Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Mon
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Highlights

Triggering a 1-2 foot thick avalanche in wind-drifted snow is possible today. The likelihood of triggering avalanches will increase throughout the day as a storm brings new snow and wind. While less likely, avalanches in terrain sheltered from the wind can't be ruled out. Small loose snow avalanches involving the new snow will be easy to trigger later in the day. 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    4-8" of new snow is forecast for the Banner Summit area by the end of the day. Snowfall will be accompanied by moderate winds blowing out of the SW. Winds will easily transport the new snow into slabs that will be increasingly sensitive as they grow in size. Over the past week, multiple large natural and human-triggered avalanches released in wind-loaded terrain in the Sawtooths and Smokys (list). Yesterday, 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, all of these slides occurred before the added load from the snow that will fall today.

    The wind slabs that you'll find today may behave in ways that surprise you. A variety of weak layers exist in the upper 1-2' of the snowpack, and recent wind slabs are resting on top of them. Because of this weak snow, today's slabs may break wider and deeper than you'd expect from a "simple" wind slab. Avalanches that break down into these deeper weak layers will be larger and more destructive, and may be triggered from a distance away from the slope that slides (remote triggering). This is exactly what happened on Wednesday, when skiers on Titus Ridge near Galena Summit remotely triggered a large avalanche that broke on one of these layers (report with photo and video here). The crown was 2 feet thick and over 500 feet wide and the debris piled up 10+ feet deep against large trees. 

    You can reduce (but not eliminate) your risk by steering clear of wind-loaded terrain. Unfortunately, today's snowfall will mask the presence of older slabs, making it much more difficult to tell if a slope has been affected by the wind. You'll be most likely to 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. If the snow you are moving through becomes deeper and denser you are likely moving through a wind slab. Snowpack collapsing or cracks shooting out from your skis or sled are clear indicators you are traveling on an unstable slab.

    ADDITIONAL DISCUSSION: The layers of weak snow in the upper 1-2 feet of the snowpack are being pushed toward their breaking points. Up to this point, we've needed the additional weight and stiffness from wind loading in order to make avalanches on these layers. However, 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 get a better sense for this problem.

Avalanche Discussion

Avalanche conditions are ripe for getting surprised right now. And not in a good way. Conditions have been changing this week and will continue to do so today. If your last trip in the mountains was last weekend, be prepared to encounter a different snowpack than the one you found then. Read on.

Storms over the past week have gradually piled up 14-18" of snow in this zone. An additional 4-8" are expected by nightfall. In areas where the wind has been transporting this snow, you'll find wind slabs 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 you might otherwise expect. This loading process has been slow and incremental, but the weight of the new snow is starting to add up and should not be ignored. As the slab over these weak layers gets thicker, the possibility of triggering avalanches in terrain sheltered from the wind comes to the table. We have seen a few small slabs that failed in largely sheltered terrain in the past few days. These are clear indicators that encountering unstable slabs in sheltered terrain is becoming increasingly likely. 

The Avalanche Danger Scale is a tool we use to communicate the conditions you'll find in the backcountry. Each danger rating covers a broad range of conditions. The danger is MODERATE , but we are in the upper end of that category. 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. 

With wind slabs sitting on weak layers and a budding persistent slab problem in sheltered terrain, there is a lot of uncertainty in the forecast. I'm managing my uncertainty by stepping back from some of the bigger, higher-consequence avalanche terrain that I was comfortable playing in last week. I'm taking the time to prove to myself that entering the specific piece of avalanche terrain in front of me is a good idea. And when evidence points to instability, I'm content choosing lower angle terrain free of terrain traps like creek bottoms and thick trees. Skiing and riding conditions are excellent right now, don't let this distract you from the hazards that hide beneath the fresh snow. Triggering an avalanche large enough to bury you is well within the realm of possibility today. 

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