Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  3

  Intense snowfall and strong winds will build wind slabs on the NW-N-NE-E-SE aspects faster than those new wind slabs can form bonds within themselves or with the old snow surfaces. Human triggering of these wind slabs will be likely today and natural avalanches involving these wind slabs will be possible especially during the periods of heaviest snowfall this morning and tonight. Wind slabs will accumulate on wind loaded and cross loaded slopes and clues like blowing snow, cornices above a slope, pillows of drifted snow, and other wind created textures can help identify where these fragile wind slabs exist. While open near and above treeline terrain will represent the most common locations for new wind slabs, they could also form in some more traditionally sheltered areas such as open slopes in below treeline terrain due to the strength of the winds at all elevations during this storm. Wind slabs could easily grow to more than 3 ft in depth today as new snow and wind combine to multiply the accumulation amounts. If these wind slabs release on slopes where a persistent weak layer exists in the the old snowpack, they could overload that deeper layer and involve even more snow. 

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2

  Persistent slabs may form on NW-N-NE aspects today as the new snow accumulates above a snowpack with weak layers of sugary snow in the bottom half of the snowpack. Data indicates that these facet layers remain weak. The additional load of new snow may push them so close to the breaking point that human triggered avalanches resulting from the failure of these persistent weak layers will become possible today. In many areas the persistent slabs remain confined by anchors poking through the buried weak layers, so these avalanches may not fracture over large areas. However, these persistent slab avalanches could fracture across wider areas and involve much more snow on slopes with smooth ground cover. Cracking, collapsing, whumphing, and recent avalanche activity can all provide clues as to where these persistent slabs may be an issue, but they could still occur on slopes without those clues. Digging into the snowpack to find this persistent weak layer and avoiding slopes where it exists, traveling on lower angle terrain, or traveling on well anchored terrain represents the best way to avoid this avalanche problem.