Boletín de aludes

Bitterroot

West Central Montana Avalanche Center
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Peligro de aludes

Wed
Alta montaña
2 · Moderado
Límite del bosque
1 · Débil
Bajo el límite
1 · Débil
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Desactualizado: este boletín caducó 4 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

Skiers and riders could still trigger very large persistent slab avalanches on upper elevation northerly slopes. Shifting winds drifted snow into wind slabs on all aspects at upper elevations. Carefully assess the snowpack before committing to upper-elevation slopes steeper than 35 degrees. 

Problemas de aludes

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Unlikely
    Tamaño
    1.5–3

    Skiers and riders could trigger very large and dangerous persistent slab avalanches on northerly aspects at upper elevations. These slides could be up to 5 feet deep and break at the ground. Triggering one is unlikely, but not impossible - it’s a low likelihood, high consequence scenario. Choosing low angle slopes under 35º, ideally under 30º, is the best way to manage these layers. If you step out onto steeper terrain, be sure to dig down and see what weak layers may be underneath you. Never ignore a poor snowpack structure, such as a weak layer buried under a slab. Avoid potential trigger spots where the snowpack is thin, like convex rolls and around rocks and windswept ridges. 

  • Wind Slabs

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

    Recent winds drifted snow into fresh wind slabs at upper elevations. The winds shifted from west over the weekend to east yesterday to southwest today. Isolated and stubborn wind slabs could be found in unusual places on all aspects due to the shifting winds. Wind slabs are often found just below ridges and in cross loaded gullies. Wind slabs can be identified by their smooth and rounded appearance. Signs of instability like cracking warn you that dangerous conditions exist. Carefully assess the snowpack before committing to upper elevation slopes steeper than 35 degrees. 

Avalanche Discussion

The avalanche hazard has remained fairly steady over the last few days. East winds yesterday created fresh wind slabs on west aspects at upper elevations. Today’s southwest winds may transport some of that snow back onto east aspects but keep an eye out for wind slabs on west aspects today. We can expect continued high pressure with more sun than clouds today and tomorrow. Tomorrow night, the weather pattern will begin to shift. A series of unorganized weather systems will impact us through the middle of next week. 

Between December 25th-28th, a strong, warm storm brought heavy rain up to 8000 ft and significant snow. We now have a several-inch thick rain crust up to at least 7000 ft that makes it difficult to trigger any weak layer beneath it. Due to this, we removed the persistent slab problem and dropped the danger rating to Low at middle elevations today. Remember that Low danger does not mean avalanches are impossible. In isolated areas with thinner crusts at middle elevations, it is still possible to impact weak layers below. 

The rain crust is thinner or nonexistent at upper elevations, and skiers and riders could still trigger failures in buried weak layers. The increase in snow depth from recent storms has promoted rounding in the snowpack. Buried weak layers are healing and becoming less sensitive. However, the healing process takes time, and poor structure remains, especially on northerly aspects. In the central and northern Bitterroot, weak basal facets near the ground are the primary layer of concern. In the southern Bitterroot, a crust facet layer just above the rain crust is the primary layer of concern. If you find any of these layers where you plan to ride, avoiding steep slopes is the best strategy. 

Low likelihood, high consequence problems like this are very difficult to manage. We don’t know exactly what it will take for a slope to fail and produce a dangerous avalanche. The weight of a skier or rider in the right trigger spot could be enough. If you are ever in doubt, take a step back to lower angle terrain. 

Public observations are a critical tool for us to produce accurate forecasts. Please help us out by submitting an observation. If you see any avalanches or experience collapses, we would love to hear about them! 

Ski and ride safe.

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