Bulletin d’avalanche

Galena Summit & Eastern Mtns

Sawtooth Avalanche Center
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Danger d’avalanche

Thu
Alpin
2 · Modéré
Limite forestière
1 · Faible
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 4 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

You could trigger avalanches where the wind drifted yesterday's snowfall. The pockets of wind-blown snow (slabs) are most widespread in alpine and upper elevation terrain. The drifts formed above slick surfaces, crusts, and weak snow on some slopes, so any avalanches you trigger could be wider and larger than expected.  

Problèmes avalancheux

  • Wind Slabs

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

    Yesterday's gusty, erratic winds may have formed slabs near ridgelines, on the sides of cross-loaded gullies and chutes, near prominent terrain features, and on aprons lower in alpine cirques. The wind slabs will be most common at upper elevations but may have formed in some middle elevation terrain.

    The relatively thin (less than a foot thick) new slabs formed above icy surfaces, crusts, small facets, and possibly surface hoar on some slopes; this could make them easier to trigger than you'd expect, and they could break both above you or wider than anticipated. Because it's difficult to know what kind of snow is buried beneath the fresh wind slabs (see Forecast Discussion below), the easy move is to steer clear of recently wind-loaded slopes steeper than about 35 degrees.

    Look for signs the wind has been at work recently. Snow pillows and dunes are great indicators. If you find the snow depth is quickly increasing or the surface is becoming stiffer, you've found a wind slab. Cracks shooting out in front of you are an obvious sign of instability. Many of the new slabs will be on the smaller side, but some may be large enough to bury people. Think about where even a small avalanche would carry you before committing to riding or skiing any recently wind-loaded slopes steeper than about 35 degrees. 

    ADDITIONAL DISCUSSION: Yesterday, observers in the Baker Creek drainage reported triggering a small loose snow avalanche in middle elevation terrain where the new snow was moist. Today's muted sunshine and thin cloud cover could trap heat, creating "greenhouse" conditions that make you feel like you're in a sauna. Pay attention to how sticky the new snow is becoming. While I don't expect widespread loose snow avalanche problems today, you may be able to trigger small, narrow avalanches in very steep terrain sheltered from the wind.  

    (3/11/2022) A heli-ski guide triggered this small wind slab on Friday while stomping cornices along a ridge. It broke 12" thick and 100' wide, running on a slick crust. Western Smokys, 9000', E aspect. Photo: Sun Valley Heli Ski

Avalanche Discussion

UPPER SNOWPACK COMPLEXITY

About 25 years ago, a longtime avalanche forecaster advised me to expect to be surprised when some avalanches release, but don't be surprised by what they fail on. Those sage words served me well over the years. Unfortunately, our current snowpack is throwing a fly in the ointment: the myriad weak layers in the upper part of the snowpack.  

You'll find multiple layers of facets (shady aspects), crusts (sunny aspects and some rain crusts on all aspects), crusts + facets (mixed sunny/shady aspects), and possibly surface hoar in the upper 1-3 feet of the snowpack. The presence and relative strength/weakness of these layers change dramatically with elevation and aspect. The weak layers' appearances will even vary within individual starting zones. Yikes. Because of the complexity and variability, we are unable to accurately predict exactly which weak layer is most worrisome at a scale large enough to cover a day riding or skiing let alone an entire forecast zone. 

So we have all of these persistent weak layers, but why don't we have a persistent slab problem? The slab. A warm storm in the Banner Summit zone dropped a quick 0.8-1.0" of SWE a couple of weeks ago, overloading weak layers on several slopes. But, our other zones have not (yet?) developed enough of a slab to cause issues in sheltered terrain. Wind-loaded slopes are a different story. There, you might see or trigger avalanches that fail on the buried weak layers and break wider than expected or surprise you by how easy it is to trigger slides. 

How do we know for sure when the slab is capable of producing persistent slab avalanches in more sheltered terrain? The same way we'll know when the war in Ukraine is over: verifiable, empirical evidence. Sadly, the answer is "when natural or human-triggered avalanches begin releasing in sheltered terrain."  

What can we do to reduce or eliminate the chances of being caught off guard by the buried persistent weak layers? Slides involving these layers are most likely on recently wind-loaded slopes that are over about 35 degrees; avoiding those will dramatically reduce the chances of triggering slab avalanches. On gentler (30-35 degrees) wind-loaded slopes, triggering avalanches is possible. On very steep (over 35 degrees) sheltered slopes, SAC staff and local avalanche professionals are playing it safe by avoiding many consequential avalanche paths where getting caught in a slide is likely to cause injury.

The moral of the story: Steer clear of very steep, obviously wind-loaded terrain. Carefully consider the consequences of triggering an avalanche if you're playing in very steep sheltered terrain or wind-loaded slopes between about 30-35 degrees.  

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