Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Tue
Alta quota
2 · Moderato
Limite del bosco
1 · Debole
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

The combination of new snow and wind will create sensitive wind drifts at upper elevations. Look for obvious signs of wind loading and steer clear of these areas where you find them. Additionally, weak snow buried near the ground continues to pose a threat. Triggering a slide on this weak snow would be unlikely, but the resulting slide would be very large and destructive.

Problemi valanghivi

  • Wind Slabs

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

    3-5” of low-density snow fell in this zone yesterday. Wind speeds picked up overnight and shifted to blowing out of the N-NE. This wind is picking up the new snow and depositing it in the form of wind slabs. These slabs will be largest and most sensitive to triggering in exposed, upper elevation terrain. Where these slabs are sitting on stiff surfaces, like crusts or older hard slabs, they are likely to pick up speed quickly if triggered. Be aware of the consequences of being carried at a high rate of speed through the terrain that you are in.

    A slightly older batch of wind slabs was buried by yesterday's snowfall. On Saturday, snow professionals on Galena Summit found small but sensitive wind drifts (photo at right). On Sunday, an observer south of Baker Creek reported encountering similar thin, hard, sensitive wind slabs (see photo here). The presence of these older slabs will be masked by the new snow on top. Triggering an avalanche involving these older slabs is less likely, but would be larger and more consequential. 

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2–3

    Weak layers of facets and depth hoar are buried near the base of the snowpack in this zone. As time passes without major loading events, triggering a slide on one of these layers becomes unlikely. But unlikely things do happen.

    On Saturday, I was in the White Cloud Mountains and watched as a  group of riders in low angle terrain remotely collapsed a large alpine bowl nearby (see photo here). Cracks traveled over one quarter of a mile from where they were playing, up to near the top of the peak above, and the entire snowpack shifted downhill...and then stopped. Had this thick slab of snow (see photo here) continued to slide, the resulting avalanche would have been large and destructive. 

    This is the sort of problem we are dealing with when we talk about weak snow at the base of the snowpack. You are most likely to encounter this problem exactly where the riders in the White Clouds did - in steep, rocky, heavily wind loaded terrain. Be aware that even if you aren’t directly in steep terrain yourself, the snow that you are traveling on may be connected to that sort of terrain.

Avalanche Discussion

When we travel through the backcountry (both professionally and recreationally), we are looking for signs of instability to help guide our decisions and snowpack analysis. As the danger decreases, these indicators are more difficult to find. We know that the likelihood of triggering avalanches on the weak layers that exist in the snowpack is decreasing, but it is hard to assess exactly how quickly that happens.

Unfortunately, on years when we have buried persistent weak layers in the snowpack, we find ourselves stuck worrying about the unlikely — not because there is a great chance of it happening, but because the outcome of the unlikely would be catastrophic. I observed exactly such a situation on Saturday in the White Cloud Mountains, read about it in the persistent slab problem discussion above. Fortunately, in this case the unlikely came without catastrophic consequences. This event serves as a great illustrator of what unlikely looks like.

Regioni coperte