Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

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

Points saillants

You are most likely to trigger an avalanche on steep slopes where the wind has helped to stiffen the snow. These slides will break 18-30" down from the snow surface on a thick layer of weak snow. Avalanches are also possible in terrain that is sheltered from the wind. Avalanches may be triggered remotely—from below, above, and to the sides of steep slopes. 

Problèmes avalancheux

  • Persistent Slabs

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

    Weak snow is buried 18-30" deep in this zone. This layer of sugary snow (known as "facets") developed during the prolonged drought at the end of November and the beginning of December. A series of storms has gradually loaded this snow, creating numerous natural avalanches with each major loading event. Look at the photo to the right and the Media Gallery below for examples. Triggering an avalanche involving this snow remains a very real possibility today. This will be most likely on wind loaded slopes but is possible in terrain sheltered from the wind as well.

    This same layer of weak snow exists across our forecast areas. This past weekend, skiers triggered 3 avalanches involving this snow in the Galena Summit area. You can find details on these slides by visiting our Avalanche Database. In the Banner Summit area, this layer is buried a bit deeper, which acts as a double-edged sword. While the likelihood of triggering it here may be slightly less, the consequences will be higher. In addition, you may receive fewer signs of obvious instability (like cracking and collapsing in the snowpack) as you travel through the mountains.

    Your best bet for managing this problem is to avoid steep slopes, particularly those with recent and past wind loading. As a bonus for your caution, skiing and riding conditions are great in this low angle terrain right now, but don't lose sight of what is above you! Given the nature of our snowpack, remotely triggering an avalanche from below, above, and to the sides of steeper slopes is a possibility. Be aware of the terrain you are in and the terrain you are exposed to from above. 

    ADDITIONAL DISCUSSION - WIND SLABSSaturday's storm graced us with nearly a foot of low-density snow in this zone.  In the days since, periods of increased wind built isolated slabs in exposed terrain. Wind speeds will ramp up again today, creating another batch of wind slabs. Though small in size, these may be touchy and will be adding stress to the weak snow described above. If you trigger one of these wind slabs it may be able to "step-down" to the layer of weak snow below, creating a much larger and more dangerous avalanche. 

    (12/23/2020) Large avalanche that released in the southern end of the Banner Summit zone. It broke hundreds of feet wide on weak facets buried in mid-December.

Avalanche Discussion

If you are reading the Forecast regularly, you are probably noticing that the danger is gradually decreasing. Continue reading to better understand why and what you should do about it:

The avalanche danger scale is based primarily on the likelihood and size of potential avalanches. It attempts to simplify the infinite complexities of the backcountry environment into 5 danger levels. During periods of HIGH and LOWdanger, the message is pretty simple. At CONSIDERABLE, and particularly MODERATE danger, the message is significantly more nuanced. Right now, the likelihood of triggering avalanches is decreasing, but it remains a real possibility. Fortunately, the danger scale isn't the only tool we have available to us to help communicate hazards about the backcountry environment. An equally important tool are what we call the Avalanche Problems. 

Different Avalanche Problems require different management strategies. Persistent Slab problems (what we have now) are among the most difficult and frustrating to deal with. I feel this way both as an avalanche forecaster and an avid backcountry traveler. Trust me, there is nothing that I love more than skiing down steep, snow-covered mountains (read, "avalanche terrain"). Unfortunately, the weak layers that create these types of problems can be very slow to gain strength and often produce avalanches that fail in surprising ways. Slides may wrap around terrain features that usually confine simpler types of avalanches and they may be triggered remotely—from below, above, and to the sides of steep slopes. 

When I sit down in the morning to write my Forecasts, I am trying to give you an accurate picture of the snowpack that you can use to make your decisions in the mountains. Today, I want you to know that the likelihood of triggering an avalanche is (very slowly) decreasing, but that the consequence aren't. And even though I can say with confidence that the likelihood is decreasing, I can't point to any individual slope and tell you if it will or will not avalanche. Nobody can. That is part of what makes dealing with Persistent Slab problems so challenging. This uncertainty has kept me and many very experienced local riders and skiers from rolling the dice in avalanche terrain.

As the danger drops my suggestion to you remains the same. Do I want you to travel cautiously and conservatively in the mountains again today? Yes. Will I be doing the same? Yes. 

Régions couvertes