Bollettino valanghe

Banner Summit

Sawtooth Avalanche Center
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Non aggiornato — questo bollettino è scaduto 7 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Fresh snowfall and moderate winds are forming fresh wind drifts a foot or more thick at upper elevations. Although small, they could be easily triggered by the weight of a skier or rider.  It'd be a stroke of bad luck, but triggering an avalanche on weak layers buried 4-6 feet deep remains a possibility on very steep, rocky, wind-affected slopes at upper elevations. 

Problemi valanghivi

  • Wind Slabs

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    Over the past week, the wind has waged a war against our snowpack. Recently, it's been losing the battle as its ammunition—loose snow available to blow around—becomes depleted. Today, reinforcements are being delivered in the form of low-density new snow. 

    Up to 8 inches of new snow may accumulate by this afternoon. This snowfall, combined with moderate westerly winds, will form fresh, soft drifts a foot or more thick in upper elevation, exposed terrain. These will be forming on a variety of slippery surfaces including icy crusts and facets, so I would expect them to be easily triggered on steep slopes. These new wind slabs will probably not be big enough to bury you, but could be plenty to knock you off your feet or sled. You'll want to use extra caution on—or avoid—steep slopes where you find fresh drifts.

    Today's avalanche conditions are directly tied to the weather forecast—which has some uncertainty. If you're finding more than 8 inches of new snow has accumulated, or the winds are blowing harder than forecast, expect the wind slab problem to be more dangerous.

    ADDITIONAL DISCUSSIONBy the end of the day—especially if the storm produces more than about 8 inches of new snow—it may become possible to trigger soft slabs within the new storm snow on very steep, wind-sheltered slopes. 

    Triggering a 4-6 foot thick persistent slab avalanche has become very unlikely. It would take a dose of very bad luck, but it may be possible to trigger one of these monsters in very steep (over 35 degrees), wind-swept, alpine terrain.  

Avalanche Discussion

Today's storm should produce less than about 4 inches of new snow in the mountains south of Galena Summit. This snowfall shouldn't have a huge effect on today's avalanche danger, but fresh, thin wind slabs may form in upper elevation terrain. Due to the old, slick surfaces they're forming on, they could be touchy but are likely to be small in size. On the other end of the likelihood/size spectrum, the persistent slab problem continues to nag us. While unlikely, triggering one of these would produce an avalanche plenty big enough to bury or kill you. 

In the Sawtooth & Western Smokys and Banner Summit zones, our attention is slowly shifting from deeply buried persistent weak layers to storm-related surface instabilities. Even though today's storm isn't looking as promising as it was a few days ago, up to 8 inches of new snow could accumulate in these mountains by the end of the day. With the low-confidence weather forecast, this storm could produce even more (or of course, less). Moderate westerly winds today will easily transport this new snow into fresh, soft wind drifts on upper elevation ridgelines and other prominent terrain features. These wind slabs will form on old snow surfaces that include anything and everything—wind-hardened surfaces, stiff drifts, sun crusts, and near-surface facets. All of these tend to be slippery, and today's fresh wind slabs will likely be touchy on steep slopes. If the storm produces more than forecast—say, greater than about 8 inches—we may begin to see a storm slab problem on very slopes that are sheltered from the wind.

It's easy to get cavalier about these surface instabilities after dealing with persistent slabs problems all winter. When faced with these "manageable" problems, you may find yourself in consequential terrain where even a small slide can be every bit as dangerous as a big, bad persistent slab. Remember that the avalanche danger is only one part of the equation, the other components are your level of exposure to the avalanche hazard and the consequences of triggering a slide. Often our overconfidence at managing surface instabilities leads us into terrain where the exposure and consequences can be dangerously high.

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