Central Sierra Nevada

Avalanche problems

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2

  A buried layer of weak sugary snow exists on most near and below treeline W-NW-N-NE-E aspects. Observers have even found it in a few isolated areas above treeline. Yesterday brought more reports of collapsing, whumfing, and unstable test results on this persistent weak layer (named the Dec. 11 facet layer). Some data indicating that this layer has started to adjust to the current load was mixed in with these signs of instability, and no one reported new avalanche activity on this layer. While these signs are encouraging, triggering persistent slab avalanches remains possible. All the right ingredients exist: the foot + of recent snow represents the slab. The Dec 11 facets represent the weak layer. A person recreating on the snowpack could provide the trigger. These avalanches could break partway down a slope and propagate farther than expected. Failures in the facet layer could travel across terrain allowing remote triggering from below, above, to the side, or from any connected terrain. These avalanches would also have serious consequences as many obstacles still exist in the early season snowpack.

  Persistent slabs represent one of the most challenging avalanche problems to manage. Sometimes cracking and whumpfing or snowpit tests can provide clues to instability; sometimes triggering a large avalanche is the only clue to instability you see. Avoiding travel in avalanche terrain where persistent slabs might lurk remains the best strategy for dealing with persistent slabs. Terrain less steep than 30 degrees that is not connected to steeper slopes offers fun snow and provides a much better safety margin.

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1

  The NE winds have redistributed snow across the ridges scouring previously loaded slopes and forming small slabs of wind drifted snow on new slopes. Wind slabs may exist on almost any aspect since the previous wind slabs may still linger on some slopes. Triggering wind slab avalanches remains a possibility today. Most of these should remain small unless they rest on the persistent weak layer mentioned above. In these cases, larger avalanches that could propagate much farther than expected and be remotely triggered could occur. 

  Cornices, wind pillows, and wind created surface textures can all help to identify which slopes may hold wind slabs. Moving into more sheltered terrain or traveling on scoured slopes provides a haven from wind slabs. Unfortunately, scoured slopes are firm and uneven, and the more problematic persistent slabs mentioned above live in the sheltered terrain. The best travel advice is the same as above: stick to slopes less steep than 30 degrees that are not connected to steeper slopes for fun snow and better safety margins.