Central Sierra Nevada

Avalanche problems

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2

  The Dec. 11 persistent weak layer is widespread throughout the forecast region on W-NW-N-NE-E aspects in below and near treeline terrain and more isolated on these aspects in above treeline areas.  Multiple human triggered avalanches have occurred on this weak layer since our last storm 3 days ago.  Several observations from yesterday continue to show whumpfing sounds, cracking, and unstable snowpack test results that point to this weak layer still being reactive and capable of producing large avalanches.  As this weak layer becomes more deeply buried, signs of instability may be harder to find.  Any whumpfing, shooting cracks, or recent avalanche activity would be huge indications of unstable snow, but the first sign could also be you or your group triggering a large slab avalanche.  

  Persistent slab avalanche problems are one of the most challenging problems for backcountry users to deal with.  Managing terrain is the easiest way to set yourself up for long term success until this weak layer goes away.  Avoid any avalanche terrain over 30 degrees and any steep connected or overhead terrain.  Remote triggering has occurred on this weak layer and remains possible.  Wide propagation with large avalanches, mid slope avalanches, and avalanches on slopes that already have tracks could also occur.    

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1

  Strong to gale force NE winds overnight have scoured many exposed slopes and created new wind slabs on SE-S-SW-W-NW aspects.  Previous wind slabs on NW-N-NE-E-SE aspects have been wind scoured in many areas from these winds.  In areas with NE wind protection or in locations where these wind slabs exist over the Dec 11 persistent weak layer, unstable snow with human triggered avalanches remain possible.  Winds will shift to the W tonight and into tomorrow ahead of our next storm.  Wind slab avalanches could be possible on all aspects in near and above treeline areas.

  Look for blowing snow, new cornice development, and fresh wind pillows.  Many slopes will have been wind loaded previously during the storm to be wind scoured recently.  Avoid areas under large cornices and steep wind loaded slopes.