Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2

  Strong southwest winds and 2 to 6 inches of snowfall overnight mean that fragile wind slabs have started forming on wind-loaded slopes. Wind slabs 1 to 2 ft. in depth could already exist on heavily wind loaded slopes this morning. Wind slabs will become more widespread and larger during the day as more snow accumulates. They could easily grow to 2 to 3 ft in depth in many areas with larger wind slabs in some areas. These wind slabs will form on top of weak old snow and potential buried persistent weak layers mentioned below. Human triggered wind slabs will be possible today and could become likely as more snow accumulates during the next 24 hours.  

  Newly formed wind slabs will exist near treeline and above treeline on NW-N-NE-E-SE aspects on slopes steeper than 35 degrees. Identify where wind slabs exist and use this information to avoid these fragile wind slabs. 

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2

  More snow load means that the persistent weak layers near the 12/15 rain crust should become easier to trigger today than they have been for the last 2 days. Human triggered persistent slab avalanches will be possible today. These avalanches could involve all of the snow down to the 12/15 rain crust. Recent observations have shown the persistent weak layers remain weakest along the Sierra Crest on the NW-N-NE aspects but they may still exist on the W and E aspects as well. Some persistent slab avalanches may also become possible in areas where the persistent weak layers are less well developed in places with deeper snowpacks at higher elevations like the Mt. Rose backcountry as new snowfall adds more load to the snowpack. Persistent slabs could exist on slopes 32 degrees and steeper. A great deal of variability exists concerning how much load these layers can support from slope to slope or even across the same slope.

  Traditional obvious clues may not provide reliable data for persistent slabs. While digging into the snowpack and snowpit tests can reveal whether or not the persistent weak layers exist, an avalanche can sometimes be the first clue that a slope is unstable. Persistent slabs can break after several people have already used slope when one person finds the right trigger point and propagate farther than expected. They can be triggered remotely or low on a slope and away from the more common ridgetop wind slab start zones. In summary, persistent slab avalanches require different terrain management techniques. Use conservative decision making and travel techniques like traveling on low-angle or well-anchored slopes or sticking to areas where the persistent weak layers do not exist.  

• 

  Loose Dry

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1

  The new snow that accumulates during this storm may not bond well to the current snow surfaces that consist of loose unconsolidated snow. Human triggered storm slab avalanches may be possible in some areas as more snow accumulates today. In some areas where the new snow does not have enough cohesion to behave like a slab layer, some loose dry sluffs may occur. The storm slab avalanches should only involve the new snow in areas where the persistent slabs mentioned above do not exist. In areas where the persistent slabs exist storm slab avalanches could step down and become persistent slab avalanches.