Bulletin d’avalanche

Central Sierra Nevada

Sierra Avalanche Center
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Danger d’avalanche

Alpin
2 · Modéré
Limite forestière
2 · Modéré
Sous la limite
1 · Faible
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Périmé — ce bulletin a expiré 9 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

Hard wind slabs continue to pose an avalanche problem leading to MODERATE avalanche danger in some near and above treeline areas. Below treeline, avalanche danger is LOW.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1

    Hard wind slabs sitting on top of low density old near surface facets is the problematic snowpack structure (see videos and photos of this in the observations section). 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. Slab thickness has been observed to range from 6 to 18 inches. Human triggered avalanches up to size D2 remain possible. A few reports have been received of recent human triggered avalanche activity in relation to the avalanche problem. Snowpit data continues to show stubborn triggering with little day to day change.

    These wind slabs are very dense, generally allowing for less than 1 inch of ski, snowboard, or snowmobile penetration into the snow surface. These slabs will allow a person to travel well out onto the slab and become fully committed to the slope prior to avalanche occurrence.

    The way this avalanche problem has set up, don't expect to have much opportunity to see signs of instability in any place other than the snowpit. Avoidance is the easiest management technique. Avoid areas of dense wind packed snow. Look for evidence of past drifting, smooth wind sculpted surfaces, and avoid areas with any type of hollow sounding snow. By far the best recreation conditions exist below treeline in areas of both shade and NE wind protection.  

Avalanche Discussion

*A couple reports of recent human triggered wind slab avalanches have been received.

* 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, and Mount Rose areas. Tests targeting these stubborn, hard wind slabs continue to show unstable results with little change day to day (ECTP's and 2m ECTP's).

* Widespread hard wind packed 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, but the snow surface conditions are becoming increasingly variable.

Snow coverage is deepest in areas above 8000 ft. in the northern half of the forecast area (north of Emerald Bay). Even at these elevations, snow coverage varies by location. Some areas at the upper elevations south of Emerald Bay still have marginal snow coverage. Below 8000 ft. most areas still have patchy snow coverage with many exposed obstacles and areas of bare ground.

 

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