Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

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

Points saillants

There's a good chance you will trigger 1-2 foot thick avalanches on steep slopes. Wind-loaded areas with drifts or stiffer snow near the surface are most suspect. While unlikely, we can't rule out the chance of triggering a huge, destructive, 4-foot thick avalanche—especially on very steep, rocky, wind-affected slopes.

Problèmes avalancheux

  • Persistent Slabs

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

    Last night's snowfall and wind formed a fresh batch of wind slabs and increased the chances of triggering avalanches on weak snow (surface hoar and some faceted snow) buried 1-2 feet below last week's storm snow. On Monday, Chris noted a wide, natural avalanche that may have failed on this layer at the tail end of last week's storm. He found surface hoar that was reactive in his snowpack tests (see video to right/below). Terrain where the wind formed drifts or stiffened the snow near the surface should be considered guilty until proven innocent, and sheltered slopes should be considered suspect.   

    Surface hoar is tricky and has a history of fooling seasoned avalanche professionals. Unfortunately, we're in the sweet spot for surface hoar avalanches in this zone right now. Some surface hoar basics:

    • Most surface hoar avalanches are 1-3 ft deep.
    • Most happen during or after the storm following the first snow event that buries the surface hoar layer (ours got buried in last week's storm).
    • Most happen within the first 3 weeks of burying the week layer (we're about a week out from burial).

    While the fresh drifts will be easy to spot and avoid, you may or may not see obvious signs of instability (cracking, collapsing) before triggering an avalanche on the weak snow 1-2 feet deep. Slides involving surface hoar can pull into relatively flat terrain above and to the sides of steeper slopes.

    It's worth poking around in the snow to look for the tell-tale gray stripe (buried surface hoar) before entering any avalanche terrain. You'll need to dig a lot of quick holes to look for it because surface hoar can be finicky when it forms; it needs just the right weather conditions, and slight changes in aspect, wind exposure, tree cover, or elevation can either enhance or prevent its formation.

    Conditions are dangerous today—keep your slope angles in check, be alert to the terrain above you, and pay attention to the consequences of the slopes you plan to ski or ride. Any small slides you trigger that involves just the new storm snow could step down and trigger larger avalanches on the surface hoar layer or possibly the deeply-buried faceted layer described below in Problem 2.

    You can read more about weak layers in the Forecast Discussion section below.

  • Deep Persistent Slabs

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

    Our snowpack is deepest in this zone, but there is a monster sleeping in the basement. A dense, 4+ foot thick slab is resting on a weak layer of sugary depth hoar near the ground. The snowpack will take time to strengthen enough that we can forget about the depth hoar lurking below. Chris looked at this layer on Friday and found it was showing few signs of healing. The new snow from last week and last night's storm added more stress to this layer, keeping the monster from going to sleep.

    Avalanches occur when we are able to collapse a weak layer, initiating a fracture that propagates through the weak snow. Because the slab over this weak snow is so thick and dense, it is difficult for us to impact the weak snow. However, if you're able to find a spot to collapse the weak layer, the resulting avalanche will be huge and probably unsurvivable. Triggering a deep slab is most likely in very steep, rocky, wind-affected terrain where "trigger points" exist. Trigger points are locations where the overlying slab is thinner and the weight of you and-or your sled can more easily impact the weak layer.  You can reduce the chances of triggering one of these beasts by sticking to slopes less than 35 degrees and avoiding craggy, uneven terrain peppered with rocks.  

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

Avalanche Discussion

THE EVOLUTION OF WEAK LAYERS: PART 2

An update on Ben's discussion on weak layers: 

Weak snow near the base of the snowpack: This snow has been buried for 50 days and is not yet showing signs of going to bed. Up until last week, this layer was responsible for nearly all of the avalanches we've observed and recorded this season (approaching 500). It produced countless avalanches during last week's storm and will probably continue to misbehave. It is now buried 3-5' deep, and any avalanche that fails on this layer will be large enough to injure or kill people.

Weak snow underneath last week's storm snow: This weak snow formed in mid to late January, and the character of this layer varies by aspect (the direction a slope faces) and elevation. You might find crusts, crusts and faceted snow, small facets, and some isolated surface hoar (more widespread near Banner Summit). 

Weak snow buried last night: Before this storm, warm temperatures and sunshine formed crusts on many south and west-facing slopes. Also, some surface hoar just got buried on a variety of slopes (observed in the Banner Summit, Galena Summit & Eastern Mtns, and Soldier & Wood River Valley Mtns zones). Surface hoar is tricky and has a history of fooling seasoned avalanche professionals. Some surface hoar basics:

  • Most surface hoar avalanches are 1-3 feet thick.
  • Most surface hoar slides happen during or after the storm following the first snow event that buries the surface hoar layer. Near Banner Summit, a surface hoar layer was buried in last week's storm, so this zone now fits this description.
  • Most surface hoar slides occur within the first 3 weeks of burying the week layer (we're about a week out from burial near Banner Summit and starting the clock in other zones where surface hoar was buried intact last night).

Wasn’t it nice when the snowpack was just a simple, hot steaming mess, and all you had to worry about was the obvious weak snow relatively close to the ground? The deeper weak layer (December facets) has shown signs of improvement in the Banner Summit area and now has enough snow above it to begin the healing process throughout our forecast area. It won’t happen overnight, or even in a week, but there may be light at the end of the tunnel. Hopefully, that light isn't a freight train. If/when the December facets finally fade into the sunset, how will our snowpack behave? The magic 8 ball says “future uncertain.”

  1. One of the newer weak layers (now buried in the top 1-3 feet of the snowpack) could take the baton from the December facets.
  2. A “yet to be named” player could emerge.
  3. The older weak layers stabilize, we get frequent small storms, and no additional ugly weak layers form. This option gets my vote, but it’s the least likely of the 3.

The snowpack has become a lot more complex, and we’re going to have a difficult time deciding when it’s safe to play on those steep slopes we’ve been patiently waiting to ski and ride all season. While we wait to see how the multiple weak layers evolve in the coming weeks, we need to focus on the here and now: it’s dangerous today.

Régions couvertes