Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

Mon
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 5 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Avalanche conditions remain dangerous. The recent storm formed sensitive wind drifts and added weight to weak layers buried in the snowpack. Similar conditions over the past few weeks have produced human-triggered and natural avalanches. Reduce your risk by sticking to gentler slopes sheltered from the wind. 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    2–3

    The recent storm added stress to persistent weak layers (facets and/or crusts, and occasionally surface hoar) buried in the top 3-4' of the snowpack. These layers have produced several avalanches over the past two weeks (Dollarhide, Galena Summit, Smiley Creek, Beaver Creek, Sawtooths, Banner Summit). They seem most reactive on sunny slopes, but two recent large avalanches occurred on shady aspects in the Sawtooths (photo above/right). 

    When faced with a complex snowpack, it helps to step back and take a more holistic view. Recent loading due to new snow and wind? Check. Weak layers in the snowpack? Check. Recent avalanches? Check. When you look at it that way, the situation becomes more clear. Adopt the K.I.S.S. principle when choosing the terrain to play in: Keep It Simple & Safe. Triggering an avalanche will be most likely on upper elevation slopes steeper than about 35 degrees. Excellent skiing and riding conditions exist, and you can enjoy the new snow in a relatively safe manner on moderate slope angles on mid-elevation slopes sheltered from the wind. 

    ADDITIONAL DISCUSSION:Today will be the first day in a trend of increasing sunshine and warming temperatures. Expect to see sluffing of the new snow as the surface warms up, especially on slopes 40 degrees or steeper. Given the large amounts of recent snowfall, these sluffs could be dangerous on their own or could trigger larger slab avalanches as they travel downhill.

    (2/24/2021) This large, wide persistent slab avalanche released last week above Hanson Lakes in the northern Sawtooths, likely as a result of new snow and strong winds.

  • Wind Slabs

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

    This zone picked up 12-18" of low-density new snow during the recent storm, and this snow was easily transported by the wind that accompanied the weather event. Yesterday, I observed more than a half-dozen natural slab avalanches in the northern Sawtooths. Most of these had released on the aprons below cliff bands (photo above/right). Owing to the near-vertical nature of the Sawtooths, the wind often blows spindrift down cliff faces where it overloads the slopes below.

    When low-density snow is blown into wind slabs and drifts, the difference in texture may be subtle. Drifted areas may still feel like powder, but may have just enough added cohesion that you could trigger a wind slab avalanche. Wind slabs may be forming atop weak old snow surfaces (facets and/or crusts) making them more reactive to your weight. Recognize that wind-loading also adds stress to the weak layers buried in the snowpack (described in Problem 1 above). Any wind slab you trigger could step down to these deeper layers, producing a much larger, more dangerous avalanche. 

    (2/27/2021) Natural avalanches in the northern Sawtooths. Several of these likely released due to loading from sprindrift trickling down cliffy terrain above. Five crowns are visible, but several are partially filled in.

Avalanche Discussion

Since the end of January, our snowpack has received nearly continuous loading from new snow and wind. In Scott's recent Weekend Update, he mentions that portions of our northern mountains received some form of loading on 28 out of the last 31 days. After a snowy and cold February, the first week of March promises a pattern shift towards sunny skies and warmer temperatures. What should we expect from this warming trend?  As is typical with avalanche conditions, it's a good news/bad news situation.

Short-term (bad news). Over the next few days, the sunshine and warming temperatures will come as a shock to the snowpack. Most of our area has significant amounts of cold, lower-density snow at the surface. Starting today, we'll likely see some of this recent snowfall sluffing from steep terrain. Over the next few days, we could see a significant "shed" cycle where large amounts of snow come unglued from steep, sunny slopes—especially in areas that received the most recent snowfall. These loose snow avalanches could trigger larger slabs as they run downhill. Another point to consider is that as the snow surface warms to near the freezing point, increased creep (the downward "flow" of snow) may add stress to buried weak layers, worsening the persistent slab problem.

Long-term (good news). This past month has led to an overworked and stressed-out snowpack. After the initial effects of the warm-up run their course, the snowpack will start to get the break it sorely needs. Over time, warmer temperatures promote the strengthening of snowpack layers. Since the more recently-formed persistent weak layers in the top 3-4' of the snowpack aren't particularly well-developed (compared to the early season facets, for example) there's reason to be hopeful that the warming trend will help these to stabilize.

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