Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  2–3

  While the best window for natural avalanches occurred yesterday, the wind slabs built by the strong winds and new snow remain unstable. Moderate amounts of fresh snow and wind today will also help them grow. If a person ventures too close to a wind-loaded slope today, they would likely trigger a wind slab avalanche that could be several feet deep. NW-N-NE-E-SE aspects in near and above treeline terrain will hold the largest and most fragile wind slabs, but some of them may also exist below treeline due to the strength of the winds. Large, delicate cornices have formed above many wind-loaded slopes.

  Blowing snow, cornices above a slope, drifted snow, and wind-created surface textures all indicate the presence of wind slabs. Route plans that give wind slabs a wide berth can help avoid getting caught in a wind slab avalanche. This planning will also steer your group into more sheltered terrain where less wind-affected snow will likely exist. 

  

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Weaknesses may still linger within the storm snow or at the boundary between storm snow layers. Triggering a storm slab avalanche while traveling on a steep slope remains possible today. New storm slabs may also form this afternoon as warmer heavier snow or rain falls. Where rain falls on the snow, areas of unstable wet snow will form, and sluffs of wet loose snow may occur. Storm slabs could involve enough snow to bury or injure a person today. 

  Watch for signs of storm slab instability, like recent avalanches or shooting cracks. Reaching into the snow to feel for heavy layers on top of lighter layers or layers of storm snow that easily separate from the snow below them can also help identify where unstable storm slabs may linger. If any signs of instability or uncertainty about a slope exist, bypass that area and switch to a less steep slope.

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2–3

  A thin layer of weak snow may exist near a rain crust buried on Jan 4. Snowpit tests on NW-N-NE aspects between 7,500 ft and 8,500 ft in the Border Ruffian and Blue Lakes areas south of Carson Pass have produced unstable results on this layer. One large avalanche may have occurred on it on 1/8 in that area. A storm day observation from Luther Pass may have found similar instability, but the storminess prevented any certainty. Observations at various elevations in the Independence Lake, Donner Summit, Pole Creek, Mount Rose, and West Shore Tahoe zones have not found evidence of faceting or weakness on this layer. Substantial unknowns remain regarding the areas where this layer is weak and problematic.

  The recent snow could overload this weak layer. An avalanche that fails on this crust/facet layer would fail several feet deep in the snowpack, likely have wide propagation, and could be triggered from low on a slope or the side. If any questions exist about whether or not this layer is present or remains unstable in the area, recreating on slopes less steep than 30 ° that are not connected to steep slopes represents the best plan.