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

CONSIDERABLE avalanche danger exists on near and above treeline slopes and MODERATE danger exists in below treeline areas. Human triggered wind slab avalanches are likely today and storm slab avalanches are possible. Dangerous avalanche conditions exist. Carefully evaluate the snowpack and terrain before committing to any backcountry route and use this information to choose appropriate terrain that avoids the avalanche problems. 

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    The fragile wind slabs that existed yesterday have grown in size and become more widespread due to more snow and wind during the night. While the best window for natural wind slab avalanches likely occurred last night, human triggered wind slab avalanches remain likely today. These wind slabs most likely exist on exposed near and above treeline wind loaded N-NE-E aspects and cross loaded NW and SE aspects on slopes 35 degrees and steeper. They could measure more than 3 ft. in depth and avalanches involving these wind slabs could have serious consequences.

    Use clues like blowing snow, cornices above slopes, wind pillows/drifts, ripples/waves across the snow surface, scoured surfaces, and other wind created textures to help identify where these wind slabs may exist and avoid the wind slabs.

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    Rising snow levels during the beginning of the storm created an "upside down" snowpack with heavy snow on top of light snow. This light snow can serve as a weak layer with the heavier snow above it serving as a slab layer.  Other changes during the storm may have created other storm snow weaknesses as well. Human triggered storm slab avalanches will remain possible today on any slopes steeper than 35 degrees where new snow exists. 

    Digging into the snow to feel for heavy snow on top of lighter snow and clues like recent avalanches or surface cracking can help identify where storm slabs may be an issue. 

Avalanche Discussion

Yesterday observations on Donner Peak, Tamarack Peak, and Powderhouse Peak all showed dense heavy snow on top of lighter layers of snow and continuous wind transport and wind slab formation on exposed leeward near and above treeline slopes. Wind slabs up to 2 ft in depth had formed on wind loaded slopes by 2 pm. In all three of these locations, tests and observations indicated that the lighter layer of snow formed a weak layer below the heavier layer. In protected areas, fractures did not propagate very well along this weakness. In wind exposed areas where wind loading had created wind slabs, fractures easily propagated along this weakness. Ski cuts and kicks on wind loaded test slopes on Donner Peak triggered shooting cracks in many places with wind slab failures on previously undercut wind loaded test slopes. On Tamarack Peak, ski kicks consistently triggered easy wind slab failures 1-2 ft. in depth.  

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