Bulletin d’avalanche

Banner Summit

Sawtooth Avalanche Center
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Danger d’avalanche

Sat
Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 6 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Avalanches are most likely in exposed, upper-elevation terrain where new snow and wind combined to form sensitive drifts. Smaller avalanches are possible on many steep slopes, even those that are relatively sheltered from the wind. Loose snow avalanches are likely in steep terrain.

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Problèmes avalancheux

  • Wind Slabs

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

    Expect to find touchy, wind-stiffened slabs on most aspects in exposed terrain. You're most likely to encounter this problem at upper elevations where snow totals are greater and wind speeds are higher. 

    Yesterday in the Sawtooths, Ben and I intentionally triggered pockets of wind slab along a wind-affected ridge. The small slabs quickly gouged into loose, weak, facets forming slides that were certainly large enough to ruin your day. In the distance, we observed a larger avalanche that failed naturally under the weight of a heavy slab of wind-drifted snow. Wind direction shifted from the south and west yesterday to the northwest and north overnight, painting these slabs onto a variety of aspects. Look for cornices, ripples, dunes, or wave-like patterns in the snow to help you identify freshly loaded slopes. 

    Numerous small, slab avalanches (Galena, Soldiers) were reported throughout the region in locations with little to no wind–a testament to the poor bond between the recent snowfall and the weak, sugary snow beneath. Expect these slabs to be touchy again today. We've observed far-traveling collapses in remarkably soft new snow. As the snow stiffens in more areas, these collapses may become more frequent and the likelihood of triggering a slide remotely–from the side, above or below a slope–will increase.

    Additional Discussion:Loose snow avalanches are likely on steep slopes. Direct sun will peel snow off of rocks, causing small natural avalanches. Human-triggered loose snow avalanches remain a real concern when riding steep, confined terrain. 

    (12/17/2020) This avalanche failed on an E facing slope at 9,500 feet after just 6 inches of snow fell. Visible are several different pockets that failed, the largest is on the left side of the frame. It appears that this heavily wind-loaded portion was able to trigger slides in more sheltered terrain as it traveled downslope.

Avalanche Discussion

For weeks we've been showing you videos of weak, sugary facets both at the ground, near the surface, and in shallow snow areas throughout the entire snowpack. This weak snow now sits buried under 5-12" of snow since Tuesday. As expected, the interface between these layers is proving to be extremely weak on a variety of aspects and elevations. 

Yesterday, reports of small slab avalanches rolled in from all corners of the forecast area including the Soldiers, Wood River Valley, Galena Pass, Sawtooths, and Banner Summit. What's remarkable is not the size of these slides, but how touchy they were given the relatively thin and soft overlying slab. As Ben and I climbed into the Sawtooths yesterday we experienced a couple of far-traveling collapses and half expected to see small crowns appear on nearby rollovers. These rumbles occurred in meadows where the new snow was still quite soft and just barely kissed by the wind. 

Higher up, the danger was more pronounced as naturally occurring wind slabs failed in heavily wind-loaded terrain. In these locations, a shallow wind slab that begins near the snow surface has the potential to break into deeper weak layers of snow near the ground. 

The stability equation prior to this storm was relatively simple. Adding a slab near the surface changes the picture entirely. We've entered into what is likely to be a long-lived and occasionally tumultuous relationship with this weak layer. Expect some ups and downs. Our first date was a bit bristly, and even confusing at times. For many of us, this is one of the weakest snowpacks we've ever seen and we should expect some surprises. 

Régions couvertes