Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1.5–3

  Unstable slabs of wind drifted snow remains an ongoing concern. Ridgetop winds shifted from SW to NE overnight and will redistribute snow on the ground today.  Wind slabs will continue to form as winds continue to drift and deposit wind-blown snow onto slopes near ridgelines and into gullies. Wind slabs will have a wide range in size today. Wind slabs that formed under SW winds during the storm are thick and the resulting avalanches could be deep. Wind slabs formed by the current, post storm NE winds will be smaller in comparison, but still large enough to present a hazard to backcountry travelers. Wind slabs could be encountered on any aspect in near treeline and above treeline terrain. Wind slabs have also formed to a lesser extent below treeline due to the strength of the SW winds during the past 24 hours.

  Human triggered wind slab avalanches are likely today with isolated natural avalanches possible. Any avalanches triggered could easily be larger than expected. Consider avoiding slopes and gullies directly below or adjacent to cornice features. Slopes near recent avalanches are suspect. Snow surface cracking is a sign of unstable snow in the vicinity. Dangerous avalanche conditions exist today, warranting well thought out terrain selection and conservative travel management.

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2–3

  While surface instabilities in the form of storm slabs or dry loose instability are expected today, deeper storm slab instability is the major concern. Various weak layers exist 6+ feet below the snow surface on NW-N-NE aspects. The most notable is lightly faceted snow, sometimes with a crust at the Feb 21 old/new snow interface. This is right at the bottom of the 6 to 10 feet of storm snow that accumulated since Feb 21. Isolated snowpack collapses, whumpfing, test slope failure, and unstable snowpit test results have been observed where relative weakness exists at the bottom of the massive storm snow load. The most likely time for this layer to fail was either over the past 24 hours during the storm or during the day today as the snowpack continues to adjust to the rapid loading/stress it just received.

  Becoming caught in an avalanche that fails deep in the snowpack may not be survivable, especially below treeline. Deliberately choosing terrain with slope angles in the 25 to 28 degree range with size and shape that is not capable of producing large avalanches is a way to reduce the risk. Keep in mind that an avalanche that fails deeper and wider than expected has a greater chance of catching more than one person. Don't rely on marginal safe zones.