Avalanche forecast

Banner Summit

Sawtooth Avalanche Center
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Avalanche danger

Mon
Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Out of date — this forecast expired 6 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Dangerous avalanche conditions exist. Skiers triggered two avalanches yesterday on Galena Summit. Triggering a large avalanche more than 2 feet thick is likely on many steep slopes—especially those that are exposed to the wind. Avalanches may be triggered from gentle terrain below, above, and to the sides of steep slopes. 

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    A series of storms since mid-December gradually loaded an extremely weak, faceted snowpack. The largest storm came earlier this week, and a significant avalanche cycle took place in the Sawtooth & Western Smokys and Banner Summit zones. Numerous large avalanches released, all breaking 1.5-2 feet deep and many fracturing hundreds of feet wide (see photos in Media Gallery for examples).

    This zone received 10-12 inches of new snow in yesterday's storm, adding insult to an already injurious snowpack. In the Galena Summit & Eastern Mtns zone, two skiers triggered separate avalanches yesterday. Triggering a large avalanche 2+ feet thick is likely on many steep slopes. As the weak layer becomes more deeply buried, signs of instability like collapsing of the snowpack may be less common. Don't take this as evidence that conditions are stable. If you need some convincing that dangerous conditions exist, dig down into the snowpack and see the loose, sugary snow that lurks beneath the dense slab.

    Today, I'd choose conservative terrain by avoiding steep slopes at mid and upper elevations. Given the nature of our snowpack, remotely triggering an avalanche from gentle terrain below, above, and to the sides of steep slopes is a possibility. 

    ADDITIONAL DISCUSSIONWhile winds yesterday and overnight were generally light to moderate, a few weather stations reported an uptick overnight. Combined with abundant soft snow, the wind may have formed small wind slabs in upper elevation terrain. These may be touchy on their own, but they are also adding more weight to the persistent slab problem discussed above.

    (12/23/2020) Large avalanche that released in the southern end of the Banner Summit zone. It broke hundreds of feet wide on weak facets buried in mid-December.

Avalanche Discussion

GALENA SUMMIT & EASTERN MOUNTAINS, SOLDIERS & WOOD RIVER VALLEY:Each time we've loaded the fragile facet layer that formed during the early winter drought, many backcountry skiers and riders temporarily ratcheted back their terrain choices. But after a few days, folks are back to riding in steep terrain. In general, this approach has been appropriate—each of the small loading events hasn't quite been enough to produce widespread instability. Unstable slopes have remained relatively isolated.

I'm afraid that with the recent storm, we've crossed a point of no return. We saw more signs of instability—and triggered slides—yesterday than after previous loading events. In the true fashion of "persistent" weak layers, this instability is here to stay. Dangerous slopes have become more widespread, and identifying which slopes are unstable has become difficult. A more prolonged conservative approach will go a long way in keeping you out of harm's way moving forward.

SAWTOOTHS & WESTERN SMOKYS, BANNER SUMMIT:A persistent slab problem also exists in our northern zones, but it's a somewhat different beast. More significant loading since mid-December produced clear-cut, widespread instability. An impressive avalanche cycle occurred in these zones - you can see photos and read details in our new Avalanche Occurrence database. Yesterday's storm loaded the snowpack with a foot of new snow, and now the weak facet layer is buried well over 2 feet deep.

As the weak layer becomes more deeply buried, it results in the classic double-edged sword. Slides will be more difficult to trigger, but much more dangerous if one takes you for a ride. Signs of instability—like collapsing/cracking and poor snowpack test scores—may become less common. Over time, the weak layer is likely to heal under the weight of the overlying slab—but we're not there yet! As a result, a conservative approach to backcountry travel is just as important in our northern zones as it is further south.

Want to catch up on what's happened this past week? Be sure to watch our latest Weekend Update, published on Friday each week.

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