Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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Non aggiornato — questo bollettino è scaduto 9 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Hard wind slabs continue to pose a difficult to trigger avalanche problem keeping MODERATE avalanche danger ongoing in some near and above treeline areas. Below treeline, avalanche danger is LOW. These near and above treeline wind slabs are very dense and could allow a person to travel well out onto the slab, becoming fully committed to the slope prior to avalanche occurrence. It is also plausible that several to numerous individuals could travel a slope before the snowpack fails.

Problemi valanghivi

  • Wind Slabs

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

    This avalanche problem is found near and above treeline most prominently on NW aspects. These wind slabs have also been found in very isolated areas on N-NE aspects where the interaction between NE wind and terrain did not result in snow surface scouring. A fair bit of variability for presence or absence of the problem exists around the forecast area, but it continues to show in numerous areas on a regional scale.

    This avalanche problem is challenging. Most areas above treeline are very wind affected and it requires a trained eye to differentiate out the areas of hard, mostly smooth wind slab that exist within of the greater areas of wind scoured snow surfaces. Informal evidence of instability is hard to come by, giving little in the way of evidence to aid decision making for the typical recreationist. Don't expect to have much opportunity to see signs of instability in any place other than the snowpit.

    If snowpit work and exact slope angle measurements are not solid skills in the tool kit, avoidance is the easiest management technique. Avoid areas of dense wind packed snow. Look for evidence of past drifting, fairly smooth wind sculpted surfaces, and avoid areas with any type of hollow sounding snow. The best snow surface conditions for recreation exist below treeline in areas with both shade and NE wind protection.

    This avalanche problem is fueled by the presence of a low density layer of old near surface facets below the hard wind slabs. This hard slab over weak layer is a problematic snowpack structure (see videos and photos of this structure in the observations section).

    Slab thickness has been observed to range widely with most of the unstable snowpit data resulting from slabs 6 to 18 inches thick. Human triggered avalanches up to size D2 remain possible.

Avalanche Discussion

*A couple reports received of human triggered wind slab avalanches this past week.

* Hard wind slabs resting above a weaker layer of older near surface facets have been observed on wind-loaded slopes in the Donner Summit,  Carson Pass, Granite Chief/Needle Pk, and Mount Rose areas. Snowpit tests targeting these stubborn, hard wind slabs continue to show unstable results in some areas.

* Widespread hard wind scoured surfaces exist on exposed N-NE-E aspects. Some areas of firm ice (rain crust) exist at the snow surface in the most heavily wind scoured areas.

* Some areas of unconsolidated snow linger below treeline in shaded areas sheltered from the E and NE winds. These snow surface conditions are becoming increasingly variable due to warming.

Snow coverage is deepest in areas above 8000' in the northern half of the forecast area (north of Emerald Bay). Some areas at the upper elevations south of Emerald Bay have marginal snow coverage. Below 8000', many areas hold patchy snow coverage with exposed obstacles and areas of bare ground. Some exceptions exist with areas of more significant snow cover down to 7,300'.

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