Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1

  Ridgetop winds increased yesterday afternoon out of the SW and drifting snow was observed near and above treeline. Plenty of recent storm snow was available on the ground for wind transport. New snow last night combined with SW winds will have further wind loaded NW-N-NE-E-SE aspects in near treeline and above treeline areas. New wind slabs are expected in these areas. Ridgetop winds are expected to shift to the W Wednesday afternoon and then back to the SW on Thanksgiving Day. Ridgetop wind speeds are expected to remain moderate to strong in speed and capable of moving snow. This avalanche problem could exist on slopes 35 degrees and steeper.

  Avoid areas of recently drifted snow by identifying cornice features and wind pillows. Avoid venturing into steep terrain in areas where human triggered snow surface cracking is observed around the edges of wind slabs.

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2

  In the northern portion of the forecast area, snowpit data points to a persistent slab avalanche problem. Along the Sierra Crest from about Tahoe City northwards, above 8,300' and in the Mount Rose area above 9,000' to 9,300', evidence of an unstable snowpack exists on N aspects. Snow that was deposited at the very end of October and during the first half of November has become faceted and weak. In areas along the northern Sierra Crest below 9,000' this faceted snow is capped by a problematic rain crust from Nov 16. This weak layer of faceted snow is sandwiched between 1 to 3 feet of recent storm snow on top and basal ice on the bottom. Where this weak layer exists it crosses over sub ridges, spines, and other features connecting multiple avalanche paths and start zones together. Given the persistent nature and depth of the weak layer, any avalanches that occur could be wide, large, and destructive. This avalanche problem could exist on slopes 32 degrees and steeper.

  Here is the rub. This high consequence avalanche problem exists in the areas that have the best snow coverage in the forecast area. Due to the depth of the weak layer, informal observations are unlikely to highlight instability until an avalanche occurs. Previous tracks are neither a sign of stability or intelligent life. Either get out the shovel and perform snowpit work to determine if this avalanche problem/weak layer exists in the particular area of interest or go the easy route and avoid N aspects that meet the location and elevation criteria for presence of this avalanche problem. Keep in mind that this is the type of avalanche problem that could be triggered from low on the slope or even remotely from low angle connected terrain.

• 

  Loose Dry

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1

  While new snow amounts Tuesday night into Wednesday morning were fairly modest at 2 to 6 inches, several reports were received of surface hoar (feathery surface snow) persisting well into the Tuesday afternoon hours in wind sheltered areas on all aspects near and below treeline. Air temperatures in many areas above 7,500' on Tuesday were insufficient to destroy surface hoar and SW winds did not increase to the strong range until around sunset. This leaves a fair bit of uncertainty as to if any areas of surface hoar persisted in an upright position and were buried under the new snow. While any associated avalanches will be small, they would be very easily triggered and could have consequences if a person were caught in an avalanche in a terrain trap such as the sidewall of a large creek bed. This avalanche problem could exist on slopes 30 degrees and steeper.