Bollettino valanghe

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Sawtooth Avalanche Center
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Pericolo valanghe

Sun
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

Dangerous avalanche conditions exist. Large human-triggered avalanches are likely on many slopes. Slides that fail at the base of the recent snow will involve 1-2 foot thick soft slabs. Much larger avalanches failing on weak layers deep in the snowpack are less likely, but would be unsurvivable.

Problemi valanghivi

  • Wind Slabs

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

    With 15-20 inches of new snow over the past few days, you can expect to find a range of new snow problems today. These broadly fall into two categories:

    SLABS: Shifting winds have blown from a variety of directions since the new snow fell. I was in this zone yesterday and I observed signs of wind drifting in many locations. You can trigger slabs that fail within this new snow, or a bit deeper on the layer of weak snow that we buried at the start of the month. Read more about this budding persistent slab problem in the Forecast Discussion below. Look for drifts, dunes, and rippled snow surfaces as an indication that the wind has been at work. Cracks shooting out from your skis or machine are a clear sign that you've found an unstable slab that could be triggered on steeper slopes. 

    LOOSE SNOW: In terrain sheltered from the wind, expect small loose snow avalanches to be easy to trigger. If you're hunting cold snow on northerly aspects these sluffs will have a dry character. On slopes where the sun is warming up and melting the snow surface, these loose snow slides will have a wet character. Thin clouds and southerly winds should help to mute some of our wet snow issues today. However, if you're finding moist/wet snow at the surface or if you are seeing wet balls of snow rolling down the slope this type of slide is coming onto the menu. Loose snow slides will start out small, but can pick up steam quickly thanks to the stiff and crusty surfaces underneath the recent snow. 

    Avalanches involving the new snow will range from small to large enough to bury you. Triggered and natural avalanches involving the new snow can be effective triggers for avalanches failing on the deeper weak layer described below. 

    This large avalanche was remotely triggered by skiers near Dollarhide Summit on Friday. It failed on an E/SE-facing slope at 9,100'. Based on the appearance of the slide, it likely failed on a weak layer that was buried at the beginning of February. 

    Photo: Sun Valley Heli Ski

  • Deep Persistent Slabs

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

    Triggering a wide, 2-4 foot thick avalanche large enough to bury people and possibly destroy a house remains possible, especially on the northern half of the compass (W-NW-N-NE-E aspects). Stout, buried crusts on sunnier aspects reduce the chances of triggering large avalanches on SW-S-SE aspects. 

    The odds you will trigger a Deep Persistent Slab are not high, but the consequences remain severe. Before triggering a slide, don't expect to see obvious signs of instability, like snowpack cracking or collapsing. You may be able to trigger avalanches from flat or low-angle terrain long distances away from steep slopes. Slopes with variable snowpack depths (windy areas, and craggy slopes with rocks or small trees poking out of the snow) are typical places to trigger Deep Persistent Slab avalanches. 

    Accurately evaluating whether a given slope is stable or unstable is challenging. Sadly, the only surefire way to manage this problem is to avoid avalanche terrain, especially slopes with terrain traps or severe consequences.

Avalanche Discussion

UPPER SNOWPACK WEAK LAYERS

For much of the season, the snowpack has been relatively simple. Yes — weak, unstable, and dangerous — but relatively simple. A widespread persistent weak layer buried near the base of the snowpack, and new snow issues closer to the surface. Snowpack complexity has been steadily increasing in the past two weeks. Storms at the start of February buried a weak layer composed of rain crusts with facets, sun crusts with facets, and generic near-surface facets and surface hoar, and we just buried another similar layer underneath the most recent snowfall.

Where are these layers coming from? Great question! We are in the process of forming another set of these crusts and facets on our snow surface currently. The sun melts the snow on southerly slopes, which freeze into crusts when temperatures drop. Cold, clear nights create strong gradients at the snow surface, which promotes faceting on all aspects. Tonight's storm will bury all of this snow, producing the latest buried weak layer.

As we bury weak layers like this, we watch to see how the snowpack responds. A growing body of evidence is pointing to the 2/1 weak layer as a persistent slab problem. The most glaring pieces of evidence have been the natural avalanches that have failed on this layer and the large, human-triggered avalanche that occurred near Dollarhide Summit yesterday. 

Travel through the mountains thinking about new snow problems, our old "friend" the January 6th weak layer, and these budding persistent slab problems. As complexity increases so does our uncertainty. The snowy world becomes less predictable and we are forced to build broader safety margins to deal with this uncertainty. Enjoy the fresh snow today and don't be afraid to splash some of it on your smile. But don't ignore the fact that you are playing in a dangerous, increasingly difficult-to-predict world. 

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