Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Wed
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
3 · Marqué
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Triggering a dangerous, 2-3 foot thick avalanche is possible on most steep slopes. Slides are most likely on slopes that have been drifted by the wind, but could also occur in wind-sheltered terrain—even at lower elevations. Choose lower-angled, wind-sheltered terrain to reduce your risk. Avalanches may be triggered remotely—from gentler terrain above, below, and to the sides of steep slopes.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    2–3

    You could trigger a large, dangerous persistent slab avalanche on most steep slopes. December storms buried multiple weak layers that are now lurking beneath 2-3' thick slabs. While weak layers are nothing new for us, what is unique is that our current problem layers exist on nearly all aspects and at all elevations. 

    Yes, our snowpack is slowly improving, and you are less likely to trigger a slide than a week ago. But on Sunday, JP and Ben received consistently unstable snowpack test scores in this zone. This, in combination with the obviously poor snowpack structure (slab over weak layer), is solid evidence that the risk remains too high—and the consequences too great— to stomach. Most savvy backcountry folks continue to avoid skiing or riding on (or even below) steep, consequential slopes.

    You could trigger an avalanche on wind-sheltered slopes at low elevations, but the chances are greatest on wind-affected slopes at middle and upper elevations. The wind over the past few days has created fresh wind slabs (see below), and the drifting and snow redistribution is stressing the buried weak layers. Enjoying the excellent powder conditions on lower-angled, wind-sheltered slopes remains a great option for reducing your risk.

    (12/12/2022) In the Banner Summit zone, the weak November snow is buried beneath a 3' thick dense slab. 

  • Wind Slabs

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

    Over the past few days, the wind has been blowing strong enough to transport our ample powder onto leeward slopes.  On Sunday, Ben observed a handful of wind slab avalanches in the Boulder and Sawtooth Mountains—some of these were plenty large enough to bury a person (photo above/right). There are three issues making these wind slabs particularly dangerous:

    1. The wind is drifting snow onto widespread weak snow surfaces (a mix of facets, crusts, and surface hoar), making wind slabs especially touchy.
    2. Wind-transported snow is adding weight to the buried persistent weak layers described above.
    3. Triggering a wind slab avalanche could step down into these deeper weak layers, producing a much more dangerous avalanche.

    Triggering a wind slab is likely, and it'll be a very good idea to avoid any steep slope with evidence of wind drifting or stiffer surface snow. Winds are expected to increase into the strong to extreme range by tomorrow morning, so expect wind slabs to remain an issue for the next several days.

    (12/18/2022) This large avalanche was observed in the Boulder Mountains on Sunday evening. It failed in heavily wind-effected terrain on a W/NW facing slope at 9,700'.

Avalanche Discussion

Observant forecast readers may notice that the likelihood of triggering a persistent slab decreased from "likely" to "possible" today. What evidence is this decrease based on? 

  • During or immediately after our last storm, there were around 6 reports of human-triggered avalanches—several of which were triggered remotely. We haven't received a report of a human-triggered slide in a week. 
  • While we are still getting reports of large, dramatic, and frankly scary collapses, the overall incidence of snowpack collapsing is decreasing.
  • It's been around a week since the last major loading event, and the snowpack has had some time to adjust. However, this is being complicated by the recent wind that's been redistributing snow and adding more stress to some slopes.

The same observant forecast readers might ask why the avalanche danger didn't drop along with the slight decrease in likelihood. Here are a few reasons:  

  • The Avalanche Danger Scale is not linear, but rather exponential. Dropping to Moderate danger would imply that conditions are significantly less dangerous than they have been. While conditions are slowly getting better, there hasn't been enough improvement to warrant a decrease to the next danger level.
  • While the persistent slab problem may be slowly improving, the wind slab problem is tipping the scales in the other direction.
  • The likelihood of triggering a slide is only one part of the avalanche danger equation. If you trigger an avalanche, there's a good chance that it would be large (D2) to very large (D3). In plain language, it would be big enough to bury you or perhaps large enough to be destructive and unsurvivable.
  • Most savvy backcountry users (SAC staff included) are, for the most part, avoiding avalanche terrain. These conservative travel practices are more in line with Considerable danger than Moderate.

Tomorrow's forecasted wind event will be a setback on the road to recovery. Even if the danger doesn't increase a level tomorrow, there's still a good chance that conditions will be more dangerous than they are today.

Régions couvertes