Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

Wed
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 5 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Dangerous avalanche conditions exist and human-triggered avalanches are likely. Weak snow buried near the ground is capable of producing large and destructive avalanches. Where this older snow exists, slides can be triggered remotely—from flatter terrain below, above, and to the sides of steep slopes. Fresh wind drifts will be less sensitive today but they are adding additional load to weak snow buried near the ground. 

Problemi valanghivi

  • Persistent Slabs

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

    There is weak snow leftover from October storms buried in this zone. Snowfall over the past 10 days has built a 2-3' thick slab of snow on top of the old, weak snow underneath.  I was in the southern Sawtooths looking for this problem yesterday, and I found it (watch the video at right). Chris was a bit further south on Saturday and he found similar conditions (take a minute to watch his video here). The existence of this layer kept both of us out of avalanche terrain where it is present and had us mindful of avoiding terrain threatened by hazard from above.

    Buried weak layers like this can be very slow to heal. We have looked at this snow across our forecast region and it isn't showing us any indication that it was to heal quickly. The additional snow that fell during the day on Sunday and the wind drifting that occurred yesterday is keeping this weak snow teetering on the edge of stability. Any avalanche that fails on this weak snow will be large and destructive. These types of slides are likely to behave in ways that surprise you. They may wrap around terrain features like ridges and may be triggered remotely—from flatter terrain below, above, and to the sides of steep slopes.

    While we've shaded the problem rose above to depict where you're most likely to trigger an avalanche, any slope harboring old snow (appearing as a mix of facets and/or crusts near the ground) is suspect. The difference between slopes that have old October snow and those that don't is fairly stark, but may be hard to identify without digging down, thanks to the thick slab of new snow. On slopes where this older snow doesn't exist, you won't find the avalanche problem I just described, but pay attention to what is going on up higher in the snowpack (read the problem described below). 

  • Wind Slabs

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

    Sunday's storm brought a quick 5-10" of snow to this zone. Gusty, moderate winds blowing out of the W/SW have reworked this snow into stiff drifts. I went out to see how these slabs were behaving yesterday and was pleased to see that most that I encountered seemed to be bonding reasonably well to the surfaces they sat on. You are most likely to trigger one of these slabs in upper elevations, where they will be the thickest and most broadly distributed.

    If you are traveling along and you feel the snow stiffen under your skis or your machine you've almost certainly found a wind slab. Look for drifts and dunes of new snow and for textures on the surface like ripples and waves to help you identify these slabs. They will be up to several feet thick may be reactive to your weight. The greater concern is that these slabs are adding weight to the weak snow underneath, exacerbating the persistent slab problem described above. Avalanches that initiate as wind slabs may step down to this weak snow, resulting in a much larger and more destructive avalanche.

Avalanche Discussion

HEADING INTO THE STORMWhile a good portion of the rest of the country languishes in record-setting temperatures, we find ourselves barreling towards a very white Christmas. The first in a long series of storms moves into our area late tonight. This round will be a prelude to what is to come, just a few inches to help freshen up the skiing and riding conditions. So why am I writing about this today? I'd like to get you to think about the snowpack conditions that you are experiencing in the mountains not just as a moment in time, but as being connected to the past and future. The snowpack that sits on the ground is a record of the season's weather, written in snow and ice. This record is written by the day.

When you head out into the backcountry today the snow surface that you are traveling on will be what tonight's storm falls on. Today is your last chance to easily look at this before it gets buried. It is much simpler to look crusts, facets, and surface hoar when they are still on the surface. We name weak layers by the date that they are buried, it helps us to keep track of them across our region and to communicate this information to others. The weak snow from October storms was buried on December 11th, so we call it 12/11. On Sunday, a quick-hitting storm buried another snow surface. This one varies in character across the region, but includes crusts, facets, and the combination of the two. This is 12/19. When the snowfall starts late tonight or early tomorrow we will have another player in the game. As you head to your favorite backcountry location today spend some time looking at this surface. Are you finding crusts? What about those pesky, sugary grains known as facets? Maybe you are seeing some surface hoar flakes that formed last night or the night before? Even though snow can be infinitely complex it is available to all of us. We don't need an expensive gadget to look at it, we can pick it up and interact with it with our hands, eyes, and tongue. 

As each additional layer gets buried the snowpack picture becomes more complicated. Fortunately, many of these layers don't end up creating long-standing problems and we can forget about them, freeing up mental space to focus on the ones that are causing problems. It will be some time before we are forgetting about 12/11, weeks and weeks at a minimum. The upcoming storm will be testing all of these layers. In times when the behavior and distribution of these layers is uncertain (right now), the amount of terrain that is safely available to us is much more limited. In addition, our focus needs to be split in multiple directions: we are paying close attention to whether or not the slope we are on has old snow underneath and we are also thinking about the presence of wind slabs at the surface and what they are resting on. When the backcountry world gets more complex taking a step back and increasing the size of our safety margins is generally a good idea. We had a good little window there where a portion of our terrain was "open" and we were enjoying excellent skiing and riding in terrain that was largely free of avalanche problems. That widow has closed significantly and is about to get slammed completely shut. For the foreseeable future, I'll be heading into the mountains with this in mind. 

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