Central Sierra Nevada

Avalanche problems

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2

  We have a deep slab problem. Four Deep Slab avalanches occurred during the past two days. The avalanches occurred at a variety of elevations on N-NE-ENE aspects. At least nine Deep Slab avalanches are known to have occurred during the recent storm and in the sunny days following. Deep Slab avalanches are a continued avalanche problem today.

  The source of the problem is a weak layer of old faceted (loose, sugary) snow buried 4 to 8 feet deep in the snowpack in many locations on NW-N-NE-ENE aspects. It can be difficult for backcountry travelers to get force down to the depth of this layer where it is deeply buried. In areas where this weak layer comes closer to the snow surface such as near exposed rocks, isolated trees, cliffs, and convex slope roll overs, large avalanches can be triggered by backcountry travelers. This can allow for numerous tracks to exist on the slope prior to an avalanche as was the case in Sunday's avalanche near Lost Lakes (Carson Pass area). Tracks on a slope are not signs of stability when dealing with this avalanche problem.

  Once a Deep Slab avalanche is triggered, propagation will be wide. The avalanche will likely be size D3 (could bury and destroy a car, damage a truck, destroy a wood frame house, or break a few trees). The avalanche could run well into areas of mature forest that might typically be considered "safe terrain".

  This is not the typical type of avalanche dealt with on a storm to storm basis during a Sierra winter. Tactics often applied for managing the common wind slab and storm slab problems may not be effective for dealing with deep slabs. For example, slope cuts are fully inappropriate for this specific avalanche problem. The only way to deal with this avalanche problem is through avoidance. When traveling on NW-N-NE-ENE aspects at any elevation, avoid travel on slopes steeper than 30 degrees and stay out from under steeper terrain above. This type of avalanche problem could be triggered from the valley floor in some cases. Do not underestimate the potential avalanche size. Previous tracks do not equate or prove stability.

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1

  Rapid warming, air temperatures climbing above freezing, and March levels of incoming solar radiation will likely create areas of loose wet avalanche problems today. Sun exposed areas on E-SE-S-SW-W aspects will become problematic as the day progresses. Avalanche size up to D2 is possible (could bury or injure a person).

  Identify and avoid steep sun exposed areas where roller balls are occurring. Roller balls may occur naturally as snow falls off of rocks, cliffs, trees, or can be human triggered.

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1

  There have been a few recent wind slab avalanches over the past several days. Winds have shifted from SW to NE to S so the potential exists for wind slabs to linger near and above treeline on just about any aspect. The largest wind slabs will likely exist on NW-N-NE-E-SE-S aspects.

  Much of the terrain for the largest wind slabs will overlap with areas of concern for deep slab. It will be challenging to manage wind slabs in these areas while at the same time avoiding deep slabs. Use a compass to keep track of aspect and take both avalanche problems into account when dealing with NW-N-NE-ENE aspect terrain.