Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Wed
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

Human triggered avalanches will remain possible today at all elevations.  Persistent slab avalanches are possible failing on the recently buried surface hoar.  Existing wind slabs and new wind slabs that form as winds increase this afternoon will also be possible.  MODERATE avalanche danger exists today.  Avalanche danger is expected to increase dramatically tonight as a large and powerful winter storm impacts our forecast region.

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Surface hoar that formed over the weekend has been buried by the Sunday night storm.  This buried surface hoar is on NW-N-NE-E aspects in near and below treeline areas.  Up to 1 feet of new storm snow exists on top of this buried weak layer.  In many below treeline areas, this storm snow is very light and unconsolidated with no slab characteristics present.  In steep areas where no slab exists, loose dry sluffing will be likely failing on the buried surface hoar.  Locations that received higher end storm totals, wind loaded areas, or places where the storm snow is stiffening and gaining strength, remain the concern for slab avalanches today.  

    Buried surface hoar will exist in open terrain in near and below treeline areas.  Avalanches could be remotely triggered from the top, the side, or below the slope.  Any signs of recent avalanches or cracking below the storm snow would be indications of unstable snow in the immediate area.   

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Additional wind slabs will develop today as SW winds begin to increase ahead of the storm tonight.  The low density existing snow will easily be wind transported as wind speeds increase into the moderate range.  Wind slab avalanches will be possible on NW-N-NE-E-SE aspects in near and above treeline areas.  Look for blowing snow, wind pillows, and cornice development as clues to where wind slabs are forming.  In some areas, these wind slabs could be building on top of buried surface hoar.  Avalanches associated with these wind slabs could have wider propagation, be easier to trigger, and larger in size.  

Avalanche Discussion

* Buried surface hoar was found on Silver Peak, in the Pole Creek area.  A small intentional skier triggered wind slab failed on the buried surface hoar.  A natural avalanche seen in the distance was also thought to be associated with the buried weak layer.  

* Clear overnight skies with calm conditions could have again formed another round of surface hoar.  If you see newly developed surface hoar, please send in an observation.

* The Dec 11 layer of faceted snow in the lower portion of the snowpack is being monitored for the possibility of a deep slab avalanche problem if the major snowfall event currently expected Tues afternoon-Thurs verifies as forecast.

* With numerous weak layers from the now buried surface hoar, crust-facet sandwiches in the upper snowpack, and the lingering Dec 11 facet layer, the existing snowpack is in poor condition to handle a major new snow load.

Regions covered