Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Pericolo valanghe

Wed
Alta quota
2 · Moderato
Limite del bosco
1 · Debole
Sotto il limite
1 · Debole
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In sintesi

Moderate winds and 4-6 inches of snow overnight have built wind slabs in upper elevation terrain. These slabs will grow in size and distribution today as snowfall continues and may be sensitive to the weight of a skier or rider. Look for these slabs near terrain features that slow the wind down and force it to drop the load it is carrying, such as ridgelines, gullies, and ribs. Cracking in fresh drifts is a sign you've found a sensitive wind slab. Avalanches failing on layers of weak snow buried in March present a less likely problem, but one that would create a larger, more destructive avalanche.

Problemi valanghivi

  • Wind Slabs

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

    4-6 inches of snow fell in this zone overnight and another 2-4 inches will fall today. This snowfall was accompanied by moderate, gusty winds blowing out of the SW-W. In exposed upper elevation terrain, this combination has built wind slabs that will continue to grow throughout the day. These slabs will be most sensitive where they are resting on weak, faceted snow crystals and facet+crust combinations. This weak snow is widespread on the northern half of the compass.

    As these slabs continue to grow, expect them to become more sensitive to your weight. Look for signs of wind drifted snow such as pillows and dune-like drifts. A rippled surface texture is another good sign that the wind has been at work. If the snow depth rapidly increases, or you move from soft, powdery snow into thicker, denser snow, you are probably pushing into a wind drift. Cracks shooting out from your skis or board are a clear indication that you've found a sensitive drift. These drifts aren't likely to be large today, but even a small slab can knock you off your feet and drag you through unfriendly terrain. 

    ADDITIONAL DISCUSSIONIn March, alternating periods of clear and stormy weather created layers of weak snow that are buried in the upper portion of the snowpack. These layers have produced natural avalanches, cracking, collapsing, and unstable snowpack test scores. And though they have been in a period of dormancy, the new snow may be changing that equation. This is particularly true where the wind is building slabs and adding extra stress to these layers. You are most likely to encounter this problem on slopes on the northern half of the compass. Be alert to this possibility today and in the coming days as new snow adds stress to these layers.

Avalanche Discussion

In all aspects of life, we sort the world into boxes to help us understand and communicate about the reality we encounter.  As avalanche forecasters, we use boxes known as avalanche problems to help describe the type of unstable snow that you might encounter in the backcountry. Different problems form in different areas and require different management strategies in order to avoid the hazard they pose. The avalanche danger is MODERATE in all of our zones today, but the avalanche problems and the hazard mitigation strategies are very different. 

In our northern areas, we are primarily dealing with a WIND SLAB problem right now. Avoiding the danger posed by a wind slab is relatively simple. (Notice the word "relatively" in there, I didn't just throw that in for fun. The backcountry environment is an incredibly complex one and we are well served to give it the respect it demands.) We can avoid wind slabs by avoiding terrain that is affected by the wind. No wind, no wind slab. If we want to cut things a bit closer, we can travel in terrain that is exposed to the wind, but where the wind has removed snow (scouring) instead of adding snow (loading). If we choose to cut our margins even closer, we can travel in terrain where the wind has been loading slopes with slabs and attempt to assess the stability of the wind slabs. Being able to do this successfully requires a great amount of experience and opens you up to high consequences if your assessment goes awry. 

In the southeastern portion of our advisory area, we are currently dealing with a PERSISTENT SLAB problem. Persistent slabs are an entirely different beast to deal with than wind slabs. They form when weak, long-lived (hence "persistent") layers of snow are buried. As you just read, there are several options available when it comes to managing a wind slab problem. In contrast, a persistent slab problem presents a problem with few management strategies. Most of the time, the only real option you have is to avoid the terrain where the problem exists. I've spent many, many days in the backcountry, digging hundreds of snowpits and assessing snowpack stability. But even though the danger is "only" moderate in the Soldier and Wood River Valey zone right now, there are many places that I wouldn't even think of venturing.  

Still with me here? When things are simple, the types of avalanche problems that you encounter in the backcountry feat neatly into the avalanche boxes. However, if you're old enough to be reading and understanding an avalanche forecast you probably know that the world we live in is rarely simple. The month of March has been dominated by alternating periods of snowfall and clear weather. Between storms, weak layers of snow formed at the snow surface and were subsequently buried by the next round of snowfall. In areas where wind slabs have formed on top of these weak layers, the wind slabs are adding additional stress to the weak snow underneath.

Is this a wind slab problem or a persistent slab problem? I suspect you all have some extra free time today. I'd like to challenge you to spend some time thinking about what type of problem you might think of this as and how you might manage it. Call your favorite backcountry partner and ask them what they think. Today is a great day to open up your brain and learn something new. The backcountry environment always challenges us and provides us with endless opportunities to learn something new. Clear your brain of your preconceived notions and make an effort to look at this idea with fresh eyes. I promise it is worth your time and effort. After all, those who think they know never learn.

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