Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

Strong winds and heavy snow will bring back dangerous avalanche conditions. The slabs of new snow will likely overload the old weak layers in sheltered areas. In wind-loaded areas, fragile slabs of wind drifted snow will exist. The complicated avalanche conditions mean that advanced route finding and decision making skills are required for backcountry travel. Traveling on slopes less steep than 30 degrees not connected to steeper slopes represents a safer choice. CONSIDERABLE avalanche danger exists at all elevations.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    Persistent slabs are back. Prior to this storm, it took several days for the persistent weak layers buried in the snowpack to adjust to the snow above them. Old weak sugary snow (the Dec 11 facets) existed about a foot below the surface on most near and below treeline W-NW-N-NE-E aspects. Observers have even found it in a few isolated areas above treeline. Observers have also reported another thin layer of weak snow closer to the surface. Now 6  to 10 inches of new, dense, heavy snow has accumulated on top of the snowpack in less than 12 hours. This new load will likely push the old weak snow to the breaking point again. Adding a person on top of that new snow would likely provide enough of a trigger to cause a persistent slab avalanche today. These avalanches could break partway down a slope, propagate farther than expected, and people could trigger them remotely from less steep terrain. These avalanches could also have serious consequences as many obstacles still exist in the shallow snowpack.

    Persistent slabs represent one of the most difficult avalanche problems to manage. Sometimes cracking and whumpfing can provide clues to instability; sometimes triggering a large avalanche is the only clue to instability you see. Avoiding travel in avalanche terrain where persistent slabs might lurk is the best strategy for dealing with persistent slabs. The best travel advice for this kind of avalanche problem: stick to slopes less steep than 30 degrees that are not connected to steeper slopes for fun snow and better safety margins.

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1

    The strong winds will move the new snow around and pile it up on the leeward slopes. These new slabs of wind drifted snow will form on top of a variety of old snow surfaces including firm wind-scoured and wind-packed snow. The new wind slabs will be fragile today since they have just formed and have not had time to bond to the existing snow surfaces. In some cases, these wind slabs may form on slopes where the persistent weak layers mentioned above exist. Human-triggered wind slab avalanches will be likely on the most heavily wind-loaded slopes. Larger avalanches that could propagate much farther than expected and be remotely triggered could occur where the wind slabs exist above the persistent weak layers. 

    Newly developed cornices, wind pillows, and wind created surface textures can all help to identify which slopes may hold wind slabs. Signs of wind slabs instability include recent avalanches, cracking, or signs of denser snow over less dense snow. Avoiding these fragile wind slabs only represents part of the strategy today since the persistent slabs mentioned above are likely to exist in many of the more sheltered terrain. Managing multiple avalanche problems at all elevations will be required today. 

Avalanche Discussion

* Strong winds have hammered the snowpack and observers have reported widespread wind-affected snow and wind drifted snow in many places over the last few days.

* Signs of instability and unstable snowpack test results on the Dec 11 persistent weak layer decreased prior to this storm. Despite this overall decrease in instability, this layer still exists and it remains variable with some areas still showing unstable snowpack test results. Data also indicated that additional snow load could reactivate this weak layer.

* Observers have also reported a second potentially problematic layer across the forecast area that was buried on Dec. 17. This thin layer of weak snow continued to show signs of potential instability as of yesterday. 

* Overall data and observations indicate that the snowpack may be too weak to support a new snow load. 

Regions covered