Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Natural and human-triggered wind slab avalanches occurred Friday, Saturday, and Sunday and are likely again today. SW ridgetop winds increased yesterday afternoon and last night reaching gale force speed. Drifting new snow will occur today, continuing to add to the current instability. Slabs of wind drifted snow exist on leeward slopes near ridgelines, in gullies, and on open slopes exposed to the wind. The majority of wind slabs exist on NW-N-NE-E-SE aspects above treeline and near treeline but expect to encounter them anywhere wind drifted snow has deposited. Today's wind slabs could easily involve enough snow to bury or injure a person.

  Clues like recent wind slab avalanches, cornices above a slope, blowing snow, snowdrifts, ripples on the snow surface, and other wind created features can be used to identify where wind slabs exist. Identify areas of suspect wind slab and move around them with an avoidance mindset. Cornices will likely remain fragile again today.

  

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Yesterday marked three days in a row of storm slab avalanches, most of them skier/snowboarder triggered in steep terrain. Recent new snow has been slow to bond keeping storm slab avalanche problems ongoing. Denser snow over lower density storm snow is the issue, sometimes with graupel (pellet snow) involved in the weak layer. Today's snowfall is upside-down, again depositing denser snow over weaker less dense snow and on top of a new layer of graupel in some areas. Storm slab avalanche size could range from small (D1) to large (D2) today depending on if just today's new snow is involved or if the snowpack failure involves all of the new snow accumulated since Thursday night.

  This avalanche problem exists in places that people gravitate towards to avoid wind slabs such as wind protected, tree-covered slopes. In these areas, avalanches could have more severe consequences if they push people into trees or into/over other obstacles. Terrain traps such as narrow gullies and creek beds can also magnify the consequences of a storm slab avalanche via deeper burial.   

  Recent avalanche activity, shooting cracks, or dense over soft feeling snow, are clues to storm slab instability. If these clues are present, move to sub 30 degree slope angle terrain.