Boletín de aludes

Galena Summit & Eastern Mtns

Sawtooth Avalanche Center
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Peligro de aludes

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

Lo esencial

You can likely trigger an avalanche large enough to bury you on many slopes. Slides running at the bottom of the recent snow will move 1-2 foot thick soft 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. Loose snow avalanches will be easy to trigger in sheltered terrain. 

Problemas de aludes

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Likely
    Tamaño
    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 this zone. Many of these slides have been large enough to snap mature trees and destroy large vehicles (see photo at right/above). 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).

    This very large avalanche on Galena Peak failed naturally sometime between 2/9 and 2/10. The crowns span 2,000 feet of terrain and the slide ran over 1,000 vertical feet. The majority of the slope that avalanched faces NW. 

  • Wind Slabs

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

    Today's 1-3 inches of fresh snow will add to the 5-15 inches that fell at the end of the week. 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

Throughout the season, you've returned to this place 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 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. Primarily we've been managing a progressively more deeply buried weak layer that is creating our widespread Persistent Slab problem. Difficult to manage due to the ease of remote triggering avalanches on this layer, the potential size of these slides, as well as being entrenched in the same mindset for much of the season since the December drought ended, this weak layer has been mostly uniform throughout our zones. Our newly buried February weak layers are weak enough to produce large slides from a distance on some slopes (remote trigger), while bonding quite well on others. 

This budding Persistent Slab problem seems to be at its worst on slopes with thin crust 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). 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. A 1-2 foot thick slab overlies these weak layers, and is growing with each storm.

While it's hard to say with certainty where these weak layers are heading, here are some data points nudging us to pay heed to them going forward:

Dollarhide remotely triggered avalanche - A large avalanche was remotely triggered by the professionals at Sun Valley Heli Ski on Friday (photo here). It broke several hundred feet wide on an E/SE facing slope at 9,100'.Profile drainage natural avalanche - A large natural avalanche was observed in the Profile drainage yesterday (photo here). It broke several hundred feet wide on an E/SE facing slope at 8,500'.Parks Peak natural avalanche  - A very large natural avalanche was observed on Parks Peak that likely failed at the tail end of the storm on Thursday on a SE-facing slope at 10,000' (photo here).Banner Summit area -  View my observation from Friday of collapsing and unstable snowpack tests involving this weak layer here.Durrance area natural avalanches - Very wide crowns involving this weak layer were observed adjacent to avalanches that failed on deeper weak layers following storms earlier in February.  

And here is a quick video from yesterday discussing this developing issue.

Complexity, uncertainty, and unpredictability have all been increasing in the snowpack over the last two weeks. The only proper way to address each of these is through giving yourself wider margins, taking attentive observations, and to continually remind yourself of the big picture. Yes, the snowpack has easy to trigger near-surface snow instabilities that may come as loose snow or wind slabs, a budding persistent slab problem that is likely to trigger 1-2 feet down, and a persistent slab probelm 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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