Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
1 · Low
Treeline
2 · Moderate
Below
2 · Moderate
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Out of date — this forecast expired 13 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

MODERATE avalanche danger still lingers on near and below treeline NW-N-NE aspects on slopes steeper than 32 degrees because persistent slabs still remain in those areas.

Human triggered avalanches remain possible. Barely covered rocks, logs, stumps and other obstacles would increase the consequences of any avalanches that do occur.

I will update the advisory again on Dec. 26 at 7:00 am.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Persistent slabs still exist on NW-N-NE aspects in near and below treeline terrain. In some areas the myriad of anchors/obstacles poking through the snowpack have kept these slabs small. In some areas these slabs have become more difficult to trigger. However, in most places data and observations continue to indicate that the additional weight of a person on top of the slope can break the weak layer (Dec 7th facets) and that the resulting fracture can still travel along that weak layer. Human triggered persistent slabs avalanches remain possible. 

    Any of these avalanches that do occur would drag a person through rocks, stumps, trees, and other obstacles because of the shallow snowpack and the fact that the near and below treeline terrain holds the most unstable snow as well as the greatest number of obstacles.

Avalanche Discussion

Yesterday, observations along Becker Ridge showed some signs of improving stability (snowpit, more info). The Dec 7th facets still exist in this area and a more cohesive slab layer still sits above them. Snowpit data and tests still indicated that the sugary snow comprising the Dec 7th facets remains weak and easy to break, but that the resulting fractures did not travel very far along the weak layer. Also no human-triggered whumphing, collapsing, or cracking occurred. In contrast to this more benign data, the majority of data from around the region still points to an unstable snowpack in which fractures can easily travel along the Dec 7th facet layer (see the observations page).

As persistent weak layers like this one gain strength, they strengthen sporadically with some areas showing no signs of instability while adjacent areas remain unstable and easy to trigger. This process leads to a great deal of variability on all scales and uncertainty about which slopes or even which parts of the same slope remain unstable. Recent observations seem to indicate some of this variability.

The weak and shallow snowpack will not support a person or snowmobile enough to prevent collisions with all of the barely covered rocks, stumps, logs, and other hard immobile objects. Collisions with these obstacles could easily break equipment and injure people.  

Regions covered