You can watch two videos describing these weak layers here: Boulder Mountain weak snow, Stanley Foothills weak snow. Wind slabs that break on these layers will be wider, larger, and more sensitive to your weight. 

    Both natural and triggered wind slabs and collapsing cornices make for "great" triggers for the persistent slab problem described below. Avalanches that step down to buried weak layers are certain to be large and destructive.

    Additional Discussion: Last night's storm looks like it dropped about as much snow in the town of Hailey as anywhere else. Winds near the mouths of canyons can produce small but surprising avalanches in the foothills around town. Be on the lookout for slabs here if you are out in this type of terrain.

    (12/31/2021) On Friday, NW winds were actively loading slopes - especially in the southern portion of the forecast area. This small wind slab in the Soldiers occurred naturally as a result. Photo: Soldier Mtn Ski Patrol

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    2–3

    A mix of facets and crusts leftover from October snowfall make up the base of the snowpack on most middle and upper elevation slopes. This layer is at its weakest on the shaded (N) side of the compass but old snow exists on upper elevation slopes that face the sun as well. Avalanches failing on this old snow have been observed with each major precipitation event this winter (photo at right/above). 

    On Sunday, the professionals at Sun Vally Heli Ski experienced a large collapse to the north of this Zone when they landed their helicopter in a safe location and collapsed this layer of weak snow. Public observers traveling in the Galena Summit area have also reported triggering large collapses involving this weak snow in recent days. In steeper terrain these collapses are avalanches. This activity occurred before we added more weight in the form of last night's storm. This ongoing storm will continue to increase the likelihood of encountering this type of avalanche problem.

    These fresh slabs and freshly loaded cornices make for very effective triggers for deeper weak layers. 

    (1/2/2022) This large persistent slab avalanche was observed above Corral Creek, near Johnstone Peak. It failed on a N/NW facing slope at 8,600'. The terrain and trees below the crown limited its size, but the crown is over 500' wide.

Avalanche Discussion

We find ourselves nosing into another multi-day storm event. Last night, a quick-hitting wave of moisture blasted through our region, bringing erratic, gusty winds and intense precipitation rates. In most areas this was short-lived and we will be sitting in the lull between storms today. Broadly speaking, there are three parts of the snowpack to direct your focus to today:

UPPER SNOWPACK The new snow is falling on a mixed bag of snow surfaces: near-surface facets, facets above and below crusts, small surface hoar, and relatively friendly decomposing snowflake particles. These surfaces will be variably reactive. Of all of the surfaces I've looked at in the past few days, the most concerning are crusts with facets above and below them. These are found on steeper slopes that face the sun. Coming in a close second are the near-surface facets that were generated by the cold weather and that exist on all aspects. These surfaces will make it easier to trigger the slabs that get deposited on top of them. Initially (today) these types of slides will be much more likely in wind-loaded terrain, but if enough snow piles up we may see avalanches failing on these weak grains in sheltered terrain as well

MIDDLE SNOWPACK We have seen two major storm events this season; one beginning on the 11th of December and the second starting just before Christmas. In the space between these storms we formed irregularly distributed layers of weak snow (facets with and without crusts, and surface hoar) that are now buried 1-2.5' beneath the snow surface. The pattern of this weak snow has been hard to pin down, but it is concerning where it exists. Broadly speaking, these layers appear to be more common south of Galena Summit. However, the largest natural avalanche we've seen failing on this weak snow (photo and limited details available here) was seen north of the Pass, just south of the Alturas Drainage. 

LOWER SNOWPACK Weak snow leftover from October storms exists on many middle and upper elevation slopes and has produced countless large and destructive avalanches already this season. I can say with a high level of confidence that we have not seen the last of these. In fact, in places with a thinner snowpack we may just be getting into the meat of this problem. Any avalanche that fails on October snow near the base of the snowpack will be large and destructive. We are talking about the kind of slides that snap trees like toothpicks. Yikes. 

Maybe you are reading this and finding yourself thinking, "Hmmm, that sounds complex." If that is the case I am doing a good job of describing the world you will find out there today. In times of increasing danger and increasing complexity, our best bet is to take a few steps back and build out our safety margins. We are moving through a period of increasing danger over the remainder of the week, I'd strongly recommend you plan your upcoming adventures in the backcountry accordingly. 

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