Central Sierra Nevada

Avalanche problems

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Very Likely

  Size

  1.5–3

  Avalanches are very likely in the backcountry today, both above and below treeline. 

  This powerful storm has brought gale-force winds, intense snow, high snow levels, and rain. Cohesive slabs of wind drifted snow and slabs of new snow have formed on top of old, weak snow, resulting in dangerous avalanche conditions. Unstable snow exists in many areas. Some of the most problematic avalanche conditions exist in the sheltered below treeline terrain due to old weak snow at the old/new snow interface. People often consider this wind sheltered below treeline terrain as "safer" during storms. Sometime this is the case, but today conditions are different. Avalanches in these below treeline areas could easily fail wider than expected. People might trigger them from below the slope, low on the slope, or while approaching these slopes from the side.

  In areas where wind-blown snow has filled in along the downwind side of ridges, gullies, and open slopes, larger unstable slabs of locally deeper snow exist.

  Even though accumulated snow has been limited due to the warmth of this storm, a substantial load of 1.6 to 5.2 inches of SWE has been added to the snowpack over the past 48 hours. One thing I have learned over the last 20 years of avalanche forecasting in this area is that the weak layers do not need to be that weak when the load is big. With more rain and snow continuing to load the snowpack today and tonight, it is certainly possible that large avalanches failing on facet/crust layers deeper in the snowpack could occur during this storm cycle. Some may have already occurred yesterday afternoon or last night. Visibility has been poor and the number of backcountry observations are limited, making it difficult to know how much avalanche activity has already occurred during this storm cycle.

   

  

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Very Likely

  Size

  1–2

  Continued heavy rain up to around 8,000' to 8,500 ft today will result in widespread areas of unstable, deep, wet snow on all aspects across a wide range of elevations. Wet snow problems will also extend slightly higher into areas where heavy wet snow falls instead of rain. Similar to yesterday, widespread wet loose avalanche activity is expected again today. Long, steep slopes can produce large, dangerous, wet loose avalanches since they entrain more and more snow as they move downhill. The heavy, wet snow/rain could cause the old, weak surface snow below it to fail, resulting in slab avalanches of wet or storm snow, especially on protected northerly aspects where soft snow lingered during the dry spell.