Bollettino valanghe

Soldier & Wood River Valley Mtns

Sawtooth Avalanche Center
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In sintesi

Steep, rocky, thinly covered upper elevation terrain where the wind has sculpted the snow is where you are most likely to trigger a slide today. Relatively small wind slabs may be reactive to your weight. Noticing stiffer pockets of snow or cracks shooting out from your skis/machine are indicators you have found a slab. Larger avalanches running on deeper weak layers in the snowpack are unlikely, but if triggered would be consequential.

Problemi valanghivi

  • Wind Slabs

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

    Be on the lookout for wind slabs in your travels today. Saturday's storm brought 2-5 inches of new snow accompanied by moderate to strong winds blowing out of the SE-S-SW, before shifting to the NW. 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 ridgelines and along 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 how a high speed fall might play out in the terrain you are thinking of entering. 

    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

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    • 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

Saturday'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. Before 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. Hard, stiff slabs may be supportable to your body weight and more difficult to identify in this zone without probing down more attentively into the snowpack. 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.

It is spring, and valley bottom locations will see highs in the high 30s 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. Being conscious of your timing by starting early and exiting before the heat of the day will help mitigate these issues. 

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