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

  SW winds and new snow have built new slabs of wind drifted snow in areas where blowing snow accumulates. Yesterday afternoon some of these wind slabs had already started to fail. The additional weight of a person on a slope where a wind slab exists would likely cause an avalanche today and some wind slabs may fail naturally. Slopes near and below ridgelines on the wind-loaded N-NE-E aspects and on cross-loaded NW and SE aspects will hold the largest and most fragile wind slabs. While near and above treeline areas will see the most wind loading, some smaller wind slabs may form in other areas due to the strength of the winds. 

  Blowing snow means wind slabs. Other signs that a wind slab has formed on a slope include cornices above the slope, wind drifted snow, and other wind-created surface textures.  While many wind slabs will behave like typical wind slabs, buried persistent weak layers also exist below the wind slabs in some areas. Avalanches in these areas would behave more the persistent slabs mentioned below. Complex avalanche conditions mean that avoiding the problems represents a great choice for today. Route-finding that focuses on lower angle sheltered terrain can help avoid avalanche issues. 

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  2–3

  No new persistent slab avalanches were reported yesterday. However, large avalanches, cracking, whumpfing, and snowpack collapse have been observed every other day since the storm. The layers of weak sugary snow buried below the new snow simply cannot support all of the weight above them. They fail under the weight of recent snow and/or people recreating on the snow. Some avalanches have been triggered from lower angle terrain near steeper terrain. Some have propagated around terrain features creating wider than expected avalanches. 

  More new snow last night means that these persistent slab avalanches will remain likely today. Some may fail naturally, but a person recreating on a slope where the persistent weak layers exist would be a more likely trigger. Observations from areas around Yuba Pass, the Mt. Rose backcountry, Luther Pass, and Carson Pass have all found the buried weak layers on NW-N-NE-E-SE aspects. Some elevations as low as 6800 ft have persistent weak layers, but the NW-N-NE aspects at higher elevations hold the most widespread weaknesses.  These weak layers almost certainly exist in other locations as well, we just haven't gotten field reports confirming their presence yet.

  Triggering a persistent slab avalanche today could have severe consequences. These kinds of avalanches often break wider and travel farther than expected. People can trigger them from lower angle terrain or from the bottom of a slope as well. Avoiding areas where these persistent weak layers may exist represents the best strategy for dealing with this problem. Focusing your travel plan on slopes less steep than 30 °  that are not connected to steeper slopes can help avoid this problem.