Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2–3

  Slabs of drifted snow linger on leeward slopes on wind-loaded and cross-loaded aspects. In some areas, newer non-wind-affected snow exists on the surface making the wind slabs more difficult to identify. Observations indicate that these wind slabs have formed decent bonds with the old snow surfaces and within the new snow and that trigging a wind slab avalanche has grown more difficult in many places. Still, human-triggered wind slab avalanches may remain possible today especially on heavily wind-loaded slopes or in complex or extreme terrain. In addition to the lingering wind slabs, large fragile cornices also exist above many slopes.

  Cornices overhanging a slope, wind-scoured areas along a ridge, drifted snow, and wind created surface textures can all help identify where wind slabs exist. Evaluate these areas carefully. If any doubt exists about the stability of the wind slabs, change plans and move into an area without the wind slab. Travel in more sheltered terrain will help avoid the wind slabs, but you will still need to assess the potential for the persistent slabs mentioned below. 

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2–3

  Triggering a persistent slab avalanche is becoming more difficult due to a combination of the old weak layers gaining strength and the difficulty of transmitting force through the thick layer of new snow. Some uncertainty still exists about these layers since we have limited data about them during and after the storm. This uncertainty coupled with the severe consequences associated with persistent slab avalanches means that this problem still warrants attention. Triggering one of these avalanches may still remain possible in some places especially with a large trigger like a big cornice or multiple people on a slope or by hitting a trigger point where the weak layers are closer to the surface. These kinds of avalanches often propagate farther and wider across slopes and can be triggered remotely. If one were to break on the Dec 11 facet layer, it could involve almost the entire snowpack. 

  Signs like recent avalanche activity, shooting cracks, or whumpfing can sometimes indicate areas of instability, but sometimes the first sign of instability is triggering a large avalanche especially when the weak layers are this deep. Careful snowpack evaluation involving lots of digging can help determine where these layers may still remain problematic. Sticking to slopes less steep than 30 degrees that are not connected to steeper slopes represents one way to avoid this problem. Areas where a shallow and weak snowpack previously existed like in the Carson and Ebbetts Pass areas hold the most lingering concerns.