Bulletin d’avalanche

Central Sierra Nevada

Sierra Avalanche Center
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Danger d’avalanche

Sat
Alpin
3 · Marqué
Limite forestière
3 · Marqué
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 6 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

New snow and wind have created dangerous avalanche conditions in the backcountry. Watch out for fresh slabs of wind drifted snow near ridgelines, in gullies, and on open slopes exposed to the wind. In more sheltered areas, slabs of storm snow may exist.  Human-triggered avalanches are likely on wind-loaded slopes and possible on steep sheltered slopes. CONSIDERABLE avalanche danger exists in near and above treeline terrain and MODERATE danger exists below treeline.

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Likely
    Taille
    1–2

    Strong winds and more than a foot of new snow in many areas have formed new slabs of wind drifted snow on leeward slopes near ridgelines, in gullies, and on open slopes exposed to the wind. Due to the wind shifting between the SW and W some wind slabs could exist in unusual places. They have not had time to form strong bonds with the old snow surfaces, and a person's weight on them would likely trigger a wind slab avalanche today. These wind slabs could measure several feet in depth and could easily involve enough snow to bury or injure a person

    Clues like cornices above a slope, drifts of windblown snow, blowing snow, ripples on the snow surface, and other wind created surface textures can help determine where the wind slabs exist. Wind slabs will be fragile today and avoiding them will be the best strategy.

  • Storm Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    In areas sheltered from the wind, soft slabs comprised of storm snow may exist. Human-triggered storm slab avalanches may be possible on some steep slopes where the most new snow accumulated. These storm slabs would fail on weaknesses within the storm snow or at the interface between the storm snow and the old snow surfaces. They should remain smaller and less sensitive than the wind slabs mentioned above, but it is possible that they could involve enough snow to bury or injure a person especially in areas with terrain traps like steep narrow gullies. In addition to the possibility of storm slabs, some loose dry sluffs may occur on steep slopes. 

    Recent avalanche activity, shooting cracks, or punchy (dense over soft) feeling snow, can indicate storm slab instability. Identify where storm slabs may exist and if there is any doubt about the stability of a slope change plans and move to lower angle terrain where the snow will still be just as soft.

Avalanche Discussion

The old weak layers like the Jan 22 crust/facet combo or the Dec 11 facets still exist in the snowpack. They are buried under strong snow and have been gaining strength. The vast majority of observations and data collected across the region since the major January storm indicate that these old deep weak layers have gone dormant. In a few isolated areas with a very weak and shallow snowpack, snowpit tests still yield some unstable results mixed in with other tests yielding stable results. While this storm does add new load to the snowpack, it is unlikely that it has added enough new weight to reactivate those deeply buried weak layers.

 

Recent Observations:

* Yesterday observers on Mt Judah ( Donner Summit) found variable snow surfaces ranging from frozen supportable crusts to soft melt-freeze snow to breakable crusts to wet unsupportable snow to dust on crust conditions. Similar variability was reported from the Blue Lakes area (Carson Pass). 

* Observers from Blues Lakes( Carson Pass), Mt Judah (Donner Summit), and Castle Peak (Donner Summit) all reported increasing winds and incoming clouds in the morning with precipitation starting a little before noon and intensifying throughout the afternoon. 

Régions couvertes