Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Non aggiornato — questo bollettino è scaduto 6 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Triggering a very large avalanche that fails on weak snow buried 3 feet below the snow surface remains possible. Triggering an avalanche of this size 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.

Problemi valanghivi

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    3

    The Banner Summit Zone is exhibiting a classic deep slab avalanche problem right now. A layer of weak snow makes up the foundation of the snowpack and a strong, dense slab is sitting on top of it. If you dig a pit here, expect to find a snowpack that "looks pretty good"....right up until you find the weak snow near the ground. This poor structure (strong snow overlying weak snow) is concerning, even in the absence of obvious signs of instability, like cracking and collapsing. 

    Where are you most likely to trigger one of these slides?

    • Very steep slopes (those over about 35 degrees)
    • Rocky areas where the weak layer is closer to the surface
    • Wind-affected terrain where the slab of snow is thinner in places

    This type of avalanche problem is what we call a low-probability high-consequence problem. 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.

    (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

I've never returned home from a day of skiing or riding and thought to myself, "I'm satisfied, I never need to go into the backcountry again." Truth be told, every day I spend in the snow-covered mountains just leaves me looking forward to the next one! In order to ensure a long future filled with snowy summits, I know that I have to behave in accordance with the dictates of the snowpack. We don't make the rules, she does.

Fortunately, every season the snowpack provides us great opportunities to learn. In fact, the years with the worst stability (like this one) are usually the ones where we are offered the most learning opportunities. Even though conditions have been quite dangerous this year, the snowpack is relatively simple: weak, early season snow is buried beneath a 1-3+ foot thick slab of snow. As you travel around different areas in our region I'd recommend pulling out your shovel, digging a pit, and seeing if you can identify these layers. This simple set-up gives us a great natural laboratory for learning about the properties of slabs, weak layers, and the interplay between them. I'll spend a little time talking about slabs today, stay tuned for some insight into weak layers in tomorrow's discussion.

SLABS: The primary driver that is creating variable Avalanche Danger across our forecast area is the nature of the slab on top of the weak snow. Let's take a quick tour through our snowpacks, from thickest to thinnest:

  • Where the snowpack is deepest (Banner Summit Zone), this slab is 3+ feet thick and very dense. When you are digging here, expect to be digging a big hole through dense snow. This meaty slab makes it difficult to impact the weak snow below it, but also makes it so any resulting avalanche would be very large and destructive.
  • Traveling south from Banner into the White Clouds, the mountains above Smiley Creek, and the mountains around the headwaters of the Big Wood River you will find a 1.5-2.5 foot thick slab that isn't nearly as dense as the slab around Banner. This set up is where you are most likely to trigger an avalanche today. The slab is still plenty strong to be "communicating" fractures, but it isn't so strong that it is difficult to impact the weak layer.
  • As you continue south, into the mountains above the Wood River Valley and north of Carey in the Little Wood drainage, you will encounter areas where the total depth of the snowpack is less than 2 feet. In most of these areas the slab just isn't thick or strong enough to be causing widespread problems...yet. A notable exception to this is in areas where the wind has thickened the slab that rests on top of the weak snow beneath. 

Looking for a quick recap of what happened last week? Be sure to check out our latest edition of the Weekend Update, my man Chris will fill you in!

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