Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

Sat
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

Dangerous avalanche conditions exist, and triggering slides large enough to bury people is possible. Weak layers exist in the top 3 feet of the snowpack—especially in the Western Smokys—and human-triggered avalanches have occurred in the past week. You may be able to trigger slides remotely: from flatter terrain above, below, or to the side of a steep slope. Large avalanches are most likely in wind-loaded terrain but are possible on sheltered slopes too.  

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Problemi valanghivi

  • Persistent Slabs

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

    Late-March storms have buried weak layers of faceted snow that formed earlier in the month. Before this most recent storm, human-triggered avalanches occurred in Owl Creek (in the neighboring Galena Summit & E Mtns zone) and in the Salmon Headwaters. The last storm dropped 18-24+ inches of new snow in this zone, and these weak layers are now buried beneath a 2 to 3 feet thick slab.

    On Wednesday, skiers on a ridgeline remotely-triggered a slide on an ENE-facing, 9300' slope on Avalanche Peak in the neighboring Galena Summit & E Mtns zone. I checked out this avalanche yesterday, and found that it released on one of these March weak layers (video above/right). On the same day, riders near Beaver Creek may have also remotely-triggered a large avalanche.

    These unstable weak layers are not as widespread as when we normally deal with them mid-winter. But that's not a good thing! This makes them more insidious, difficult to manage, and more likely to catch you by surprise. What's more, they're capable of producing dangerous avalanches large enough to bury a skier or rider. These layers are most common in the Western Smokys on slopes that face E-NE-N-NW, but may exist in portions of the Sawtooths as well. On these aspects, it's best to assume that these layers exist and stick to lower slope angles. 

    Remember that persistent slab avalanches can be triggered remotely—from flatter terrain above, below, and to the sides of avalanche starting zones. 

  • Wind Slabs

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

    Expect to find soft and stiff, 1-4 foot thick wind slabs in exposed terrain. They will be most widespread near ridgelines at upper elevations, but the recent storm's erratic winds may have built slabs in unusual places lower on slopes as well. In the Sawtooths, huge amounts of snow collect in cirques and on aprons below large, steep cliff faces. While these wind drifts are becoming less touchy, slopes that were loaded by the wind are more likely to break down to the buried weak layers described above. Use extra caution on any slope that shows signs of wind loading—including a pillowed appearance, stiffer or denser snow, or a rippled texture.

Avalanche Discussion

What's old is new again. After dealing with widespread persistent weak layers for the first part of the winter, an extremely dry February left us with a stable snowpack and LOW danger by March. But now persistent slabs are back on the problem list—déjà vu anyone?

Many folks think of weak, faceted snow as an early-season phenomenon, but the clear nights and temperature swings of spring can be perfect for forming facet layers at the surface of the snowpack. Often times, these facet layers are mixed in with crusts formed by the sun and warming. During the first half of March, an alternating pattern of small spring storms and clear, cold weather created several of these weak layers. More consistent snowfall in the second half of March has now buried these persistent weak layers beneath 1.5-3+ feet thick slab.  

These persistent weak layers exist throughout the forecast area, but seem to be most common and reactive in the Galena Summit area and Smoky Mountains (including the Western Smokys and Salmon Headwaters). In these areas, there have been three, possibly four, human-triggered persistent slab avalanches since late-March. A number of natural and earthquake-triggered avalanches in recent days appeared to break down to these layers as well. Snowpack bservations from our northern mountains have been sparse, but these weak layers may be less common in the Sawtooths and the Banner Summit area—but I wouldn't rule them out.  In the Soldiers & Wood River Valley, there has been less recent loading on these weak layers.

Spring persistent slab problems make me nervous, especially when they follow an extended period of relative stability like we experienced in late-February into early-March. We all seem to understand that these problems are "normal" during the early-winter, and we get in the habit of choosing mellow terrain. But by spring, we expect a more stable snowpack and smaller, more manageable avalanche problems. Quite often that's the case—but not always. In fact, our area has had two avalanche fatalities in April, one major injury in late-March, and a number of late season close calls.

This isn't a typical spring, not on a global scale and not on the scale of our snowpack. There are a number of reasons to ratchet it back out there—and triggering a dangerous persistent slab avalanche is one of them.

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