Central Sierra Nevada

Avalanche problems

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  2–3

  Buried surface hoar, near crust facets, and older facet layers all existed in the snowpack before this storm. The new load likely already broke these layers in some areas and those slopes have since refilled. In other areas, this new load has brought these weak layers to the breaking point, and adding a person to the top of the snowpack would likely trigger a large persistent slab avalanche today. These kinds of avalanches often propagate farther and wider across slopes and can be triggered remotely. Some of them may even break down on the Dec 11 facet layer and involve almost the entire snowpack, especially where a shallow and weak snowpack previously existed like in the Carson and Ebbetts Pass areas. 

  Persistent slab avalanches could run long distances and be very large and deep. Signs like recent avalanche activity, shooting cracks, or whumpfing can sometimes indicate areas of instability, but sometimes the first sign of instability is triggering a large avalanche. The best plan to avoid this problem is to stick to slopes without the weak layers (hard to know which slopes these are) or to stick to slopes less steep than 30 degrees that are not connected to steeper slopes (a much more certain and easier to accomplish plan). 

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  2–3

  Gale force winds shifting between the S and SW combined with several feet of new snow have left deep slabs of drifted snow on leeward aspects. Due to the shifting winds, wind slabs may exist in some unusual places in addition to the more traditional locations. Many wind-loaded slopes likely avalanched during the storm, have since refilled, and are ready to slide again. While the best window for natural wind slab avalanches occurred during the more intense periods of the storm, large human-triggered wind slab avalanches will be likely today if people chose to venture onto wind loaded slopes. Large fragile cornices also exist above many slopes.

  Look for where wind slabs exist using clues like drifted snow, cornices above a slope, and wind created surface textures. These wind slabs will be fragile today and avoiding them will be the best strategy. Travel in more sheltered terrain will help avoid the wind slabs but will not avoid all the avalanche problems since the persistent slabs and storm slabs could still exist in those areas. 

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  2

  Warming temperatures, fluctuating snowfall rates, shifting winds, and other changing conditions during the storm have created weaknesses within the storm snow. While these weaknesses tend to consolidate faster than other weak layers, storm slabs remain a problem. A person recreating on top of the snow can trigger avalanches involving slabs of new snow that fail on the storm snow weaknesses today. These storm slab avalanches could involve part or all of the storm snow. Storm slab avalanche problems could exist on any steep slope on any aspect. In areas where the storm snow lacks cohesion loose dry sluffs could continue to occur today. 

  Recent avalanche activity, shooting cracks, and feeling light snow under more dense snow can all help identify where storm slabs may be more prevalent. Due to the combination of all the avalanche problems and the difficulty in telling where a storm slab or a persistent slab may release, avoiding steep slopes still represents the best travel advice today.