Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Tue
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
3 · Marqué
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Périmé — ce bulletin a expiré 3 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

A snowmobiler was killed in an avalanche in the Sawtooth Mtns yesterday. Similar human-triggered avalanches involving dense, 1-4 foot thick slabs of snow are likely today. Dangerous avalanche conditions exist and conservative terrain selection is encouraged. Avalanches may be very large and destructive and can be triggered from long distances away from steeper slopes. 

An AVALANCHE WATCH has been issued. The likelihood of both natural and human-triggered avalanches will rise overnight and throughout the day on Tuesday. Very dangerous avalanche conditions are anticipated.

Problèmes avalancheux

  • Persistent Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    2–3

    Weak layers are buried underneath dense, 1-4+ foot thick slabs of snow here. This combination can produce very large avalanches capable of destroying a house, and was responsible for yesterday's avalanche fatality in the Sawtooths. If you dig down, you'll find facet+crust combinations on slopes that face the sun and thicker stacks of weak facets on shaded slopes. These weak layers were producing large avalanches even before the weight of the big storm this weekend was added to the snowpack (photo at right/above). The slab overlying these weak layers will be even thicker in wind-loaded terrain (see Problem 2 below).

    Avalanches that fail on these layers can break very wide, wrapping around multiple aspects and connecting multiple avalanche paths. These slides can run the length of the avalanche path, impacting flat terrain long distances away from steeper slopes. Avalanches may break much further above you on the slope than you expect and can be triggered from long distances away from steeper slopes. This happened yesterday when snowmobilers triggered a large avalanche near Smiley Creek while riding on the valley floor below. Tracks on a slope are not an indicator of stability—slides can be triggered after multiple riders of skiers have been up or down a slope. You aren't likely to get much or any direct feedback from the snowpack with a problem like this. The first indication of instability may be the large avalanche you trigger.

    Managing persistent slab problems is difficult. If you want to completely eliminate the risk of triggering one of these beasts you'll have to avoid avalanche terrain and terrain exposed to hazard from above.

    (3/12/2023) This avalanche failed on a SE slope at 9000' in Lola Creek. Based on the aspect and the appearance of the crown, this slide likely failed on a layer of facets below a melt freeze crust.

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    Storm totals of 3-6" are expected by this afternoon. Snowfall will be accompanied by increasingly strong winds blowing out of the S-SW. This combination will build a batch of relatively small but reactive wind slabs. You're most likely to find a sensitive wind slab in upper elevation terrain. Wind slabs commonly form along ridgelines and near mid-slope terrain features like ribs and gullies. Wind slabs extending down into middle elevation terrain are possible if the wind blows hard enough today.

    Look for signs that the wind has been at work. Pillows, drifts, dunes, cornices, and rippled textures on the snow surface are all indicators that the wind has been transporting snow. If the snow you are traveling through increases suddenly in depth you are likely moving through a wind slab. Cracks shooting out from your skis or machine are clear indications that the slab you've found is unstable. Wind slabs and cornices can be very effective triggers for the destructive persistent slab avalanches described above in Problem 1. 

    (3/10/2023) This very large slab avalanche failed in a heavily wind loaded location on a NW-facing slopeat 6,800' above Croy Canyon. Large amounts of snow are being transported in this photo.

Avalanche Discussion

It is with heavy hearts that we report the death of a snowmobiler in our area yesterday. The rider was climbing the slope when the avalanche released, burying them in at least 4-5 feet of heavy debris. The avalanche broke above the rider on a slick, icy crust buried underneath a very dense, 3+ foot thick slab. The other two members of the party had already climbed the slope that avalanched. The group had not experienced any signs of instability during their ride. Read that again, all of the details are important. 

A widespread set of weak layers developed in our area during the drought in early February. Snowfall returned by mid-February and our area has seen a steady accumulation of snow over the past month. This snow has settled into thick, dense slabs that are delicately perched on the weak layers below. The addition of your weight may be all that is needed to push these weak layers to their breaking points. Wind slabs and failing cornices can be very effective triggers for these layers as well.

Avalanche conditions were dangerous yesterday. The additional loading from new snow and wind overnight and during the day today is only making things worse. Triggering an avalanche large enough to bury you or worse is likely today. Spend a few minutes thinking about yesterday's accident. The characteristics of that avalanche are important, as the types of avalanches that threaten you today have the same characteristics. They may break deeper and wider than you expect and they will be large and destructive. These types of avalanches can be triggered from long distances away from steeper slopes, including from the valley floor. Avalanches may fail on slopes that already have multiple sets of tracks on them. You aren't likely to get much or any direct feedback from the snowpack with a problem like this. The first indication of instability may be the large avalanche you trigger. 

Our deep, heartfelt condolences go out to the family and friends of the rider and to everyone involved in the accident and rescue. These are not hollow words. Please be safe out there my friends, when you head into the backcountry today you are entering a dangerous world. If you just want to be outside in the mountains and come home safely tonight there are lots of great options; the skiing and riding are excellent in low angle terrain right now. 

Régions couvertes