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 continues near treeline and below treeline on NW-N-NE aspects on slopes 32 degrees and steeper due to ongoing persistent slab instability.

Current conditions are very conducive to human triggered avalanches. Use low angle (sub 30 degree) slopes for protection. Stay clear of areas connected to steeper slopes above. Conservative terrain choices and effective communication are critical given the current instability.

Problèmes avalancheux

  • Deep Persistent Slabs

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

    Over a week has passed since the last snowfall and persistent slab instability continues. Near treeline and below treeline slopes on NW-N-NE aspects that resisted scouring by the Nov 21-23 ENE gale force wind event hold significant amounts of faceted snow below the more recent storm snow. This is the Dec 7 persistent weak layer with an overlying slab. This weak layer is easily collapsed under the weight of a person and has shown the ability to propagate from tens of feet to hundreds of feet. Sizable human triggered avalanches are possible with triggering occurring either high on the slope, low on the slope, or remotely.

Avalanche Discussion

Persistent is certainly the word to describe the current snowpack instability. Observations show little to no change in stability from one day to the next. Observations made yesterday in the Deep Creek Drainage provided ongoing evidence of the instability associated with the Dec 7 faceted persistent weak layer. Widespread collapsing with audible whumphing was observed on near treeline and below treeline northerly aspects from 7,200' up to the high point of travel at 7,850'. Some of the collapses were so pronounced that a wave was visible traveling along the snow surface as the collapsing weak layer propagated away from the trigger point. Some cracking with visible vertical displacement of nearly 1 inch was also observed (see below for more info, photos, pit profile).

Barely covered rocks, stumps, and/or logs that exist on most slopes present an additional hazard to backcountry travelers. There are a few areas where the shallow snowpack has enough structure or strength to keep a person from hitting these objects. In many other areas, the snow cover serves to camouflage their presence.

Régions couvertes