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

Moderate avalanche danger will exist for most of today due to a wind slab avalanche problem.  Human triggered wind slabs will be possible in near treeline and above treeline terrain.  After the onset of snowfall this afternoon, avalanche danger will quickly rise to considerable due to wind slab, storm slab, and deep persistent slab avalanche problems.  Dangerous avalanche conditions will exist.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1

    Strong to gale force S/SW winds overnight will increase through today and build reactive wind slabs on W-NW-N-NE-E-SE aspects in near and above treeline terrain.  In many areas there is substantial available snow for transport on windward slopes to form windslabs today.  Wind slabs will continue to build throughout the afternoon and evening as snowfall begins.  Human triggered avalanches are possible today with natural wind slab avalanches likely this afternoon and evening as new snowfall intensifies.

    Look for blowing snow, new cornice formation, and wind pillows as clues to where wind slabs are forming.  Avoid steep wind loaded terrain.

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1

    Storm slabs will become likely on all aspects in near and below treeline terrain after the onset of snowfall this afternoon.  Surface hoar along with shallowly buried surface hoar has been observed in several locations throughout the forecast area.  This surface hoar has been found in open areas in near treeline and below treeline terrain where winds have not destroyed it.  Buried surface hoar is capable of being remotely triggered and can fail on lower angle slopes.  Storm slabs could also have weakness within the new storm snow or at the old snow/new snow interface. 

    Look for cracking around skis, shooting cracks, or any signs of cohesion within the new storm snow.  These storm slabs will be in wind protected near and below treeline terrain.  Avoid areas that show signs of unstable snow.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    2

    The weak faceted layer of snow that has affected our forecast area over the last several weeks is now buried under 2 to 3'+ of snow and has become a deep persistent slab avalanche problem.  This problem exists in isolated areas on NW-N-NE aspects in near treeline and below treeline terrain.  Although avalanche activity has been limited, concern exists that this layer could fail with a large load of new snow.  Any avalanche due to this deep slab avalanche problem would have large consequences due to increased slab size involved. 

    Deep slab avalanches are unpredictable and hard to collect additional information on.  Tracks on a slope will not be an indicator of stability.  Look for collapsing, cracking, and whumpfing sounds.  Remote triggering is possible, along with failure mid to low slope.  A storm snow related avalanche could have the potential to step down into these weak old snow layers.  Avoid steep avalanche terrain and runout zones where this problem may exist. 

     

Avalanche Discussion

* Buried surface hoar has been observed on Andesite Peak (Donner Summit area), Silver Peak (Pole Creek area), Lincoln Ridge (Yuba Pass area), and Squaw Ridge (Carson Pass area).  This layer of surface hoar is buried 1 to 2'' below the snow surface.  Many areas also had surface hoar on the snow surface up to yesterday afternoon.

* In isolated areas, the persistent weak layer continues to show unstable results with snowpack tests.  This layer is now buried 2 to 3'+ deep in the snowpack.

* Windward slopes in many areas had substantial available snow for wind transport.

* A mix of rain and snow occurred up to at least 8200 ft. in some areas on Monday, leaving a thin rain crust or slightly heavier snow on the snow surface.

Regions covered