Boletín de aludes

Galena Summit & Eastern Mtns

Sawtooth Avalanche Center
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Peligro de aludes

Wed
Alta montaña
3 · Notable
Límite del bosque
3 · Notable
Bajo el límite
2 · Moderado
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Desactualizado: este boletín caducó 6 years ago. Estás viendo condiciones pasadas, no el boletín vigente.

Lo esencial

Dangerous avalanche conditions exist and triggering a large avalanche remains likely on many slopes. Avalanches will fail on a layer of weak snow that is buried about 2 feet below the snow surface. You can reduce your risk of encountering such a slide by sticking to lower-angled terrain that is not threatened by steeper slopes above. Avalanches may be triggered remotely—from below, above, and to the sides of steep slopes. 

Avalanche danger will rise overnight as an intense winter storm impacts our area. If you are out in the early morning hours on Wednesday, the likelihood of both human-triggered and natural avalanches will have increased significantly, particularly in the southern portion of this zone (Pioneer Mountains).

Problemas de aludes

  • Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Likely
    Tamaño
    2

    Weak snow is buried beneath a 1-2.5' thick slab of snow in this zone. This layer exists across our Forecast Area and has been the culprit in almost all of the avalanches we've observed in our region this season. In the past six weeks we've recorded nearly 400 avalanches—the vast majority broke on this persistent weak layer. Around 20 of these were human-caused. 

    Even though we are a week out from a major loading event the snowpack is continuing to give us clear evidence of instability, here is a quick look at some of the red flags from the past few days in this zone:

    • Yesterday, skiers near the mouth of Baker Creek experienced several large collapses and chose to stick to low angle slopes.
    • Riders in the White Clouds on Sunday experienced shooting cracks that shattered entire slopes.
    • In the Pole Creek drainage on Sunday, Chris found an unstable snowpack structure and his snowpack tests returned unstable scores (video here).
    • I was in the Owl Creek drainage on Sunday and found an unstable snowpack, experienced cracking and collapsing, and got repeated unstable snowpack test scores in my pit.
    • Observers on Galena Summit continue to report unstable snowpack test scores and a poor snowpack structure.

    Well, why aren't people out there triggering avalanches then? Put simply, it is because most folks have been avoiding avalanche terrain. If you go out in this zone today, I'd recommend you continue this pattern. These ongoing signs of instability make it clear that the odds of triggering a slide are not decreasing significantly. You can reduce your chance of being caught in an avalanche by avoiding slopes over about 35 degrees, especially those that have been affected by the wind. Given the possibility of remotely-triggering slides, avoid terrain with steep slopes above you. 

Avalanche Discussion

Yesterday, I used my Forecast Discussion to talk about slabs (link here if you want to get caught up on the two-part mini-series). Today, we will be thinking about weak layers. 

WEAK LAYERS: Up to this point in the year we have mainly had just one weak layer to worry about: the thick stack of facets that we buried in December.  In the professional snow community, we name the layers in the snow based on the date that they are buried. On the 11th of December, after a 3 week stretch of cold temperatures, light winds, and clear skies, this thick layer of weak, early season snow was finally put to bed. And thus, 12/11 was formed. Ever since its burial, 12/11 has caused us nothing but headaches. The general weather pattern that created this weak layer (early season snowfall + early season drought) isn't unusual for our area. However, the weak, faceted snow that developed this year is unique, both in how reactive it has been and how long it has remained reactive. In the six weeks since this layer was buried, it has been responsible for nearly all of the 400 avalanches we've recorded thus far this season. 

There is an interplay between slabs and weak layers. The weight and the characteristics of the overlying slab affect the development of a weak layer after burial. In our northern areas (Banner Summit) the weak snow has been buried beneath a 3+ foot thick slab of snow for several weeks now. During this time, the weak snow has been mechanically compacted by the weight of the overlying slab. In addition, deeper snowpacks tend to create conditions where faceted grains in the snowpack can round and gain strength. Because of this, we are beginning to see signs of healing on the 12/11 weak layer in this area. 

As you travel further south, the slab overlying the weak layer is thinner and the weight has not been enough to significantly change the properties of the weak snow. Also, since the snowpack has remained thin, strong gradients in the snowpack have helped this weak layer to stay very weak. As if that weren't enough, we are also suffering from complications due to the nature of the loading. Instead of putting one big load of snow on at once and giving the weak layer time to adjust, we have gradually built a slab in small doses. This loading style, known as cumulative or incremental loading, tends to help a weak layer stay reactive for extended time periods.

As I write this, a deep low-pressure center is making landfall on the California coast. This slow-moving storm will be impacting our area for the next few days, putting the weak layer to the test. Where the snowpack is the weakest the storm is predicted to be the most intense. If the storm verifies I have no doubt that the snowpack will not be "passing this test". Further north, where less snow is anticipated and the snowpack is stronger, many places may be able to bear the weight of the additional load. Of course, much of this depends on how this storm plays out over the coming days. Stay tuned, we will keep you posted.

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