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
2 · Modéré
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Périmé — ce bulletin a expiré 5 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Layers of weak snow are buried in the upper 2-4' of the snowpack. Triggering an avalanche involving this weak snow is possible, particularly in steep, rocky terrain and during the heat of the day. Make your terrain selection accordingly and be alert to the consequences of being caught in a slide where you are traveling. You can significantly decrease your likelihood of triggering an avalanche by sticking to lower-angle terrain and exiting the mountains when the day heats up.

Problèmes avalancheux

  • Persistent Slabs

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

    Warm temperatures and intense sunshine are causing significant settling in the upper portion of the snowpack. As the snow from last week's storms settles into a denser slab, we focus our attention on the persistent weak layers that are buried underneath the newer snow. There are two broad categories:

    1. Last week's storm buried a variable snow surface. We observed a few avalanches (both natural and human-triggered) that failed on this interface during the storm (photo at right). Warm temperatures and direct sunshine today will help this storm snow to settle into a denser slab, which will continue to make triggering an avalanche involving this snow possible today. It will take some time to see how this interface will behave, it is worth having it in your mind as you travel.
    2. Weak layers that formed at the end of January and during the month of February are now buried 2-4' deep. These layers have produced avalanches across most of our forecast area in the past two weeks (Dollarhide, Galena Summit, Smiley Creek, Beaver Creek, Sawtooths, Banner Summit). The pattern of activity outlined by these avalanches points to these layers being most unstable on slopes that face the sun, where they are accompanied by crusts, but unstable surface hoar has also been observed in this zone. 

    If I were to go and try to make an avalanche today I'd be looking in steep, rocky terrain where the snowpack is thinner and where it is being heated by the sun. As days pass without loading, the snowpack is getting some time to rest, which will gradually decrease the likelihood of triggering an avalanche on these weak layers. However, the possibility of triggering a slide here will be on the table for some time to come. If you choose to enter avalanche terrain, be alert that at least one of these weak layers almost certainly exists in the snowpack underneath you. Even as the danger decreases we still advise cautious and conservative terrain selection. Be alert to the consequence of being caught in a large avalanche in the terrain you are recreating in.

    ADDITIONAL DISCUSSION:Today's warm temperatures and bright sunshine are a perfect combination for loose snow avalanches. These sluffs can be dangerous on their own and may trigger larger slab avalanches as they travel downhill. Scott observed this exact combination yesterday in the Boulder Mountains (observation here, photo here). The heat is also affecting the large cornices that have formed throughout the season. These beasts sag during the heat of the day, eventually tumbling off the ridges they are attached to. This makes for a "great" trigger for the persistent weak layers described above.

    (2/26/2021) Small natural avalanche observed on Banner Summit on Friday. This slide likely ran on a weak layer at the old snow surface.

Avalanche Discussion

PERSISTENT WEAK LAYERS AND DECISION MAKING

Everything is much easier at the far ends of the danger scale. When conditions are either clearly stable or clearly unstable it is quite simple to make decisions in the mountains. In the gray area in between safe and dangerous things get tricky very quickly. When we add persistent weak layers (PWL) into the mix this trickiness is multiplied. And that's exactly where we are now.

As stability gradually improves and the likelihood of triggering avalanches gradually decreases, I find myself drawn towards avalanche terrain. After all, I became an avalanche forecaster because I truly love spending my time in steep, snow-covered mountains. But only when I can do so safely. As I think about the terrain that I might be able to safely enter on any given day, the presence of PWLs in the snowpack significantly limits the terrain that is available. In order to feel comfortable that a given slope is reasonably safe to play on, I have to be able to prove to myself that the PWLs are either absent or entirely unreactive. For each slope, I re-create the seasonal weather history that formed the different PWLs, think about the loading events that have stressed these layers and the patterns of natural avalanche activity that those loading events have caused.

If that all sounds complex then I think I am doing a good job of describing the process. However, the complexity doesn't end there. At the moment, we find ourselves in a gradual warming trend, with each day slightly warmer than the last. Those warming temperatures are causing wet snow issues in the upper snowpack and adding stress to buried weak layers as the snowpack creeps downhill. So, in addition to the PWL problems we are seeing a diurnal pattern of fluctuating hazard overprinted on top. At the Sawtooth Avalanche Center, we forecast for an area of over 3,000' square miles. Within that area, I can think of very few pieces of avalanche terrain that I would confidently enter right now. Is that frustrating and unsatisfying? Absolutely. But trust me, I'm right there in the same boat with you.

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