Bulletin d’avalanche

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Danger d’avalanche

Tue
Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 6 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Today's new snow and wind will create another batch of sensitive wind slabs on exposed, upper elevation slopes. Triggering a 1-2 foot thick avalanche is likely in this terrain. These slabs may be able to trigger older generations of wind slabs as they travel downslope, creating larger a larger avalanche. And while unlikely, we can't rule out the possibility of triggering an unsurvivable, 3-5 foot thick avalanche on very steep, rocky, wind-affected slopes. 

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    Today's 4-8" of fresh snow will be accompanied by moderate winds blowing out of the SW and then NW. This combination will create a fresh batch of 1-2' thick wind slabs in upper elevation terrain. These slabs will be sensitive to your weight today and can produce avalanches that are large enough to bury a person. Before today's storm, well over a foot of snow had already accumulated in favored portions of this zone, accompanied by moderate winds. These conditions generated an older batch of wind slabs that will be masked by today's snow. These older slabs are likely to be less reactive today, but any fresh slab you trigger could "step-down" into these older generations of wind slabs.

    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 steep and rocky terrain that is common in the 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

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Unlikely
    Taille
    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

WEATHER FORECASTING Every morning I generate my own weather forecast before reading what the professional weather forecasters at the National Weather Service have to say. As avalanche forecasters, it is important to be proficient in our understanding of both the snow and the weather. After all, the snowpack is nothing more than a record of the weather written in snow and ice. However, there is a lot of uncertainty involved in both of these fields and our true expertise lies in the world of the snow. 

When weather models are in good agreement and the weather patterns are stable it is possible to have high confidence in one's weather forecast. However, when there is a lot of variability between models and we are dealing with shifting weather patterns it is much more challenging to be confident in a weather forecast. This morning, as I looked through numerous weather models and ran them through my own mental model I found myself dealing with a high level of uncertainty in my weather forecast. Different models were indicating different amounts and timing for snowfall and frontal passages. I turned to the weather professionals, and was greeted to this as the first three words of their Area Forecast Discussion: "Another difficult forecast..."  Well, at least I wasn't alone in my uncertainty!

When you are traveling in the backcountry today, think about the uncertainty involved in today's weather forecast. If you were only expecting to see a few inches of new snow and you are finding closer to a foot, think about how that difference may change the likelihood of encountering avalanche problems. If strong winds are blowing and slabs are rapidly building in front of your eyes remember that that observation is more important than the danger rating that you saw in the morning. The avalanche forecast is meant as a starting point for your backcountry travel but it should be a long way from the endpoint. Use the forecast to guide your understanding of the world that you are stepping out into but be prepared to update that model based on your real-world observations. 

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

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