Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Thu
Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 6 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

The chance of triggering avalanches will increase throughout the day, and the danger will rise to CONSIDERABLE by sunset. Expect to trigger relatively small avalanches in windy areas this afternoon, and natural avalanches may occur. Triggering a very large avalanche that fails on weak snow buried 3 feet below the snow surface remains possible, especially in very steep, rocky, wind-affected terrain and the slopes beneath this type of terrain.

The avalanche danger will rise tonight as the storm progresses. Be very careful if you're out after dark. 

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    Today's weather forecast calls for 4-8" of low-density snowfall. Moderate winds will have no trouble forming thin wind slabs in exposed terrain. The new slabs should be most widespread near upper elevation ridgelines and will grow thicker and wider this afternoon.

    The fresh wind slabs will form on a variety of surfaces: weak faceted snow, crusts and facets, surface hoar, wind-hammered surfaces, and decomposing snow from last week. Expect some drifts and slabs to be very sensitive to triggering, and the chance of both human-triggered and natural activity will increase this afternoon. 

    Watch for cracks shooting out from your skis or sled. If you experience cracking on small, safe test slopes, think long and hard before entering consequential or large avalanche paths. 

  • Deep Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    3

    A layer of weak snow makes up the foundation of the snowpack here, and a dense slab is sitting on top of it.  If you dig a pit, expect to find a snowpack that "looks pretty good"....right up until you find the weak snow near the ground. This dense, 3+ foot thick slab makes it difficult to impact the layer of weak snow that is sitting below it. If you jump off your sled, chances are you won't even sink in past your boot tops. Grab the throttle and pull and your track likely won't dig down to the weak snow underneath. That is until you find the wrong spot on the slope where the slab is thinner. 

    Deep slab avalanche problems are challenging to deal with. This poor structure (strong snow overlying weak snow) is concerning, even in the absence of obvious signs of instability, like cracking and collapsing.  Because the weak layer is "protected" by a thick slab of snow, it is difficult to collapse the layer and initiate a propagating fracture (trigger an avalanche). However, if you do trigger one of these beasts, it will probably injure or kill you. These types of avalanches are often triggered after many riders have been on a slope. You are most likely to trigger one of these slides where the thickness of the slab varies and thin spots exist where you can impact the weak layer. These trigger points are most common in steep, rocky, wind-affected terrain. 

    (1/18/2021) This very large avalanche occurred in our Banner Summit Zone, likely during last week's storm. Based on the depth and appearance of the crown it probably failed on facets buried deep in the snowpack.

Avalanche Discussion

Whether I’m in the avalanche forecasting hot-seat or deciding how and where to go play, I usually ask myself three questions to begin my morning routine:

  1. What were the baseline conditions yesterday, and how much uncertainty existed?
  2. Did the overnight weather conditions do anything to change the stability equation?
  3. How is today’s weather going to affect snow stability, and how much uncertainty exists?

The Bottom Line and Avalanche Problem sections of the Forecast speak to these questions, but here’s a bit more nuance:

  1. Baseline: We have an especially ugly, resilient weak layer (facets buried on Dec 11th). It has produced countless natural avalanches with each decent storm, remote-triggered avalanches for many days after each storm, and continued significant collapsing this week (12 days after the last storm and 45 days after the weak layer was buried). The weak layer is widespread across mountain ranges, elevations, and most aspects. It’s more difficult to trigger where buried at least 2.5-3 feet deep, and it hasn’t been a major concern (yet!) where less than a foot of snow lies above it. I don’t trust this weak layer at all where a reasonable slab exists above it, and I’m quite certain of that. Also, we have some weak snow (small facets, crusts and small facets, and isolated surface hoar) at or near the surface. The near-surface weak layers are spatially variable; they change, appear, and disappear as you move to different aspects and elevations. Not all slopes have weak snow near the current snow surface. The weaker snow near the surface is a distant second on my list of weak layers to worry about, but it’s worth remembering as we come into a prolonged storm event.
  2. Overnight weather: Some areas picked up 1-3” of snow (0.05-0.1” SWE), a small load. If it quit snowing now, we’d be looking at pretty much the same hazard picture as yesterday.
  3. Today’s weather: By Thursday morning, the answer is easy in the Soldier &WRV and Galena & Eastern Mtns zones – stay the heck away from all avalanche terrain! Today is tricky, and uncertainty will grow as snow accumulates:
    • Smaller avalanches: Today’s snowfall will be fairly low density (blower powder), promoting sensitive wind slabs near ridgelines and sluffs (dry loose avalanches) in very steep terrain. On some slopes, these small avalanches will naturally release before the weight of the new snow stresses the old faceted weak layer to the point of failure.
    • Larger avalanches: The smaller, “new snow” avalanches moving down slopes could trigger large, destructive slides on the December facets. Also, the weight of the new snow could push the December facets over the brink on some slopes. While I expect this to become more likely from tonight through Friday as snow piles up and temperatures climb, I would not be surprised to see some “foreshadowing” events today.

So where does all of that leave us?

  • Don’t let powder fever get the best of you, especially in the Wood River Valley and Soldier Mountains where everyone has been “jonesing” for face shots all year. When the skiing and riding is really fun, our judgment can become clouded or disappear. We’re effectively hardwired to think “something this good can’t be dangerous.” Keep your head on a swivel.
  • With poor to no visibility, I would not consider playing in or crossing under any terrain with overhead hazard (large slopes over about 30 degrees above you).
  • Small, new snow avalanches (wind slab and dry loose) will become more and more likely as the storm progresses today, especially on slopes steeper than about 35 degrees.
  • The chance of triggering large, scary avalanches on the Dec facets layer – either remotely or while on a steep slope – was already enough to keep me out of most avalanche terrain before today. Adding a steady load from snowfall+drifting and the potential triggers from small slides running down slopes will increase the odds of seeing large, destructive avalanches.
  • MAKE A PLAN TO REEVALUATE YOUR DECISIONS AT SPECIFIC TIMES. Just because you crossed under a path or rode/skied a slope in the morning does not mean it will be safe in the afternoon.

 

Régions couvertes