Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Human-triggered wind slab avalanches remain possible today. The shifting winds have compacted some of the existing wind slabs, making it easier for them to break above a person on the slope. The most fragile wind slabs linger on NW-N-NE-E-SE facing slopes exposed to wind-loading. Expect to find slabs of wind-blown snow on slopes just below ridges, in gullies, and on open slopes. Most of this terrain exists near and above treeline, but some isolated areas below treeline may also hold wind-affected snow. Some of today's wind slabs may still break wider than expected. 

  Recent avalanches, blowing snow, cracking in wind-blown snow, wind-created surface textures, and cornices above a slope can help determine where these wind slabs may exist. Recreating in more sheltered areas without signs of wind-loading can help avoid wind slabs, but you will still have to deal with possible lingering weakness in the upper snowpack mentioned below.

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Unseasonably cold weather has allowed weaknesses in the upper snowpack to persist longer than usual, especially on steep NW-N-NE aspects. These weaknesses include graupel layers, lingering storm snow weaknesses, and a variable buried surface hoar layer. Firm snow or rain crusts below the recent cold snow have provided efficient bed surfaces for snow to slide on when the lingering weak layers break. While the lack of avalanches failing on surface hoar since Monday indicates this layer may be adjusting to the load above it, human-triggered avalanches and unstable test results on the graupel layers and other storm snow weaknesses each day since Thursday indicates avalanches remain possible.

  Identifying which slopes or weak layers may or may not fail will be difficult. Recent avalanches, unstable snowpit test results, snow surface cracking, snowpack collapse, or whumpfing can help identify unstable snow, but sometimes hitting a trigger point like a convex rollover and starting an avalanche may represent the first clue that a slope remains unstable. If you see any signs of instability or have any doubt about a slope, move to lower-angle, non-avalanche terrain that is not connected to steeper slopes.