Central Sierra Nevada

Avalanche problems

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2–3

  A skier triggered persistent slab avalanche occurred on Monday afternoon on Powderhouse Peak that resulted in an avalanche fatality.  This avalanche occurred around 8800', on a N-NE aspect, with wide propagation.  Below the 2 to 4 feet of recent storm snow from last Thursday/Friday's storm, a layer of weaker faceted snow, buried surface hoar, and/or a facet/crust combination exists.  Large to very large natural and human triggered avalanches have been reported failing on this weak layer since the storm began last Thursday.  Reports and signs of instabilities have been decreasing since the weekend along with snowpack tests showing this layer gaining strength and bonding into the existing snowpack.  The snowpack does not stabilize at the same rate in all locations and will need time for this persistent slab avalanche problem to fully go away.  While this layer is becoming stubborn and harder to trigger, large to very large persistent slab avalanches with severe consequences remain possible.

  To avoid this avalanche problem, travel on SE-S-SW aspects where this avalanche problem does not exist or on sub 30 degree terrain with no other steeper connected terrain above or to the side.  If traveling in areas where this problem may exist, reduce your exposure as much as possible.  Travel one at a time, limit time in avalanche terrain, and regroup in safe zones in the event of a large and unexpected avalanche.  

  

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Southwest winds will increase today to strong to gale force along exposed ridges and peaks.  Light snowfall along with available snow for transport on windward terrain will cause blowing snow and wind slabs to form below ridges, in gully features, and in open areas.  Terrain where more snow is available for wind transport will have larger wind slabs.  Lower elevation terrain where melt freeze crusts have developed or recent rain on snow has occurred, will have less snow available for wind slab development.  

  Look for blowing snow, new cornice formation, and wind pillows as clues to where these wind slabs are forming.  Snow surface cracking in wind loaded terrain will indicate reactive wind slabs are forming.  Identify where these wind slabs are forming and move around any areas of concern.