Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1.5–2.5

  Some wind slab avalanches have occurred over the past two days as blowing and drifting snow deposits along the down wind side of ridgetops and other slope features. Look for areas of blowing and drifting snow. Snow surface clues usually exist that can indicate areas of scouring vs deposition. Identify the locations where wind drifted snow has filled in to form wind slab, often directly below cornice features. Consider avoiding the areas you identify as potentially unstable wind slab, especially if you trigger snow surface cracking or see recent wind slab avalanches.

  Large amounts of low density, recent new snow were available for drifting at the start of this wind event. Snow on the ground has been redistributed to form this avalanche problem. In many areas, these wind slabs have formed on top of lower density recent storm snow, making for an effective slab over weak layer setup.

  

• 

  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 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. For this type of avalanche problem, inconsistent snowpit test results across a zone are common. It is also common for signs of instability to appear to decrease on a regional scale, only to have another persistent slab avalanche occur days after the last one, so don't get fooled.

  We received no new reports of snowpack collapses or audible whumpfs over the past two days. Observations suggest that some areas of instability linger across NW-N-NE aspect terrain. The sugary, Feb 10 facet layer that exists below all of the recent new snow across NW-N-NE aspects varies widely around the forecast area. It remains weak in some areas while presenting as stronger in other areas. Avalanches failing on this deep weak layer would involve all of the recent new snow, potentially making them large to very large in size. They could break wider than expected, run long distances, and could be triggered from mid-slope or below.

  

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–1.5

  Maximum daily air temperatures are forecast to warm even further today, reaching into the 40s in many locations across the forecast area. Signs of surface wet snow instability, such as rollerballs and pinwheels, are expected to form, especially in wind protected areas. Small wet loose avalanches are possible. Move cautiously around avalanche terrain in areas where signs of surface wet snow instability are present.

  There is uncertainty in how thick and how low the cloud cover gets today. If it remains higher and thinner, surface wet snow formation is expected to focus on wind protected E-SE-S-SW-W aspects. Lower, thicker cloud cover could allow for surface wet snow instability to spread to all aspects in wind protected areas.