Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Tue
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 3 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Large human-triggered avalanches will likely involve 1-3 foot thick slabs of wind-blown snow in exposed middle and upper elevations. While less likely, avalanches in terrain sheltered from the wind can't be ruled out. Small loose snow avalanches involving the new snow will also be easy to trigger.

Problemi valanghivi

  • Wind Slabs

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

    What is it? Expect to find sensitive 1-3 foot wind slabs in upper elevations. Over the past week, multiple large natural and human-triggered avalanches released in wind-loaded terrain in the Sawtooths and Smokys (photo above/right, list).  On Saturday, a rider in the Pole Creek drainage triggered a large wind slab that broke above them on the slope they were riding (observation here). Importantly, this slide occurred before the added load from recent snow and wind.

    Where is it? You'll likely find slabs in upper-elevation terrain, near ridgelines, and mid-slope terrain features like ribs and gullies. Slabs may extend down into middle-elevation terrain in areas where the wind has been blowing harder.

    How can you recognize it? Look for shooting cracks extending out from your skis or sled, which is an obvious sign of instability. Watch for drifts, dunes, and ripples on the snow surface. Wind 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 on or closer to the surface. Some of these slabs might be masked by recent snow and be more hidden.    

    How can you manage it? You can significantly reduce your risk today by steering clear of wind-loaded terrain. Today's slabs might not behave like your "typical" wind slab. Various weak layers exist in the snowpack's upper 1-2 feet (watch the video at right/above). Because of this weak snow, today's slabs may break wider and deeper than expected from a "simple" wind slab.

    ADDITIONAL DISCUSSION: The weak snow layers in the snowpack's upper 2-3 feet are being pushed toward their breaking points. Up to this point, we've needed the additional weight and stiffness from wind loading to make avalanches on these layers. Yesterday, a skier near Galena Summit steered away from skiing steeper avalanche terrain. When she returned back to the same slope, she stomped around the top and "caused the whole thing to spiderweb with cracks so vetoed that line." Later she got a different view of the bottom of the slope and noticed multiple crowns in the aprons. Triggering avalanches in sheltered terrain is becoming a real concern as the slab of snow overlying these weak layers piles up. Watch the video at right/above to better understand this problem.

Avalanche Discussion

Avalanche conditions are significantly changing from where we were last week. Slabs have been incrementally loading over the past nine days. Skier and rider-triggered avalanches and some early indicators from natural avalanche activity have slowly increased. Storm totals in the last 24 hours were most impressive in the Sawtooth and Western Smoky mountains and Banner Summit. Weather stations recorded 6-9 inches of new snow. Be prepared to encounter a different snowpack. 

In areas where the wind has been transporting this recent snow, wind slabs will be resting on layers of weak snow. Because of this weak snow underneath, these slabs may display more of a persistent slab character. What does this mean? It means they may be more sensitive and break wider and deeper than expected. As the slab over these weak layers gets thicker with more snow, the possibility of triggering avalanches in terrain sheltered from the wind comes to the table. In the past few days, we have seen a few small slabs that failed in largely sheltered terrain. These are clear indicators that encountering unstable slabs in sheltered terrain is becoming increasingly likely. 

The Avalanche Danger Scale is a tool to communicate the conditions you'll find in the backcountry. Each danger rating covers a broad range of conditions. The danger is MODERATE in the Soldier Mountains & Wood River Valley and Galena Summit & Eastern Mountain. However, we are at the upper end of that category. The further north and west you travel, the danger is CONSIDERABLE in the Sawtooth & Western Smoky Mountains and Banner Summit. These areas have seen more snowfall and crossed over the burly line of moderate danger into the more obvious realm of it being likely to trigger large avalanches in wind-affected terrain.

Today is not a good day to enter avalanche terrain lightly. Spend some time thinking about and looking at the snowpack you are encountering. It's a safe bet that you'll find weak layers in the upper snowpack and a patchwork of wind slabs resting on top. This week's human-triggered and natural avalanches are obvious signs that the snowpack isn't adjusting well to this new load. 

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