Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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In sintesi

Triggering a large, destructive avalanche is likely today. 2-3 foot thick slabs are sitting on weak snow and can be triggered from long distances away. These weak layers exist on all aspects and elevations—avoiding avalanche terrain is the only way to avoid them. Wind drifts and slabs will be reactive and may be avalanching naturally today, which can trigger much larger avalanches on deeper weak layers. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2–3

    You can trigger large, dangerous avalanches on just about any steep slope you encounter. Yes, even at lower elevations and in terrain sheltered from the wind. Widespread weak layers are now buried beneath 2-3 foot thick slabs. 

    Yesterday, JP and I were in this zone. We received repeated unstable snowpack test scores in our 3 pits. This, in combination with the obviously poor snowpack structure (slab over weak layer), was enough to keep us out of all avalanche terrain. 

    Persistent weak layers produce avalanches that behave in surprising ways. They can break wider than you'd expect and can be triggered remotely—from gentler terrain above, below, and to the sides of steep slopes. Plan your day expecting to be able to trigger these types of avalanches. Thanks to the widespread nature of the weak layer+slab combination, avoiding slopes steep enough to slide is the only way to ensure a safe return home. Playing on or below steep terrain continues to be a risky dice roll.

    (12/12/2022) In the Banner Summit zone, the weak November snow is buried beneath a 3' thick dense slab. 

  • Wind Slabs

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

    Avalanches in wind-drifted snow are likely today and may be large enough to bury you. These will be easy to trigger and may release naturally. Saturday, a public observer reported finding sensitive wind slabs on Copper Mountain. These slabs have only grown larger in the time since. Yesterday, I observed a handful of slides in the Boulder and Sawtooth Mountains that had failed in the previous 12 hours. Some broke wide and piled up significant amounts of debris (photo at right/above). Wind slabs are most widespread at upper elevations, but extend down into middle elevation terrain as well, thanks to the strength of the wind. If you are finding stiff snow at the surface and see cracks shooting out from your skis or board, you've found a reactive wind slab. You don't want to find one of these in avalanche terrain. 

    Wind slabs that fail (both naturally and ones you trigger) can act as effective triggers for the deeper weak layers described above. The result will be a much larger, much more destructive avalanche. 

    (12/18/2022) This large avalanche was observed in the Boulder Mountains on Sunday evening. It failed in heavily wind-effected terrain on a W/NW facing slope at 9,700'.

Avalanche Discussion

Snow from our big storm ended almost a week ago. However, snow falling from the clouds isn't the only way to load a snowpack. Wind speeds picked up significantly yesterday and very large amounts of snow were transported from one place to another. From the perspective of the snowpack, it "feels" like it is snowing right now, and our weak snowpack is struggling to bear the weight. Read on. 

Near the surface: Wind slabs are building and are growing in size and distribution. Thanks to the strength of the wind, these slabs are stiff and dense, and extend down into middle elevation terrain. In the calm, cold, clear weather following our big storm, the snow surface weakened significantly, and these new slabs are building on this weak snow surface. Yesterday, I observed a handful of natural avalanches that had failed in wind-loaded terrain. I expect that more happened overnight and will continue to happen today. 

Deeper in the snowpack: The weight of the new wind slabs is adding stress to the layers of very weak snow in our snowpack. These weak layers are well-developed and widespread—go dig a hole on a slope in our forecast area and you will find them. These weak layers produced a remarkable avalanche cycle just a week ago. Many hundreds of large avalanches occurred, on slopes from the valley floors all the way to the mountaintops. The weak layers are now experiencing significant stress from the added weight of the wind slabs on top. These slabs are also adding structure to the slab that already existed on top of our weak layers. 

Our very weak snowpack has been teetering on the edge since the storm ended last week. Reports of very large, rumbling collapses have come into us every day, and we've been out there experiencing them with you. This was all before the wind went to work. Add in the effect of the wind and you've got yourself a recipe for some hazardous conditions. Picture yourself triggering a small wind slab and watching it step down to weak snow near the ground and ripping out an entire alpine bowl. Yikes. We are a long way from being out of the woods, it's a dangerous world out there and it's important you travel with that in mind. 

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