Central Sierra Nevada

Problemi valanghivi

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Likely

  Dimensione

  3

  Ridgetop winds continue to increase and transport snow with one natural wind slab avalanche large enough to bury or injure a person reported yesterday on Carpenter Ridge (Independence Lake area) on a N aspect in near treeline terrain at approximately 8,700'. As gale force SW winds combine with high intensity snowfall today, wind slabs will become larger and more widespread. The vast majority of wind slabs will form in near treeline and above treeline terrain on NW-N-NE-E-SE aspects, but winds will be strong enough to form wind slabs in some below treeline locations as well. Uses clues such as blowing snow, cornice formations, wind pillows, and cracking along ridgeline edges to determine where wind slabs exist. Unlike other avalanche problems, wind slabs are constrained by terrain and often have well defined visual boundaries, making them easier to avoid.

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Possible

  Dimensione

  3

  Rapid new snow loading over the next 24 hours will add significant stress to the existing snowpack and its associated weak layers. In areas along the Sierra Crest where a persistent weak layer of collapsed surface hoar capped by the Dec 13 rain crust exists, large destructive natural and human triggered avalanches could occur. This weak layer is now located about 1.5 to 2.5 feet below the snow surface and is expected be buried by an additional 1 to 3 feet of snow over the next 24 hours. In specific locations where this weak layer exists, snowpit tests have produced repeatable results indicating that propagation is likely to follow failure of this weak layer. This weak layer has been observed over the past week along the Sierra Crest in open areas both near treeline and below treeline on NW-N-NE aspects. This weak layer is known to exist in the Ebbetts Pass, Echo Summit, West Shore Tahoe, Ward Canyon, and Donner Summit areas and may exist in other locations as well.

  Conservative, lower angle terrain choices are the best way to mitigate this avalanche problem. Without performing numerous snowpits in the area of question to determine presence or absence of the weak layer, entering steep terrain will be a decision made without using any amount of sound decision making. It doesn't mater if you were in the same terrain yesterday or previously during large storms. This is a locally uncommon weak layer that could lead to some large destructive avalanches below treeline.

  There have been some indications of a problematic weak layer of near crust facets forming in the Mount Rose area around the Dec 10 rain crust both above and below treeline on NW-N-NE aspects. Potential exists for this weak layer to become overloaded during this storm cycle resulting in large destructive avalanches.

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Unlikely

  Dimensione

  2

  Snow levels are currently expected to hold in the 5,500' to 6,000' range. If this comes to fruition, loose wet avalanches will not be a problem. If snow levels rise to 7,000' or higher, widespread loose wet avalanche activity is expected to occur on slopes 35 degrees and steeper as rain is deposited on top of new snow on all aspects. Large roller balls and pinwheels in areas of rain on snow will be indications of this avalanche problem developing.