Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Tue
Alpin
2 · Modéré
Limite forestière
1 · Faible
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 7 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Yesterday's 6-8" of snow and continued winds overnight created wind drifts in exposed, upper elevation terrain. Although small, they could be easily triggered by the weight of a skier or rider.  It'd be a stroke of bad luck, but triggering an avalanche on weak layers buried 4-6 feet deep remains a possibility on very steep, rocky, wind-affected slopes at upper elevations. 

Problèmes avalancheux

  • Wind Slabs

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

    Yesterday, 6-8" of snow fell in this zone. Chris went for an afternoon ski at upper elevations and found that NW winds were building wind drifts that were sensitive to his weight. He reported that the slabs that he saw were small and relatively thin — not big enough to bury a person, but plenty to knock you off your feet or sled in steeper terrain. Continued winds blowing out of the NW last night helped these slabs to grow in thickness and distribution. Yesterday's snow fell on a variety of surfaces, including old, hard wind slabs and icy crusts on slopes that face the sun. These stiff surfaces mean that any slide you do encounter will pick up speed quickly as it runs downslope. Read more about the wind slab problem in the Forecast Discussion section below. 

    In areas that are not affected by the wind, yesterday's gently stacked snow poses a different potential hazard — loose snow avalanches. The forecast is calling for cold temperatures and for clouds to build quickly this morning. However, if the sun stays out for long I'd expect to see small, natural loose snow avalanches running in steep, rocky terrain. These sluffs will also be easy to trigger if you're riding in steep terrain, particularly where stiff surfaces exist underneath the new snow.

    ADDITIONAL DISCUSSIONTriggering a 4-6 foot thick persistent slab avalanche has become very unlikely. It would take a dose of very bad luck, but it may be possible to trigger one of these monsters in very steep (over 35 degrees), wind-swept, alpine terrain.  

Avalanche Discussion

Unlike the persistent slab problems that we've dealt with for much of the winter, the stability of wind slabs can change rapidly — in both directions. Right now, we're watching a tug-of-war match play out. Winds blowing out of the NW continued to transport snow for a few hours after sunset last night. This wind increased the thickness and the distribution of the slabs that formed yesterday. However, these slabs have now had some time to bond to the surfaces that they were deposited on. So, what does that mean for you? 

If you were out yesterday and you go out again today you'll probably notice two things: the wind slabs you encounter will be thicker than what you saw yesterday, but they may be showing fewer signs of obvious instability. In the absence of a persistent weak layer, such as facets or depth hoar, wind slab stability can increase relatively quickly. At the same time as these slabs have been "working" to bond to the snow they are sitting on, they were also growing in size, due to the ongoing winds. As the winds die off today, the snow transport portion of this equation will be eliminated and the slabs will be able to better bond to the surfaces underneath.

The bottom line is this: slab sensitivity is decreasing but the size of these slabs has increased. You're less likely to trigger one of these slabs than you were yesterday, but it will be larger and the consequences will be greater. Keep this in mind today if you're traveling in areas where these wind slabs exist.

Régions couvertes