Bulletin d’avalanche

Central Sierra Nevada

Sierra Avalanche Center
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Danger d’avalanche

Alpin
1 · Faible
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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Périmé — ce bulletin a expiré 13 years ago. Vous consultez des conditions passées, pas le bulletin en cours.

Points saillants

MODERATE avalanche danger still exists near and below treeline on NW-N-NE aspects on slopes steeper than 32 degrees due to lingering persistent slab instability.

Even though these slabs have become more difficult to trigger, human triggered avalanches remain possible in places where consistent snow cover exists.

Problèmes avalancheux

  • Deep Persistent Slabs

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

    Persistent slabs still exist on the NW-N-NE near and below treeline aspects. The persistent weak layer near the base of the snowpack has shown some signs of strengthening in some areas. In other areas the overlying slab has eroded away. However, in many areas a slab still sits on top of a fragile weak layer waiting for the right trigger to break it loose. A person's weight on the snowpack in the right spot can still trigger an avalanche.

    The increase in variability and uncertainty associated with the persistent slabs makes it very difficult to accurately access snowpack stability. Even though most observations may point to a stable slope, it only takes finding that one hidden trigger point to start a failure that could take down the whole slope. Using the terrain to avoid the avalanche hazard by recreating on lower angle slopes that remain disconnected from steeper and deeper terrain represents a fun and safe plan when conditions like this exist.

    Between the lingering persistent slabs and the shallow snow cover over rocks, logs, and other objects, significant hazards exist in the backcountry.

Avalanche Discussion

Observations on Rubicon Peak (more info, video) yesterday showed that weak sugary snow grains (Dec 7th facets) still exist in the bottom half of the snowpack. Above this persistent weak layer, more consolidated slab-like layers of snow existed in most places between 7600ft. and 9000ft. In some areas these more consolidated layers had started to lose strength and become less slab like. Tests on the snowpack yielded unstable results indicating that fractures can still travel easily through the Dec 7th facets as well as results that indicated that fracture propagation has become unlikely.  Often these contradicting tests occurred right next to each other on the same slope. Tests also indicated that fractures are more likely to start and propagate from areas where a shallower snowpack exists or from trigger points like shallowly buried rocks. Trying to find these trigger points on a snow covered open slope would be quite difficult without x-ray vision. 

Overall data from around the forecast area indicates snowpack whose stability or instability varies greatly from slope to slope or even across an individual slope.

Régions couvertes