Bollettino valanghe

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

Tue
Alta quota
1 · Debole
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

Thanks to a lack of snowfall, avalanches are unlikely and most slopes are free of avalanche-related hazards.  Be on the lookout for areas where hard wind-drifts are sitting on weak, sugary snow. This scenario is most likely in upper elevation terrain that is exposed to the wind. 1" of new snow fell in this zone overnight, and strong winds may build very small, isolated wind slabs from this new snow.

Problemi valanghivi

  • Persistent Slabs

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

    I skied in upper elevation terrain just east of this zone on Saturday and found a very thin and weak snowpack (video here). A similar snowpack exists in much of this zone. Don't believe me? Walk out to an undisturbed snowpack in your yard, break through whatever crust you find on top, and try to make a snowball with the snow you find underneath. This is the type of snow we are describing when we talk about "weak, sugary facets." 

    With just a few inches of fresh snow in the past two weeks, most slopes lack a slab on top of this weak snow. The last reported avalanche in this zone occurred over 2 weeks ago when ski patrollers at Bald Mountain triggered an avalanche during mitigation work before opening the ski area (photo at right). The slide failed on a wind-loaded slope harboring weak, sugary, faceted snow in the lower part of the snowpack. This exact snowpack scenario—hard, wind-drifted snow overlying weak, faceted snow—is where it may remain possible to trigger a slide. While triggering a slab avalanche is unlikely, I'd still pay attention in very steep, wind-loaded terrain, especially at elevations over about 9000'.

    ADDITIONAL DISCUSSIONRoughly 1" of snow fell in this zone overnight, accompanied by strong to extreme winds blowing out of the W-NW. This small amount of snow isn't enough to drastically change the stability picture. Without a lot of ammunition for the winds, any fresh slabs that are forming should remain small and isolated, but these slabs may be touchy due to the weak, faceted snow surfaces they are deposited on. While small, encountering one of these slabs in very steep and rocky terrain could have high consequences. 

    Read a bit more about crusts, facets, and wind slabs in the Forecast Discussion section below.

Avalanche Discussion

Let's spend a minute thinking about crusts, facets, and the way the wind transports the snow:

CRUSTS:Crusts in the snowpack form when dry snow is melted and then frozen again. The most common ways these crusts form are with warm air temperatures, direct solar input, or rain. Crusts are often associated with faceted snow grains, which are described below. The combination of crusts and facets commonly form weak interfaces in the snowpack.

FACETS:Snow grains are in a constant state of change. Even before they hit the ground the structure of these crystals is rearranging in order to satisfy the whims of the laws of physics. Under weak gradients (the change in a given property over a certain distance), snow trends towards a more rounded state. Rounded grains tend to pack nicely with each other and form strong bonds. However, when placed under strong gradients, snow trends to a more angular state (facets). Cold, clear nights and days, like the ones we saw last week, create conditions that are conducive to the formation of faceted snow grains, or facets for short. We call them facets because they have sharp, angular edges on them, much like a cut gemstone. Facets are problematic because they don’t bond well to the snow around them, which makes them a “great” weak layer. This is a significant simplification of the processes involved here, but I hope it helps you grasp the basic points.

WIND:When the wind is sufficiently strong, it picks up snow and transports it to another location. While the snow particles blow through the air or bounce along the snow surface, they break up and become smaller. The smaller grains pack well into drifts once they're deposited. And just like that, you've got a wind slab. The speed of the wind plays a critical role in this process. Steady, moderate winds are great at building widespread, thick slabs. But shifting, variable, gusty winds don't do nearly as good of a job. If the wind doesn't blow hard enough it can't pick up much snow, but if it is blowing too hard it will throw the snow up into the air where it sublimates, never to rejoin the snowpack. The winds blowing yesterday and last night have been more on the gusty, and strong end of this spectrum. 

Keep these two patterns in mind — an overlapping patchwork of crusts, facets, and wind slabs. Today, triggering a wind slab is most likely where the presence of a crust+facet combination coincides with a fresh wind slab. Because of this, the problem will be isolated to specific terrain. In many areas, you won't find any touchy wind slabs. But in the right terrain, you will. Be alert to this as you travel through the backcountry today, and don't let the lack of touchy slabs in one location lead you to believe that this problem doesn't exist anywhere.

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