Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
3 · Considerable
Treeline
3 · Considerable
Below
2 · Moderate
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Highlights

This advisory is valid until 12/26 at 7am. Wind slabs in wind-loaded near and above treeline areas mean that human-triggered avalanches remain likely today and will remain possible tomorrow. In near and below treeline areas, human triggered storm slab avalanches will be possible in some areas both today and tomorrow. Evaluate the snowpack and terrain carefully and use this data to inform your route planning. Cautious and conservative decision-making represents the best plan for recreating in the backcountry. 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    Strong winds and new snow have allowed wind slabs to form on the wind-loaded N-NE-E and cross-loaded NW and SE aspects on slopes steeper than 34 degrees. In addition to the wind slabs that formed during the 12/23 storm, shifting winds on 12/24 and 12/25 could cause some smaller wind slabs to form on additional aspects including S facing slopes. All of these wind slabs rest on old snow surfaces consisting of loose weak snow (near surface and near crust facets) and some more firm wind scoured surfaces. Human triggered wind slab avalanches will be likely today and will remain possible on 12/25. The best window for natural avalanche activity involving these wind slabs occurred yesterday evening and into the night, but some natural avalanches involving these wind slabs may remain possible this morning. The potential for natural avalanches involving these wind slabs should continue to decrease during the next 48 hours. In many areas, wind slabs could measure 1.5 to 3 ft in depth with even larger wind slabs on the most wind loaded slopes. They could propagate across a slope and could break above the person who triggers them.  

    Use clues like cornices, blowing snow, snow drifts, wind pillows, scoured surfaces, ripples in the snow, and other wind created textures to help determine where wind slabs may exist. Use this knowledge to find terrain without wind slabs to recreate on. 

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    In sheltered near and below treeline areas, the new snow has fallen on top of loose, weak, old snow (near surface and near crust facets) that exists above the 12/15 rain crust. Tests and observations yesterday at the start of the storm and before the storm indicated that the new snow may not bond well to this loose, weak, old snow. In some areas, this new snow may have enough cohesion to behave as a slab layer and a storm slab avalanche problem may now exist. In these areas, human triggered storm slab avalanches will be possible today and tomorrow. While most of the possible storm slab avalanches should break at the old snow surface and involve all of the new snow, some of them could break just above the 12/15 rain crust and potentially involve the new snow and some of the old snow. They may measure 18 inches or deeper in some areas if they do break below the old snow surface. Other areas may not have received quite enough new snow or the new snow may not be quite slabby enough for this problem to exist. Storm slabs could exist on any aspects on slopes steeper than 34 degrees. 

    Clues like other avalanches, shooting cracks, collapsing, and an upside down snowpack can all help in figuring out where storm slabs may exist. Avoiding slopes with potential storm slabs by sticking to lower angle slopes or well-anchored slopes can allow for a more fun and safer day recreating on the snow. 

Avalanche Discussion

Observations made at the beginning of the storm on 12/23 on Tamarack Peak in the Mt. Rose backcountry, showed significant wind transport and wind slab formation on exposed near and above treeline wind loaded slopes. Even though only 4 inches of new snow had fallen by the early afternoon, these wind slabs already measured more than a foot in depth in some areas. Ski cuts and kicks on wind loaded test slopes caused shooting cracks and wind slab failures. In sheltered areas, new snow accumulation had not yet reached a point where storm slabs had become widespread, but observations did indicate that the new snow was not bonding very well to the old snow surfaces. In these areas, soft new snow rested on a more dense layer of new snow that all rested on top of the old loose weak snow (near surface facets). 

Regions covered