Bulletin d’avalanche

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Danger d’avalanche

Wed
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 3 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

You can likely trigger an avalanche large enough to bury you on many slopes. Slides running on near surface weak layers will move 1-2 foot thick panels of snow downhill. More destructive larger avalanches failing on deeper weak layers that involve 2-4 foot thick, dense slabs are still possible — potentially breaking all the way to the ground. All of these slides can be triggered from long distances away. Wet snow avalanches may become possible as the sun comes out in the afternoon.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    2–3

    Under the recent storm snow, you will find thick, very dense slabs resting uncomfortably on layers of very weak snow. This combo has produced dozens of recent avalanches in the neighboring Galena Summit and Eastern Mtns zone. Many of these slides have been large enough to snap mature trees and destroy large vehicles. A similar snowpack exists in many portions of this zone. If you dig down, you'll find thick piles of facets with or without surface hoar on shaded aspects, and crust+facet combinations on slopes that get more sun. Recent avalanche activity has been much more widespread on shady slopes, but a few large avalanches failing on slopes that face the sun indicate that this problem exists in those areas as well. 

    Because the slabs on top of this weak layer are so dense, and the new snow is so soft and fluffy, you aren't likely to encounter many or any signs of instability before triggering a large, potentially unsurvivable avalanche. Keep this weak snow at the front of your mind when you are moving through the mountains. The only guaranteed way to avoid this problem is to steer clear of avalanche terrain and slopes threatened by overhead hazard. Any avalanche that fails on this weak layer will be large and destructive, and slides can be triggered from long distances away (remote triggering).

    ADDITIONAL DISCUSSION - WET SNOW ISSUES: On slopes where the sun is warming up and melting the snow surface this afternoon, loose snow slides are possible and will have a wet character. If you're finding moist/wet snow at the surface or if you are seeing wet balls of snow rolling down the slope this type of slide is coming onto the menu. Loose snow slides will start out small, but can pick up steam and run quickly thanks to the stiff and crusty surfaces underneath the recent snow. 

    (2/6/2024) The Sun Valley Ski Patrol ski-triggered this large avalanche Tuesday morning in the Cold Springs Chutes in the Sunrise Expansion Area. This area has not yet been opened to the public. It failed on a NE aspect near 8000'. The slide was 2-4 ft tall, 100 ft wide, and broke branches during its 900 ft vertical run. The debris piled up 4-7 feet deep against trees. Photo: Sun Valley Ski Patrol

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    Today's 1-3 inches of fresh snow will add to the 5-10 inches of recent snow in the eastern portion of this zone and 10-20 inches in the western portion. You can expect to find a wide range of problems involving this recent snow. These broadly fall into two categories:

    SLABS: Periods of shifting winds have built several generations of wind slabs at the surface. Look for drifts, dunes, and rippled snow surfaces as an indication that the wind has been at work, but expect much of this evidence to be masked by today's snowfall.  Cracks shooting out from your skis or machine are a clear sign that you've found an unstable slab that could be triggered on steeper slopes. Even in terrain that is apparently sheltered from the wind, you can trigger slabs that fail at the base of the February snow (see photo at right/above). The variable layers of weak snow that are generating this problem appear to be at their worst where either sun crusts or rain crusts exist at this interface. Read more about the budding persistent slab problem in the Forecast Discussion below. 

    LOOSE SNOW: In terrain sheltered from the wind, expect small loose snow avalanches to be easy to trigger. If you're seeking out cold, crust-free snow on northerly aspects these sluffs will have a dry character. On slopes where the sun is warming up and melting the snow surface this afternoon, these loose snow slides will have a wet character. If you're finding moist/wet snow at the surface or if you are seeing wet balls of snow rolling down the slope this type of slide is coming onto the menu. Loose snow slides will start out small, but can pick up steam and run quickly thanks to the stiff and crusty surfaces underneath the recent snow. 

    Avalanches involving the new snow will range from small to large enough to bury you. Avalanches failing near the surface can be effective triggers for avalanches that fail on the deeper weak layer described above. 

    This large avalanche was remotely triggered by skiers near Dollarhide Summit on Friday. It failed on an E/SE-facing slope at 9,100'. Based on the appearance of the slide, it likely failed on a weak layer that was buried at the beginning of February. 

    Photo: Sun Valley Heli Ski

Avalanche Discussion

Complexity, uncertainty, and unpredictability have all been increasing in the snowpack over the last two weeks. Throughout the season, you've returned to the Forecast Discussion to see a similar vein in a lot of our messages about the snowpack: buried persistent weak layers can fail underneath newer snow to cause avalanches. While this remains true today, things are changing and becoming more complex, as we have a budding Persistent Slab problem. February storms have buried new weak layers across our zones that are taking many forms on different slopes underneath 1-2 foot thick soft slabs, and taking on hard-to-predict characteristics. These have produced a large, remotely triggered slide in addition to multiple other large natural avalanches in the Sawtooths, collapsing and unstable test results near Banner Summit, and very wide natural avalanches near Durrance, but also seem to be bonding well on other slopes. 

This complex budding Persistent Slab problem seems to be at its worst on slopes with thin crusts with facets on either side (a crust + facet combo). Avalanche activity involving these weak layers has been focused on slopes that get more direct morning and evening sun ("solar margins", or slopes facing E-SE and WSW-W) but still has a lot of uncertainty in its exact distribution. A widespread rain crust that extends to above 10,000' exists here as well, producing more signs of instability like collapsing and unstable snowpack test scores. Here is a quick video from a couple of days ago discussing this developing issue.

This emerging persistent slab issue increases our snowpack's complexity, uncertainty, and unpredictability. It also adds each of these to our decision-making when traveling in the backcounty as well. The only proper way to address each of these is through giving yourself wider margins, taking attentive observations, and continually reminding yourself of the big picture. Yes, the snowpack has easy to trigger near-surface snow instabilities that may come as wet snow or Wind Slabs, a budding Persistent Slab problem that is likely to trigger 1-2 feet down, and a Persistent Slab problem that is harder to trigger 3-4 feet down. But the big picture? It is a hard to predict world out in the hills right now. Adjust your travel plans accordingly, and come home to tell us about the wild slides you saw, from a safe distance, that you never would have expected.

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