Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Mon
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 7 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

It remains possible to trigger a dangerous avalanche, especially on slopes with wind-stiffened or drifted snow. Although buried weak layers are improving, lingering signs of instability—such as snowpack collapsing—indicate we're not out of the woods yet. Conservative terrain choices remain the best bet.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    By now, you've heard plenty about the layer of surface hoar buried 1-2 feet deep throughout our area. It was responsible for a large number of natural and human-triggered avalanches last weekend. Since then, obvious signs of instability have decreased and it's been a week since we've received any reports of triggered slides.

    Banner Summit received more new snow and wind from Thursday's storm, and as a result, remains a bit more touchy than our other areas. Yesterday in this zone, the snowpack collapsed as I approached a snowpit site yesterday, and test scores indicated that the buried surface hoar remains a concern. So while the chance of triggering an avalanche is decreasing, it's definitely not zero. Surface hoar is a notoriously tricky weak layer (see Forecast Discussion below), and it deserves—no, demands—conservative terrain choices.

    ADDITIONAL DISCUSSION:We've now had a few days of gusty southwesterly winds, and you may find small, fresh wind slabs today in exposed terrain. These may be sensitive on their own, but are also adding weight to the buried weak layer described above. 

Avalanche Discussion

As many of you know, surface hoar is essentially the equivalent of frozen dew. Clear, cold nights will cause moisture in the air to freeze onto the snow surface, forming delicate frost feathers. Once formed, surface hoar is very fragile and can melt or blow away before it gets covered by new snow. 

On December 7th, the right combination of conditions conspired to not only form surface hoar uniformly across our area, but also to bury it before it was destroyed. While our area is known for some wicked facet layers, a widespread surface hoar layer isn't particularly common. Sure we get them occasionally, but they tend to be more "pockety" in nature and rarely produce such impressive avalanche activity as the one we have now. Surface hoar can be extra tricky—in fact, some studies have shown that it's the weak layer that produces the most avalanches that catch professionals. 

So while we may not be happy to have this layer beneath our skis, board, or machine, it does present a unique opportunity for those that are curious about snow. Surface hoar is often difficult to identify in snow pits, but that is not the case right now—you'll almost certainly find an obvious horizontal stripe about 1-2 feet beneath the snow surface. Perform a snowpack test or two and see how the layer responds. Even if your tests don't give results, just seeing the layer there will likely give you pause.

So don't just take our word for it, dig down and see it for yourself. You may find that getting your eyes on the surface hoar layer makes it easier to be extra cautious in your terrain choices. As they say, seeing is believing.

Regioni coperte