Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Winds increased last night and were strong enough to blow and drift snow well before sunrise. Look for blowing and drifting snow along ridgetops. Identify the locations were wind drifted snow is filling in. This is the location where new, potentially unstable wind slabs are forming today. Consider avoiding the areas you identify as potentially unstable wind slab, especially if you see or trigger snow surface cracking.

  There are large amounts of low density, recent new snow available for drifting, as snow on the ground is redistributed to form this avalanche problem. In many areas, these slabs will form on top of lower density recent storm snow, making for an effective slab over weak layer setup. This avalanche problem is expected to increase in both potential avalanche size and distribution as wind speeds increase and drifting snow continues through today and into tonight.

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2–3

  These avalanches will be easiest to trigger in locations where the old snow/recent storm snow interface is closer to the snow surface. This could be in areas with overall less new snow (say 3 ft instead of 5 ft+) or in areas around rocks and cliffs where weak layers are often brought closer to the snow surface. Consider avoiding avalanche terrain in the area if you trigger snowpack collapses or audible whumpfing. Slopes where this avalanche problem lingers are presenting inconsistently across terrain, leading to increased uncertainty. These layers are deep and often not well assessed by compression based snowpit tests. Keep in mind that snowpit tests in general can never prove stability. They can only reveal indications of instability. This is an important distinction.

  Snowpack collapses, and audible whumpfs have been reported from throughout the forecast area over the past two days, giving clear indications of ongoing instability. These collapses and whumpfs have been reported across N-NE-E-SE-S-SW aspects. The sugary, Feb 10 facet layer that exists below all of the recent new snow on NW-N-NE aspects has been a player in some but not all of these events. Avalanches failing on deeper weak layers would involve all of the recent new snow, making them large to very large in size. They could break wider than expected, running long distances, and could be triggered mid-slope or below.