Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Sat
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

Human triggered avalanches are possible today in multiple areas.  Strong winds continue to form slabs of dense windblown snow.  Recent storm snow may not have bonded to the old snow/new snow interface.  Both of these problems produced avalanche activity yesterday which could continue today.  Avoid slopes when signs of instability are present.

Avalanche problems

  • Wind Slabs

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

    Wind exposed terrain has been hammered by recent winds over the last 2 days.  Gale force SW winds with gust over 100mph started late Wed night into Thursday morning with the recent storm follow by over a day of NE winds gusting near 100mph.  These strong NE winds will continue through most of today.  A variety of wind scoured, hard wind slab, dense wind slab, and firm scoured surfaces exist.  Some of these wind slabs have formed on top of now buried surface hoar.  Where buried surface hoar does exist, these wind slab avalanches could be easier to trigger, propagate wide, and run long distances.  Wind slabs could exist in many near and above treeline areas on most any aspect, with many of these being hard slabs. 

    Look for recent avalanche activity, blowing snow, wind pillows, cornices over a slope, and snow surface clues to help identify where these wind slabs exist or may be forming.  Steep wind loaded terrain today will be suspect for human triggered avalanches.  Cornices are very large and should be given a wide safety margin.

    (1/19/2023)

    Shallow crowns observed from natural avalanches with wide propagation.   Tbought to be buried surface hoar related.  Castle Peak, Donner Summit area.

    Photo: Steve Reynaud

  • Storm Slabs

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

    Up to a foot of new storm snow fell on Wed night into Thursday morning.  This low density snow has been slow to bond to itself and the old snow surface.  In some areas, this new snow has now buried a layer of surface hoar.  We suspect that this layer of buried surface hoar has been responsible for multiple avalanches yesterday, but we do not have any confirmed observations at this point.  As this new storm snow settles and gains strength, these storm slabs may be able to propagate more efficiently and cause more avalanches.  Remotely triggered avalanches are possible along with avalanches running on the lower end of avalanche terrain slope angles. 

    While traveling through terrain, look for recent avalanche activity, cracking, collapsing, and shooting cracks.  Cracking and shooting cracks in the upper snowpack could indicate that the surface hoar layer is present and a cohesive slab has formed above it.  If any signs of instability exist, pull back into lower angle terrain.

Avalanche Discussion

  • 2 people were caught in an avalanche yesterday and had to be rescued off of Monument Peak, South Lake Tahoe area.  Limited information is available at this time. 
  • Surface hoar formed on Monday and Tuesday nights under mostly clear and cold conditions.  Some of this surface hoar has been buried by the recent Wed night/Thurs morning storm which deposited 4 to 12'' of storm snow.  Some of the avalanches that occurred yesterday are thought to be caused by this layer of buried surface hoar, but no reports have been confirmed.  Limited data along with uncertainty exists on how widespread and reactive this layer is in our forecast region and if it will transition into a persistent slab avalanche problem.
  • Consistent ECTP results occurred on the buried surface hoar layer on Castle Peak, Donner Summit area yesterday.  Other informal observations of surface hoar instability were limited.

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