Central Sierra Nevada

Avalanche problems

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  2

  Evidence of a low probability, high consequence persistent slab avalanche problem continues to emerge from snowpit data collected in three locations as recently as yesterday. Isolated areas of a difficult to trigger (stubborn) but problematic snowpack structure exist in isolated locations around the forecast area. The layer of concern for this avalanche problem is loose grain (sugary) faceted snow generally found 1 to 2.5 feet deep in the snowpack. There is significant spatial variability around the forecast area regarding the presence or absence of this avalanche problem. No clear explanation exists for the variations in stability between similar terrain at different locations in regards to this avalanche problem.

  Difficult to trigger (low probability) large avalanches size D2 to D3 in treed terrain (high consequence) are a concern near treeline and below treeline on NW-N-NE aspects. Areas with fair to good protection from NE wind scouring are the most suspect. Plan accordingly for and take measures to manage the potential for avalanche triggering to occur in the mid to lower portions of the slope for this particular avalanche problem.

  Avoid avalanche terrain in any areas where snowpack collapse with a possible associated whumpfing sound or unstable snowpit test results are occurring. Keep in mind that snowpits dug high on the slope may miss evidence of a problem that exists in the mid to lower portions of the slope.

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Unlikely

  Size

  1

  Some signs of wind slab instability were reported yesterday along ridgelines near cornice features and wind pillows. Improving stability is expected today, but a lingering unstable wind slab is not impossible near treeline or above treeline in a recently wind loaded area on a NW-N-NE-E-SE aspect.

  Look for and identify areas of concern adjacent to or below cornices and wind pillows. Human triggered cracking near the edges of the wind slab are often the easiest signs of instability to identify.