Bollettino valanghe

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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In sintesi

Large human-triggered avalanches failing on layers of weak snow are possible. A skier-triggered slide near Galena Summit yesterday is a good reminder that these slabs are possible in both wind-affected and sheltered terrain. Recently formed wind drifts and slabs are likely to be small but may be sensitive to your weight. The consequences of being caught in even small slides can be high in terrain with rocks, cliffs, trees, and terrain traps.

Problemi valanghivi

  • Persistent Slabs

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

    Several weak layers exist in the snowpack. You're most likely to trigger avalanches on weak layers that are closer to the surface, but slides on deeper weak layers remain a real possibility here. Yesterday, a skier near Galena Summit triggered a 1-1.5 foot thick slab that appears to have failed on the layer of surface hoar described below. It failed on a N/NE-facing slope at 9,100'. The distribution, sensitivity, and depth of these layers vary across the zone and from slope to slope. Let's take a closer look:

    • Surface hoar is buried 1-1.5' deep on many shady (E-N-W) slopes. On slopes that face the sun, you'll find crusts with facets on top at this depth. In the mountains above the Wood River Valley, this layer is most problematic where recent wind loading has created a stiffer slab in the upper snowpack. In the western portion, from Dollarhide to the Soldiers, more snow has fallen and the slab overlying this surface hoar is thick enough to produce avalanches in both sheltered and wind-affected terrain. Read more about the variability across this zone in the Forecast Discussion below.
    • Older weak layers—crusts and/or facets, and spotty 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. You're most likely to trigger a slide involving these deeper weak layers in rocky, wind-affected, alpine terrain. Slopes like these have variable snow depths, with thinner areas of the snowpack that allow us to impact deeper weak layers. The recent persistent slab avalanche in the Pioneer Mountains is a great illustration of this terrain (above/right, details)

    Overlapping, variable weak layers make avalanche conditions hard to predict. You can reduce (but not eliminate) your risk of triggering a large persistent slab avalanche by avoiding slopes greater than about 35 degrees where the wind has shaped the snowpack. Look for obvious wind textures like drifts and dunes that indicate that the wind has been at work. Very stiff snow surfaces that support the weight of you and your machine are a clear sign the wind has been transporting snow.

    (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.

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–1.5

    You'll find a patchwork of new and old wind slabs at and near the surface. A multi-day wind event earlier this week constructed stiff, dense wind slabs. 1-4" of new snow+wind over the past two days built another batch of softer slabs on top of these older slabs. Today, wind slabs will be variably reactive, and the most recent slabs are likely to be the most sensitive to your weight. The Dollarhide area appears to have picked up the most snow and fresh wind slabs there are likely to be larger and more reactive. Slabs are most likely in exposed upper and middle elevation terrain.

    Multiple wind slab avalanches were observed in this Zone last week (photo at right/above). Though relatively small, these slides picked up speed and snow as they traveled downslope. Be on the lookout for fresh slabs—if the snow you are moving through becomes deeper or denser you've likely found one. Cracks shooting out from your skis or sled are a clear sign that you've found an unstable slab. Triggered slides will pick up speed quickly thanks to the stiff/dense snow underneath. Small slides can also pile up debris deeply in terrain traps like road cuts or creek beds (photo here). Consider the consequences of being knocked off your feet and sent downslope in the terrain you are in.

    (1/24/2023) This small wind slab avalanche occurred recently above the Hyndman Creek trailhead, likely as a result of the N-NW winds. As the avalanche moved downhill, it appears to have entrained loose surface snow. 7500', ESE.

Avalanche Discussion

Early season storms brought roughly equal helpings of snow across our forecast area. This helped to build a snowpack that was somewhat homogenous across the region. However, over the past month, snowfall has favored our northern and western mountains and we've seen a significant divergence in the character of our snowpacks. In a broad sense, you can lump them into two categories: shallow and deep. The Soldier and Wood River Valley Forecast Zone straddles the boundary between these categories. 

On the eastern side of this zone and in the mountains above the Wood River Valley, the snowpack remains thin (around 3' deep) and much of the snowpack is faceting. When this happens, the slabs in the snowpack lose strength and triggering an avalanche on buried weak layers 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' deep) and the slabs over the weak layers remain intact. Weak layers buried in November and December are resting underneath very dense, heavy slabs of snow. Impacting these weak layers with our weight is less likely. However, the prominent weak layer that was buried in January is sitting under a 1.5-2 foot thick slab and impacting it with our weight remains a real possibility. This layer has proven itself to be at its worst on shaded slopes (those on the northern half of the compass), where standing surface hoar can be found under this dense slab. However, a pattern of unstable snowpack test scores on sunnier slopes—where a crust is accompanied by facets and some surface hoar—is worth keeping in mind as you travel through the mountains. 

On top of these weak layers we have a few generations of wind slabs that formed this week. Recently constructed slabs are likely to be softer and more reactive to your weight. Older, harder wind slabs from last week's wind event will be less reactive, but triggering a slide involving one of these slabs remains a possibility. Triggered wind slabs 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. Spend some time thinking about the consequences of being caught in a slide as you are choosing your terrain for the day. Today's very cold temperatures add another layer of hazard to the mountain environment. The consequences of even minor injuries or mechanical issues with your machine can be greatly amplified by arctic temperatures. Pack an extra layer or two and some extra food today.

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