Boletín de aludes

Banner Summit

Sawtooth Avalanche Center
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Peligro de aludes

Wed
Alta montaña
2 · Moderado
Límite del bosque
2 · Moderado
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

Triggering a very large avalanche that fails on weak snow buried 3 feet below the snow surface remains possible. If you trigger an avalanche of this size it is likely to be unsurvivable. You can reduce your chances of encountering one of these beasts by avoiding very steep, rocky, wind-affected terrain and the slopes beneath this type of terrain.

Avalanche danger will begin to rise overnight as an intense winter storm impacts our area.

Problemas de aludes

  • Deep Persistent Slabs

    • Alta montaña
    • Límite del bosque
    • Bajo el límite del bosque
    Probabilidad
    Possible
    Tamaño
    3

    A layer of weak snow makes up the foundation of the snowpack here, and a dense slab is sitting on top of it.  If you dig a pit, expect to find a snowpack that "looks pretty good"....right up until you find the weak snow near the ground. This dense, 3+ foot thick slab makes it difficult to impact the layer of weak snow that is sitting below it. If you jump off your sled, chances are you won't even sink in past your boot tops. Grab the throttle and pull and your track likely won't dig down to the weak snow underneath. That is, until you find the wrong spot on the slope where the slab is thinner. 

    Deep slab avalanche problems are challenging to deal with. This poor structure (strong snow overlying weak snow) is concerning, even in the absence of obvious signs of instability, like cracking and collapsing.  Because the weak layer is "protected" by a thick slab of snow, it is difficult to collapse the layer and initiate a propagating fracture (trigger an avalanche). However, if you do trigger a slide it is unlikely to be the sort of avalanche you will survive. These types of avalanches are often triggered after many riders have been on a slope. You are most likely to trigger one of these slides where the thickness of the slab varies and there are thin spots where you can impact the weak layer. These trigger points are most common in steep, rocky, wind-affected terrain. 

    (1/18/2021) This very large avalanche occurred in our Banner Summit Zone, likely during last week's storm. Based on the depth and appearance of the crown it probably failed on facets buried deep in the snowpack.

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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