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Out of date — this forecast expired 13 years ago.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

On slopes steeper than 32 degrees, MODERATE avalanche danger exists on near and below treeline NW-N-NE aspects due to lingering persistent slabs. Some isolated areas of unstable snow may still remain on above treeline slopes as well. Human triggered persistent slabs remain possible in near and below treeline terrain. Remotely triggered avalanches can occur under these conditions.

In addition to the avalanche danger a myriad of season-ending obstacles still lurks just below the surface. Conservative terrain selection, careful snowpack evaluation, and conservative decision-making are essential.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Human triggered avalanches remain possible in near and below treeline terrain on NW-N-NE aspects where the Dec. 7th facet layer exists. While the vast majority of these persistent slabs exist in near and below treeline terrain, the possibility remains that some isolated areas above treeline where snow did not get scoured away earlier this season could pose a problem. Currently the snow that fell on Dec. 7th represents a soft slab sitting above the older layers of weak sugary snow. Even though some data indicates that the snowpack has become less fragile, the addtional weight of a person on a slope where this persistent weak layer/soft slab combination exists can still cause the slope to fracture. Unstable conditions like these take a long time to change and can easily catch people off-guard.

    Since fractures can travel through the persistent weak layer, people can trigger persistent slabs remotely while standing in low angle terrain or on ridgelines or anywhere that the snowpack under their feet connects to steeper and/or deeper terrain. Wide propagation and avalanche activity on lower angle slopes to around 32 degrees remains possible below treeline.

Avalanche Discussion

Yesterday some collapsing and whumpfing continued to occur on Donner Peak. As in other areas these collapses happened when the weak sugary snow (Dec. 7th facets) below the recent new snow broke and a fracture propagated along the weak layer. These collapses were less consistent and smaller than those that occurred in other areas over the weekend and earlier this week. Snowpit tests did indicate that if this layer does break the resulting fracture can travel along this persistent weak layer. Observations and snowpit data seem to indicate that while the Dec. 7th facets remain weak they have started to adjust VERY SLOWLY to the load above them. Data also indicates that the snow layer above the Dec. 7th facets still represents a soft slab.

Barely covered rocks, stumps, logs, and other hard, immobile, season-ending, equipment-breaking objects still exist on all slopes. The shallow snowpack does not have enough structure or strength to keep a person above all these things. It only camouflages their existence.

Regions covered