Bulletin d’avalanche

Galena Summit & Eastern Mtns

Sawtooth Avalanche Center
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Danger d’avalanche

Sat
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 5 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Dangerous avalanche conditions exist. You're most likely to trigger a large avalanche on shady (NW-N-NE-E facing) upper and middle elevation terrain where weak, sugary snow exists near the base of the snowpack. Slides can be triggered remotely—from flatter terrain below, above, and to the sides of steep slopes.

Problèmes avalancheux

  • Persistent Slabs

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

    The simplest way to reduce the odds of triggering an avalanche is by avoiding steep slopes where old snow sits at the base of the snowpack. Chris and Ben highlighted this point again yesterday in a video they made near Ketchum.  We've shaded the problem rose above to depict where you're most likely to trigger an avalanche on this layer. That said, any slope that had snow on the ground before the recent storm is suspect. For example, Wednesday in the Boulders, Ben and I got unstable snowpack test scores and experienced snowpack collapsing on a SW-facing slope at middle elevation. The weak layer will vary based on your aspect and elevation but most often presents as facets or a combination of facets above a crust. 

    You can find out whether old snow exists by quickly digging a hole. The old snow will be crumbly, slightly grey, and may contain one or more ice crusts. It could take weeks for the snowpack to adjust to the huge load from this storm. Pay attention to the terrain above you as remotely triggering an avalanche is a very real possibility.  Fortunately, there are still plenty of areas to find great skiing and riding. You can significantly reduce the chances of getting caught in an avalanche by sticking to terrain where the storm snow fell on dirt or bare ground.

    Additional Discussion:Light, low-density snow, and periods of moderate wind in the last 24hrs have created small sensitive wind drifts. These drifts often form downwind of abrupt changes in terrain like ridges or rock outcrops. A slab will feel stiff and may sound hollow under your feet. If you see cracks shooting out from your boards or sled, you've found a sensitive slab. 

Avalanche Discussion

Here We Go Again

If you're familiar with Central Idaho winters, the message we're sending won't be new. In fact, I'd forgive you if you feel like you've heard a very similar message before—because you have. Early season snows, followed by relatively dry weather and a cold snap preceding our last big storm, formed a weak snowpack riddled with facets and crusts. Toss an epic 3-5' of snow (3-4" SWE) into the mix and the result shouldn't be surprising.

Wednesday, Ben and I climbed high into the Boulder Mountains to glass and snap some photos before the afternoon snow showers arrived. Immediately we picked out several persistent slab avalanches in the Smokys, some of which were visible with the naked eye from over 5 miles away. Later, after sitting down to his computer to take a closer look, Ben described the aftermath as "As widespread of a cycle as I've seen there." Avalanches wrapped hundreds of feet "wall-to-wall" across several shaded, wind-loaded, upper elevation starting zones. With a minimum of 3' of new snow at their disposal, these avalanches were several feet thick. Even more impressive to think about are all the slides that went undetected due to poor visibility during a prolonged, multi-day storm.

In stark contrast to the Smokys, Chris observed only a few large avalanches while glassing the Sawtooths from the Stanley Ranger Station, north to Copper Mountain. Given the snow amounts that fell on a similarly weak, faceted snowpack, avalanches were all but certain. It's possible that these slides occurred earlier in the storm, and subsequent snow masked the crowns and debris. Precipitation rates also slowed in the Sawtooths and Banner in the latter half of the storm as compared to the Smokys, another factor that helps explain the apparent dividing line in avalanche activity late in the storm. 

Although weak snow near the ground, and the attendant persistent slab avalanche problems that arise are nothing new, no two years are the same. Weak layers form, evolve, and heal at different rates depending on aspect, elevation, weather, and mountain range. These patterns take time to uncover and require diligent monitoring. The most obvious pattern we've observed is the prevalence of avalanche activity and continued signs of instability on slopes that held snow prior to this last major storm. Chris and Ben observed this stark contrast again yesterday near Ketchum (video). The old snow provides a weak layer and serves to "smooth out" surface roughness like stumps, rocks, and sage providing a bed surface for avalanches to release and slide. As a result, you're most likely to trigger a persistent slab avalanche on shaded middle and upper elevations, but all slopes with old snow near the ground should be suspect. 

Keep an eye on our Observations Page to see what folks are finding. Consider submitting an observation yourself. Tracking and mapping this weak layer is as simple as digging below the recent snow and looking for an ice crust, or weak, crumbly, grey snow near the ground. Better yet, snap a photo of any avalanches you see and listen/look for collapses and cracking in the snow surface. The more data we get, the better we can define the problem, and the easier it is to collectively make safer decisions on where to play.

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