Bollettino valanghe

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

Fri
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2 · Moderato
Limite del bosco
2 · Moderato
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1 · Debole
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In sintesi

You're most likely to trigger a large avalanche in recently wind-loaded terrain. Avalanches are much less likely in sheltered terrain where the snowpack is thinner and softer. Avalanches may be triggered from gentle terrain below, above, and to the sides of steep slopes.

Problemi valanghivi

  • Persistent Slabs

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

    You're most likely to trigger large avalanches where stiffer wind-affected snow overlies weak, sugary, faceted snow or a series of crusts and sugary facets. Yesterday, I experienced two notable collapses, one of which shook loose a wind slab near the surface and cracked down deeper to the weak facets below.   

    The Wood River Valley received only a fraction of the recent snow amounts seen to the north and west. Weak faceted snow layers in the lower half of the snowpack are widespread, but many slopes are lacking a thick, dense slab above the weak layer. Slopes with a foot or more of recent snow above the crumbly facets are most problematic. This is more likely the case along the southern Smokys near Dollarhide Summit and further west in the Soldier Mountains. 

    Persistent slabs can release in surprising ways and can be triggered from gentler terrain below, above, and to the sides of steep slopes. These slides can break wider than expected, spanning terrain features that often confine storm slabs and wind slabs. Steer clear of steep slopes if you see cracks shooting out from your skis or sled, or if you feel the snowpack collapse or hear whumpfing noises. Pay attention to the terrain that you are in as well as the terrain that you are connected to. 

    (12/23/2020) This small drift of wind-loaded snow released about 50 feet from me after the slope I was on collapsed. The bed surface was the melt-freeze crust that formed on the solstice. There were cracks in the bed surface that reached to the December facet layer.

Avalanche Discussion

And Now the Hard Part

Our role as forecasters (and backcountry travelers) can be fairly straightforward on days when the avalanche danger is HIGH or on days that it's LOW. On either end of the danger scale, we often have a greater degree of certainty and our travel advice can be simple. In the middle ground, when the danger is MODERATE or CONSIDERABLE, conditions are often much trickier, variable, and nuanced. This is especially the case now, as we sort out the complexities dished out by a "once in a decade" weak layer, diverging snow depths from zone-to-zone, and a rain event on the Solstice. 

The Winter Solstice storm brought heavy snow and rain favoring the southern Salmon River Mountains, those around Banner Summit, and the western portions of the Sawtooths and Smokys. This event prompted our first AVALANCHE WARNING of the season. Since then we've observed the aftermath of what turned out to be a widespread avalanche cycle. Much of the observed activity occurred on slopes that face N-NE-E-SE. On these slopes, the combination of weak underlying facets and intense wind-loading proved too much to handle. At the tail end of this storm, some of the precipitation fell as rain or a wet snow and rain mix. Clearing skies and frigid cold air from the north froze this moisture into a rain crust that we've found on most slopes up to at least 9,200'.

In the last few days, light, dry snow continued to fall in the northern mountains, accompanied by NW wind. These fresh wind slabs sit atop the rain crust and add additional weight to the weak facets buried deeper in the snowpack. 

It can be easy to get lost in the weeds here, tracking weak layers and crusts, but thankfully (or not?) there is one constant–weak facets lower in the snowpack. Whether you're in the Wood River Valley, the Smokys, or at Banner Summit, this weak layer exists. What varies from zone-to-zone is the thickness and composition of the overlying slab which is generally thicker in the western and northern mountains that were favored with recent snow. Generally speaking, you're more likely to get yourself into trouble in areas where this overlying slab has been stiffened. This may be in areas exposed to the wind, or in areas sheltered from the wind which saw more recent warm, heavy snow and a bit of rain.

We're distancing ourselves from the heavy loading events of Sunday and Monday, but heavy snow and the added weight of wind slabs this week will keep the snowpack on edge. Mild, calm weather today and tomorrow will give the snowpack a rest, but persistent weak layers are slow to adjust to load. Signs of instability may become less frequent, but the ingredients for a large avalanche remain. 

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