Bollettino valanghe

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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In sintesi

Triggering a dangerous avalanche is most likely on steep, wind-affected slopes—especially those with stiff or hard drifts of windblown snow. A large avalanche was triggered yesterday on Galena Summit on a slope with these characteristics. You are unlikely to trigger a slide if you stick to wind-sheltered terrain. Avalanches can be triggered remotely—from below, above, and to the sides of steeper terrain.

Problemi valanghivi

  • Persistent Slabs

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    Probabilità
    Possible
    Dimensione
    2–3

    December facets near the base of the snowpack remain the primary stability concern in this zone. In the western portions of this zone (Soldiers, Dollarhide area) and at upper elevations, the snowpack depth is greater and this layer is buried beneath a 2-3' thick slab—making it more difficult to trigger. In the Wood River Valley, the snowpack is a bit thinner and the facets may be more easily affected by your weight. Throw in the effects from last week's wind event, and you have a highly variable snowpack. There are two general scenarios where you are more likely to trigger a slide:

    1. Steep (over 35 degrees), rocky/variable-depth , wind-affected, "prime-time" terrain. On the western side of this zone where the snowpack is deeper, this type of terrain will have more trigger points where your weight can more easily impact the deeper weak layer. Yesterday, a skier in the adjacent Galena Summit & Eastern Mtns zone intentionally triggered a large avalanche (photo above/right, more photos here) on a slope with these characteristics. By stomping around on the ridge crest, he initiated a hard wind slab that formed during last week's wind event, which then "stepped down" to the December weak layer. Ben checked out this slide yesterday afternoon—watch the video.
    2. Where you find old, harder wind slabs from the recent wind event. Especially where the snowpack remains thinner, these stiff wind slabs may be sitting over weak, faceted snow. Yesterday, a rider between Wells and Dollarhide Summits triggered an avalanche on a slope that likely had a snowpack that matched this description. Avoid steep slopes where the snow feels hard, hollow, or drum-like.

    ADDITIONAL DISCUSSION:Soldier Cat Ski guides reported that up to 10" of new snow has accumulated in the Soldiers since Thursday. Combined with moderate to strong W to NW winds, this new snow has likely been blown into fresh wind drifts and slabs along upper elevation ridgelines. In wind-exposed terrain, avoid slopes that have a drifted or pillowed appearance. These fresh wind slabs are likely to be somewhat softer than the old, hard wind slabs that formed last week.

    (2/12/2021) This avalanche was triggered by a rider NE of Wells Summit in the Soldier & Wood River Valley zone. The slide broke around 40' wide and carried the rider about 100 yards. A section of the cornice also released in the slide.

Avalanche Discussion

ON THE IMPORTANCE OF SNOWPACK DEPTHOne very simple, yet powerful, observation you can make in the backcountry is snow depth. It turns out you're already carrying the measurement device—your avalanche probe—and it takes no time at all to pull it out and measure the total depth of the snowpack. You're also carrying another important tool—your eyeballs. Visual clues tell you about the snow depth: Are you in a deep area that is generally covering rocks, small trees, and other surface features? Or do you see a lot of these poking through the snow or giving it a "lumpy" appearance? Is the snow depth variable or relatively uniform?

Why is the depth of the snowpack important? I'm glad you asked:

  • Deeper snowpacks are warmer and more insulated from cold temps at the surface. I don't think you want me to dive into temperature gradients, but processes that strengthen the snowpack are more pronounced when the snowpack is deep. What's deep? In the avalanche world, rules of thumb are problematic at best, but let's say "deep" means a total depth greater than around 4'. Snowpacks less than 4' deep tend to have more weak snow, and the weak snow improves slowly.
  • It's not uncommon to see a divergence in our snowpack conditions in the middle of winter when our deeper areas surpass this threshold. For example, the snowpacks in our deeper zones—the Sawtooths & Western Smokys and Banner Summit—are quickly becoming very different from our southern zones where the snowpack is less deep. In these areas, triggering the December weak layer has become unlikely, and surface problems due to new snow and wind are becoming the primary concern. In our southern zones where the snowpack is closer to the 4' depth, there is still a real threat of triggering an avalanche on a persistent weak layer.
  • On a smaller scale, how snow depth varies across a slope plays a critical role once a weak layer is buried deeper than about 3'. Once a weak layer becomes deeply buried, it's more difficult to impact with our weight. But on a slope with variable snow depth—which is common in steep, rocky, and especially alpine terrain—shallow areas exist where the weak layer is closer to the surface. We call these areas potential trigger points.

Next time you go into the backcountry, try putting more observational focus on snow depth, both on a large scale (how does the average depth compare to other forecast zones) and on a small scale (how much depth variation do I see on a particular slope).

Be sure to catch up on the happenings this past week in Ethan's latest Weekend Update.

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