Bollettino valanghe

Sawtooth & Western Smoky Mtns

Sawtooth Avalanche Center
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Pericolo valanghe

Sun
Alta quota
3 · Marcato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Recent snowfall and several days of wind are creating dangerous avalanche conditions at upper elevations and in alpine terrain. You’re most likely to trigger a slab avalanche in freshly formed wind drifts. Slopes where you could trigger weak layers buried 4-6 feet deep are becoming few and far between, but it may remain possible on very steep, rocky, wind-affected slopes at upper elevations.

Problemi valanghivi

  • Wind Slabs

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

    Since the middle of the week, the Sawtooth & Western Smokys zone has picked up more than a foot of new snow. That would be well and good except that the snowfall has been accompanied by persistent moderate to strong W to NW winds. As a result, fresh wind slabs have formed on the lee sides of ridges and other exposed terrain features. Over the past several days, several small natural and triggered wind slab avalanches have been reported in the central Sawtooths, near Galena Summit, in the Boulder Mountains and in the Wood River Valley.

    Warm temperatures yesterday and overnight have helped these wind slabs bond to underlying snow surfaces, but more snow and wind today will create a fresh batch that may be sensitive to your weight. Fortunately, these wind slabs will be easy to identify. Watch for snow that is denser than snow found on wind-sheltered slopes and tell-tale clues such as surface texture and a pillowed, drifted appearance. In the alpine terrain of the Sawtooths, spindrift trickling down cliffs often forms slabs on the aprons below. Avoid steep slopes where you find these indications of recent wind loading.

  • Persistent Slabs

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

    Triggering a 4-6 foot thick persistent slab avalanche is unlikely. The weak layers we've struggled with all winter have not gone away, but they've become less reactive to the weight of a skier or rider. Due to their depth, they've also become very difficult to impact with your weight. Are there still slopes where it remains possible to trigger these layers? Maybe, but it would likely take a dose of bad luck to find them. But don't completely write them off—especially since the consequences of a triggering a 4-6 foot thick persistent slab would be catastrophic. I continue to be wary of very steep (over 35 degrees), alpine terrain that is more likely to have thin spots where your weight may reach the weak layers.

Avalanche Discussion

The wind is probably one of our least favorite winter weather factors, second only to the "r" word. But since last Monday, it has been the primary force behind our avalanche conditions. As you likely know, when the wind is sufficiently strong, it picks up snow from one spot and puts it in another. While the snow particles blow through the air or bounce along the snow surface, they break up and become smaller. The smaller snow grains more easily pack into dense, stiff, or even hard wind drifts once they're deposited. And—ta-da—you have a wind slab.

In areas where these wind slabs form atop snow surfaces that aren't persistent weak layers, such as softer new snow, older wind slabs, and crusts, the period of instability is often short-lived. Warmer temperatures can help to speed up the process of stabilization. The trouble is that consistent winds over the past several days formed fresh, touchy wind slabs even as old wind slabs stabilized. With any luck, today will be the last day of the persistent winds and wind slabs will get a chance to heal once and for all.

The more complex issue with the recent wind loading is where the wind slabs are adding additional weight to buried persistent weak layers. Where our snowpack remains shallow and especially weak—for example the windswept Boulders and the Wood River Valley—the recent wind loading kicked off a round of natural and triggered avalanches. This problem is likely to persist longer than the "pure" wind slab problem and has the ability to produce a much larger avalanche.

Fortunately, areas with recent wind loading are easy to identify with "look and feel"-type observations. The snow surface on wind-affected slopes usually has a different texture and will be denser and more cohesive than the snow found on sheltered slopes. Pillows and dunes indicate areas where the wind has deposited and drifted the blowing snow. Keeping your eyes open for these clues, and avoiding steep slopes where you find them, will help you avoid most of the avalanche risk that currently exists in our area. This is especially true if you find yourself in alpine areas or where the snowpack remains on the shallow side.

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