Central Sierra Nevada

Problemi valanghivi

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Likely

  Dimensione

  1–2

  Winds have been blowing and drifting snow, depositing it 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. These are the locations where new, potentially unstable wind slabs have formed. 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

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Possible

  Dimensione

  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. 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 yesterday.  Collapses and whumpfs were reported across N-NE-E-SE-S-SW aspects Feb 19-20. Snowpit test results from yesterday continued to suggest instability in some locations and not in others. The sugary, Feb 10 facet layer that exists below all of the recent new snow on NW-N-NE aspects varies widely around the forecast area and is much weaker in some areas than in others. Avalanches failing on deeper weak layers 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 mid-slope or below.

  

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Possible

  Dimensione

  1–1.5

  Maximum daily air temperatures are on a day to day warming trend. Signs of surface wet snow instability, such as rollerballs and pinwheels, are expected to form, especially as new snow falls off rocks and trees onto slopes with direct sun exposure. Small wet loose avalanches are possible on sun exposed slopes. Move cautiously around avalanche terrain in areas where signs of surface wet snow instability are present.