Bollettino valanghe

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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1 · Debole
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In sintesi

Freshly built wind slabs will be relatively small, but reactive to your weight today. Cracks shooting out from your skis or machine are clear indicators of an unstable slab. Triggering larger avalanches on weak layers buried in the upper few feet of the snowpack is much less likely, but the consequences would be grave. These larger slides are most likely in very steep, rocky, upper elevation terrain where the wind has sculpted the snowpack and the weak layers are easier to impact with your weight.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–1.5

    Yesterday's storm brought 2-5 inches of new snow accompanied by moderate to strong winds blowing out of the SE-S-SW. Overnight, winds eased and shifted to blowing out of the NW. Be on the lookout for freshly formed wind slabs in your travels today. Surface textures like dunes and drifts indicate the wind has been at work. If the snow you are moving through quickly becomes deeper, stiffer, and more dense, you have found a wind slab. Cracks shooting out from your skis or machine are a clear sign that the slab you found is not well bonded.

    Look for these slabs near terrain features that slow the wind down and force it to drop the snow it is transporting. Typical locations are along ridgelines and the sides of mid-slope gullies. Slabs will be at their largest, most reactive, and most dangerous in upper elevation terrain. Even small slabs can knock you off your feet and send you for a nasty ride if you encounter them in ugly terrain. Consider the consequences of an unplanned, high-speed trip down the slope you are playing on. 

    ADDITIONAL DISCUSSION: Loose snow avalanches involving the recent snow will be small but easy to trigger on steep slopes. These will run quickly downslope on the stiff surfaces underneath. As periods of bright spring sunshine melt the new snow, these loose snow slides will take on a moist/wet character. 

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–2.5

    Weak layers buried under dense slabs in the upper few feet of the snowpack have had some time to rest and are trending towards unreactive. However, the consequences of triggering one of these beasts would be severe. If you dig down, you'll find facets and bands of surface hoar on shaded slopes and icy crusts with associated facets on slopes that face the sun. Slides are more likely on shaded slopes, but upper elevation sunny slopes remain suspect as well. Scott and Ethan found layers of buried surface hoar in the Soldiers and above the Wood River Valley in recent days (photo at right/above).

    Your first sign of instability involving these weak layers would be the large avalanche you trigger. These slides may break well above you on the slope and fail widely. You can trigger slides like this from a distance away from the slope that avalanches (remote triggering). You can drastically reduce your risk of encountering a slide like this by steering clear of very steep, rocky, heavily wind-affected terrain. 

    (3/21/2024) An unstable test result on a layer of buried surface hoar in a snowpit on a N aspect near 7200' in the Elk Ck drainage in Croy Canyon. Photo: Sawtooth Avalanche Center

Avalanche Discussion

Spring Weather and a Transitioning Snowpack

Yesterday's storm snow fell on a variable snowpack in the throes of a spring transition. The snowpack did a lot of melting and settling during the 10-day string of hot weather that was ended by this storm. Thick, icy crusts formed at the snow surface on all slopes that face the sun. Water was pushed deeply into the snowpack in these areas. Shaded slopes at middle and upper elevations remained cool, dry and largely crust-free.

The new snow fell on a variety of surfaces. Prior to this storm, the most concerning surfaces that we observed were on shaded middle and upper elevation slopes where slick, wind-polished surfaces have undergone some faceting. Without much snow for ammunition, the slabs you find today will generally be isolated and relatively small. Pay attention to how much new snow you are moving through. If the snow quickly becomes deeper, stiffer, and more dense, you have found a wind slab. Look for cracking in these slabs as a sign of instability, and consider the consequences of getting tossed downhill in the terrain you are playing in. In sheltered terrain, small loose snow avalanches may be easy to trigger and may run downslope quickly on the stiff crusts underneath. Extended periods of sunshine will melt the snow and these loose snow slides may have a moist/wet character. 

Beneath the new snow, a variety of weak layers exist in the upper few feet of the snowpack. The character and sensitivity of these layers varies from slope to slope. We observed some wet slabs failing on these layers during the major warm-up,  but the most recent triggered dry slab avalanche failed in the Sawtooths on March 14th (photo here). The likelihood of triggering a slide involving these layers has decreased significantly, but hasn't gone away. You'd be most likely to trigger an avalanche involving these layers on very steep slopes in rocky, upper elevation terrain. Here, the snowpack is variably thick and thin, allowing for better access to these weak layers. Avalanches that fail on these layers would be difficult to escape unscathed.

It is spring, and valley bottom locations will see highs in the mid 40s F and extended periods of sun today. The snowpack at lower elevations has experienced many cycles of melt and freeze and these warm temperatures and rain are unlikely to produce widespread wet snow issues. As always, if you are finding a wet pile of snow on the slope you are playing on its worth considering the possibility of starting small wet snow avalanches. 

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