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

Dangerous avalanche conditions continue with persistent slab and wind slab avalanche problems.  An unstable weak layer is buried in a large part of the usable terrain in our area.  CONSIDERABLE avalanche danger exists at all elevations.  Advanced route finding and decision making skills are required to avoid todays avalanche problems.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    The Dec. 11 persistent weak layer is now buried under 6 to 12'' of recent storm snow.  Many reports of loud whumpfing sounds and cracking came in yesterday across the entire forecast area.  This weak buried facet layer, that formed during our long dry spell from Nov 18 to Dec 11, is widespread below treeline and near treeline on W-NW-N-NE-E aspects. It has also been noted in isolated areas above treeline.  This persistent weak layer is showing unstable signs with the additional storm load, it may only need the added weight of a person to trigger an avalanche.  These avalanches could have wide propagation and be remotely triggered from below, above, to the side, or from any connected terrain. 

    Advanced terrain management and route finding skills are required.  This persistent slab avalanche problem lives in the same terrain that also has the most snow coverage for recreation.  Utilize slopes that are well under 30 degrees in steepness and not connected to any steeper terrain.  Whumpfing sounds or cracking will be major signs of instability.  Consequences of an avalanche remain high with our shallow early season snowpack with many exposed obstacles to potentially hit.        

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1

    Wind slabs developed during the recent storm in near and above treeline terrain from gale force SW winds.  Unstable wind slabs have been reported over the last 3 days with at least 1 natural avalanche occurring yesterday.  Human triggered wind slabs are expected to remain reactive today as well as cornice fall.  Where wind slab are deposited on top of the persistent weak layer, these avalanches could propagate wide, be remotely triggered, and create much larger avalanches.

    Look for newly developed cornices and wind pillows towards the upper part of slopes and below ridges.  Any cracking or signs of denser snow over less dense snow in these areas will be an indication of unstable wind slab.  Cornice fall could be large triggers for avalanches today.  Managing multiple avalanche problems at all elevations will be required today.       

Avalanche Discussion

*  Unstable wind slabs have been reported over the last 3 days.  Yesterday, a natural cornice collapse and wind slab avalanche was reported from Silver Peak.

*  The Dec 11 persistent weak layer was formed by the faceting process during our extended dry period from Nov 18 through Dec 11 and is now buried 6 to 12'' below the snow surface.  This buried faceted weak layer is widespread below treeline, near treeline, and is some case above treeline on W-NW-N-NE-E aspects.  Widespread whumpfing, cracking, and unstable snowpack test results have been reported throughout the forecast region.

 

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