Bollettino valanghe

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Fri
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2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

You're most likely to trigger avalanches where the wind recently formed drifts or slabs. In areas more protected from the wind, triggering a 1-2 foot thick avalanche is not out of the question. Even small slides can have big consequences on slopes with cliffs, trees, and terrain traps.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    Recent NW-N-NE wind formed soft and hard slabs in exposed terrain. A handful of small natural wind slab avalanches occurred over the weekend (Galena Pass, Banner). Monday, we spotted a large wind slab in the Soldier Mtns (photo). 

    This problem is most dangerous at upper elevations where slabs are larger and more widespread. Look for them near ridgelines, on the sides of gullies and chutes, and near prominent terrain features. 

    Cracks shooting out from your skis or sled are an obvious sign of instability (example). Watch for drifts, dunes, and ripples on the snow surface. Older hard slabs can have a smooth, chalky appearance and feel and sound hollow, like a drum. Wind-deposited snow will be stiffer and keep your skis or snowmobile closer to the surface.

    (1/30/2023) This large avalanche in the N Fork Soldier Ck drainage released on a SE aspect near 9000', likely over the Jan 28-29 weekend.

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1.5–2.5

    Multiple weak layers exist in the snowpack, and triggering a persistent slab avalanche remains possible. The distribution, sensitivity, and depth of these layers vary across the zone. Here are two distinct scenarios:

    1. Surface hoar: A skier triggered a slide near Galena Summit Sunday (view report). The 1-2' thick avalanche failed on a layer of buried surface hoar. This layer is responsible for several skier and rider triggered avalanches over the past two weeks. You're most likely to encounter this problem in the Soldier Mtns and near Dollarhide Summit. In the Wood River Valley mountains, most slopes lack a slab above this weak layer. This layer is most problematic on slopes facing NW-N-NE that are steeper than about 35 degrees.
    2. Older weak layers: Crusts and/or facets, and occasionally surface hoar, exist deeper in the snowpack. These are buried beneath dense, 2-4' thick slabs—making them difficult to trigger but capable of producing very dangerous avalanches. While triggering them is unlikely, it can't be ruled out in rocky, wind-affected, alpine terrain (photo above/right). Slopes like these have variable snow depths, with thinner areas of the snowpack that allow us to impact deeper weak layers.

    Overlapping, variable weak layers make avalanche conditions hard to predict. The single best way to reduce your risk is by avoiding slopes greater than about 35 degrees. Consider the consequences of going for a ride. Even small slides can have bad outcomes if they lead to trees, cliffs, or a creek bed. 

    (1/24/2023) This recent large avalanche occurred on the shoulder of Duncan Ridge in the Pioneer Mountains. It appears to be a heavily wind-loaded slope that broke on a persistent weak layer. A smaller wind slab avalanche is visible further right in the image. 10,100', WSW.

Avalanche Discussion

On the eastern side of this zone and in the mountains above the Wood River Valley, the snowpack remains thin (around 3 feet deep); much of the snowpack is faceting. In the past week, we've seen a few "facetlanches",  dry loose snow avalanches involving old, weak faceted snow at the surface instead of the more typical "powder" dry loose avalanches. As the entire snowpack facets, both weak layers and slabs are losing strength. With the decrease in slab stiffness, triggering slab avalanches becomes more difficult. Here, you are much more likely to trigger a slide where recent winds have built slabs on top of the weak snowpack.

On the western side of this zone (Dollarhide and west to the Soldiers), the snowpack is thicker (4-6 feet deep), and the slabs above the weak layers remain intact. Weak layers buried in November and December lie beneath very dense, heavy slabs of snow. Impacting these weak layers is less likely. However, the prominent weak layer that was buried in January is sitting under a 1-2 foot thick slab; impacting it remains a real possibility. This layer is ugliest on wind-protected, shaded slopes (those on the northern half of the compass) where surface hoar formed. However, a pattern of unstable snowpack test scores on sunnier slopes—where facets and surface hoar lie above a crust—is worth noting. 

On top of these weak layers, a few generations of wind slabs formed in the past week. The most recently formed slabs are likely to be softer and more reactive to your weight. However, the large, recent slide in the Soldier Mtns probably involved rock-hard snow that drifted onto the slope last week; triggering beasts like that is a possibility. Triggered wind slab avalanches can be effective triggers for deeper weak layers in the snowpack. 

The consequences of triggering avalanches can vary significantly. Triggering a small slide in terrain free of rocks, trees, and terrain traps is much less dangerous than triggering a slide in extreme, rocky, treed terrain with a terrain trap at the bottom. Think about the consequences of being caught in a slide before committing to skiing or riding in sizeable avalanche paths. 

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