Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Sun
Alpin
3 · Marqué
Limite forestière
3 · Marqué
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

Dangerous avalanche conditions exist. Triggering an avalanche 1.5-2 feet thick is likely on most steep slopes—especially those that are exposed to the wind. Avalanches may be triggered from gentle terrain below, above, and to the sides of steep slopes. 

Problèmes avalancheux

  • Persistent Slabs

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

    A series of storms since mid-December have gradually loaded an extremely weak, faceted snowpack, and now most of this zone now has a stout 16-20 inch thick slab atop a nasty persistent weak layer. The largest storm came earlier this week, and a significant avalanche cycle took place in the Sawtooth & Western Smokys and Banner Summit zones. Numerous large avalanches released, all breaking 1.5-2 feet deep and many fracturing hundreds of feet wide (see photos in Media Gallery for examples).

    Our northern zones are forecast to get the brunt of today's snowfall, and this will add insult to an already injurious snowpack. Storm totals by the end of the day could approach 10 inches in places, and this will increase the likelihood of you triggering a dangerously large avalanche 2+ feet thick. As the weak layer becomes more deeply buried, signs of instability like collapsing of the snowpack may be less common. Don't take this as evidence that conditions are stable. If you need some convincing that dangerous conditions exist, dig down into the snowpack and see the loose, sugary snow that lurks beneath the dense slab.

    Today, I'd choose conservative terrain by avoiding steep slopes at mid and upper elevations. Given the nature of our snowpack, remotely triggering an avalanche from gentle terrain below, above, and to the sides of steep slopes is a possibility. 

    ADDITIONAL DISCUSSIONWhile there's not a ton of wind accompanying today's snowfall, there could be enough to form small wind slabs in exposed terrain. I'd expect these to be much less of a concern than the persistent slab problem described above, but fresh wind slabs may be poorly bonded to underlying crusts leftover from the recent rain event. 

    (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

THE SLOW LOADING GAME

Although it wouldn't be as catchy on a license plate as "Famous Potatoes", our area is also famous for the slow, incremental loading of very fragile facet layers. These processes are really two sides of the same coin—which is that we live in a dry climate. Prolonged dry spells produce these persistent weak layers, and since small storms are more common than big ones in our area, they get loaded at a snail's pace. If our facet layers get loaded quickly by a large storm (as they occasionally do), the results are predictable. When we get the more common pattern of incremental loading, the predictability factor goes way down.

Years ago I had a close call that I attribute to the slow loading of a weak layer. My memory of the details is fuzzy (could be age-related), but it was a situation not unlike what we have now with a thin, slowly developing slab over a persistent facet layer. The consensus was that there just wasn't enough slab for widespread instability. I do remember one important detail about the day: there was only 4 inches of new snow. Our group triggered an avalanche, and while no one was caught, it was too close for comfort. We had totally underestimated the "straw that breaks the camel's back" factor.

In our northern mountains the loading since mid-December has been more substantial, and the warm and wet storm earlier this week produced an impressive avalanche cycle. But in our southern zones, instability has been less widespread and many folks have been exploring steeper terrain. Is today's snowfall going to be that straw? I still haven't learned how to answer that question. But I do know one thing: it's not a matter of "if" but "when" our weak snowpack will reach the breaking point. Today's storm could be it, or it could be the next one. Either way, I plan to dial back my terrain choices today. Getting surprised once was enough!

Want to catch up on what's happened this week? Be sure to watch our latest Weekend Update!

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