Central Sierra Nevada

Problemas de aludes

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta montaña
  • Límite del bosque
  • Bajo el límite del bosque

  Probabilidad

  Likely

  Tamaño

  2

  In many areas, significant snow remained on the ground and available for wind transport on windward aspects prior to last night's increase in wind speed. Using this snow on the ground, new wind slabs have already begun to build on NW-N-NE-E-SE aspects. Following the onset of snowfall, these wind slabs will continue to build in size and distribution throughout the day. Gale force winds have the ability to deposit wind slabs far down slope below traditional avalanche start zones as well as creating some areas of wind slab below treeline.

  Wind slabs tend to be more easily identifiable than other dry snow avalanche problems as they tend to form in open areas and are confined by terrain features. Cornice formations, pillows of drifted snow below ridgelines, pillows of drifted snow in lee areas behind rocks and trees, or entering areas of suddenly denser new snow are all indications of wind slabs in the immediate area. Avoiding these areas with clues of wind slab formation and/or traveling only in areas less than 30 degrees in slope angle without steeper terrain above are simple effective ways to avoid wind slab avalanches.

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta montaña
  • Límite del bosque
  • Bajo el límite del bosque

  Probabilidad

  Possible

  Tamaño

  3

  In some areas the existing snowpack is in good condition to handle new snow loading, in other areas it is not. Persistent weak layers in the form of near crust facets around the Dec 10 rain crust and old rounded facets at the bottom of the snowpack that remain loose grained have been observed in various locations around the forecast area. The presence or absence of this avalanche problem will vary widely from one portion of the forecast area to the next. Suspect areas are NW-N-NE aspects mainly near treeline and below treeline with more isolated potential above treeline.

  Failure of facets that are newly formed around the Dec 10 rain crust have the potential to propagate far and wide, wrapping around corners and across drainages. This is because a rain crust is much more uniformly created across the colder NW-N-NE aspects than other typical snowpack layers. Collapsing and whumpfing in low angle terrain are about the only indications of instability that will likely be observed prior to avalanche activity. In most cases it will require shovel, hands, and feet in the snow to deliberately seek out information indicating the presence or absence of problematic faceted weak layers in specific areas at slope scale.

  Surface hoar is rarely buried in this part of the world, but when it is it is a major avalanche problem. It frequently forms but is almost always destroyed prior to snowfall by warming air temperatures and strong to gale force winds. Widespread surface hoar was observed in numerous locations yesterday afternoon and observed to escape damage from warming air temperatures. Significant uncertainty exists as to whether or not surface hoar survived last night's winds. If it did, it will remain in large open areas below treeline on NW-N-NE aspects with a good open view of the sky and good shelter from strong to gale force SW winds. If in fact surface hoar survives intact and upright until burial today by rapidly accumulating new snow, significant avalanche problems will ensue. Isolated, localized areas of natural and human triggered avalanche activity on low angle slopes down into the 25 to 30 degree range may occur. Significant cracking, collapsing, and/or whumpfing will occur if this avalanche problem forms today. Quick hand pits are a good tool for searching for buried surface hoar at the old/new snow interface.