Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
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In sintesi

Recent snowfall and wind-drifting are creating dangerous avalanche conditions at upper elevations. Triggering a 1-2 foot thick avalanche is likely in steep, wind-loaded terrain—especially in the northern Sawtooths where more new snow has fallen. While unlikely, we can't rule out the possibility of triggering a very large, 3-4 foot thick avalanche on very steep, rocky, wind-affected slopes. 

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Since Thursday, the Western Smokys and southern Sawtooths received 6-10" of new snow. The northern Sawtooths may have picked up as much as 18". This snowfall has been accompanied by periods of moderate W to NW winds, which likely formed fresh wind drifts and slabs in wind-exposed terrain. These will be larger and more dangerous as you head north into the Sawtooths where more new snow has accumulated. An observer in the Sawtooths Friday reported seeing a medium-sized debris pile, likely from a natural wind slab avalanche. 

    Avoid steep and/or consequential slopes where you see evidence of wind loading—such as a drifted or pillowed appearance, or subtle changes in snow texture. You're most likely to find fresh wind loading along exposed, upper elevation ridglines. In the craggy Sawtooths, snow trickling down cliff faces tends to accumulate on the aprons below, forming "spindrift slabs." 

    The ongoing snowfall has been accompanied by periods of moderate westerly winds, and you're likely to find fresh wind drifts and slabs along upper elevation ridgelines.

  • Deep Persistent Slabs

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

    In this zone, the December facet layer is buried beneath a very dense, 3-4'+ slab. A unique characteristic of this weak layer is that despite having a crushing amount of weight above, it is not getting "squashed" like a more normal layer of facets would.  When a layer is buried this deeply, stability tests are time-consuming and really don't work that well—but don't worry, we did the work for you.

    It would take being in the wrong terrain, as well as a dose of bad luck, to trigger this layer. But if you did trigger a deep slab, you'd need all the luck in the world to survive it. So what's the wrong sort of terrain? Steep (over 35 degrees), rocky, wind-affected, "prime-time" terrain. You can avoid most risk of triggering a deep slab by sticking to sheltered, mid-elevation slopes where a more uniformly deep snowpack exists.

    (2/9/2021) A natural deep slab avalanche on McDonald Peak in the southern Sawtooths. It occurred last Sunday during the ongoing wind event.

Avalanche Discussion

ON THE IMPORTANCE OF SNOWPACK DEPTHOne very simple, yet powerful, observation you can make in the backcountry is snow depth. It turns out you're already carrying the measurement device—your avalanche probe—and it takes no time at all to pull it out and measure the total depth of the snowpack. You're also carrying another important tool—your eyeballs. Visual clues tell you about the snow depth: Are you in a deep area that is generally covering rocks, small trees, and other surface features? Or do you see a lot of these poking through the snow or giving it a "lumpy" appearance? Is the snow depth variable or relatively uniform?

Why is the depth of the snowpack important? I'm glad you asked:

  • Deeper snowpacks are warmer and more insulated from cold temps at the surface. I don't think you want me to dive into temperature gradients, but processes that strengthen the snowpack are more pronounced when the snowpack is deep. What's deep? In the avalanche world, rules of thumb are problematic at best, but let's say "deep" means a total depth greater than around 4'. Snowpacks less than 4' deep tend to have more weak snow, and the weak snow improves slowly.
  • It's not uncommon to see a divergence in our snowpack conditions in the middle of winter when our deeper areas surpass this threshold. For example, the snowpacks in our deeper zones—the Sawtooths & Western Smokys and Banner Summit—are quickly becoming very different from our southern zones where the snowpack is less deep. In these areas, triggering the December weak layer has become unlikely, and surface problems due to new snow and wind are becoming the primary concern. In our southern zones where the snowpack is closer to the 4' depth, there is still a real threat of triggering an avalanche on a persistent weak layer.
  • On a smaller scale, how snow depth varies across a slope plays a critical role once a weak layer is buried deeper than about 3'. Once a weak layer becomes deeply buried, it's more difficult to impact with our weight. But on a slope with variable snow depth—which is common in steep, rocky, and especially alpine terrain—shallow areas exist where the weak layer is closer to the surface. We call these areas potential trigger points.

Next time you go into the backcountry, try putting more observational focus on snow depth, both on a large scale (how does the average depth compare to other forecast zones) and on a small scale (how much depth variation do I see on a particular slope).

Be sure to catch up on the happenings this past week in Ethan's latest Weekend Update.

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