Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

Wed
Alta quota
2 · Moderato
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

Continued wind loading yesterday and overnight built sensitive wind drifts in exposed terrain. Look for signs of wind loading and steer clear of these areas where you find them. Underneath these drifts, 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

    Yesterday, the combination of moderate to strong N-NE winds and a few inches of new snow was enough to create natural avalanche activity to the SE of this zone in the Boulder Mountains (photo). We did not receive any observations of avalanche activity from this zone, but I’d expect that similar slabs were built in exposed terrain here. These slabs have had a bit of time to bond to the surfaces they were deposited on, but continued loading last night may have kept them sensitive to your weight. Be on the lookout for signs of wind loading such as a dune-like texture on the snow surface, and avoid these drifts where you find them. These drifts are likely to be fairly stiff today, thanks to yesterday’s strong winds.

    The wind doesn’t generally blow out of the NE in our region so expect to see different patterns of wind loading than you normally encounter. Slopes where you often find sheltered snow may have slabs built on them today. Wind slabs will be most sensitive where they are sitting on stiff surfaces, like crusts or older hard slabs. In these areas they are likely to pick up speed quickly if triggered, thanks to the firm surface underneath. You are most likely to encounter these slabs near exposed terrain features such as ridgelines and along rocky ribs and gullies in the middle of slopes.

  • Persistent Slabs

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

    Yesterday, in the White Cloud Mountains, I looked at the aftermath of a large avalanche that failed on weak snow near the ground on Sunday (photo at right). This avalanche broke over 500 feet wide, 4-6 feet deep, and wiped out the majority of the season’s snowpack, covering snowmobile tracks from the previous day. It is unclear if this avalanche failed naturally or remotely triggered from below. The snowpack isn’t identical in this zone, but similar layers of weak snow exist near the ground in this area. 

    When we talk about unlikely events with high consequences, this is exactly the sort of thing we are talking about. You are most likely to encounter this problem exactly where this avalanche in the White Clouds released - 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 above you.

Avalanche Discussion

You might have felt like our snowpack was tricky to deal with already, but yesterday’s weather showed us that things can always get worse. A touch of snowfall on Sunday, coupled with an intense wind event on Monday have complicated the avalanche picture. We are dealing with two very different hazards right now:

  1. Despite the lack of fresh snow, strong winds blowing from an unusual direction (NE) were enough to put a large amount of snow “into play” yesterday. These winds quickly built slabs, many of them in locations where you might not expect to find wind slabs with more usual wind directions. Natural activity involving these wind slabs was observed in the Boulder Mountains.
  2. Buried deeply in the snowpack are several layers of weak snow. These persistent weak layers create difficult to predict and manage avalanche hazards. An avalanche on depth hoar near the ground in the White Cloud Mountains on Sunday (described above) is a clear indicator that avalanches involving this weak snow are possible.

Dealing with either of these problems individually is challenging in its own right. But currently these beasts are dwelling in the same lair, and they are interacting. The fresh, stiff wind slabs are adding additional weight to the weak layers below. But that's not all; they are also adding structure, providing a medium that can help to “communicate” a fracture across features that otherwise might constrict the dimensions of a slab. Do I think it is likely that we will see natural or human triggered avalanche activity on our buried weak layers today? No. But will I be travelling with the knowledge that these difficult to predict weak layers could potentially fail and cause a large, unsurvivable avalanche. Absolutely.

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