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

  1–2

  Unstable wind slabs were reported yesterday from the Carson Pass zone with a few small skier triggered avalanches. Slabs of wind drifted snow were deposited on top of weak faceted (sugary) snow in near treeline and above treeline terrain. These wind slabs are expected to continue to be reactive this morning. Following the onset of snowfall today, gale force SW ridgetop winds will drift more snow and deposit larger and more widespread wind slabs near treeline and above treeline on NW-N-NE-E aspects across the forecast area.

  Any slopes near ridgetops where blowing and drifting snow is depositing are suspect. Any slopes with newly formed cornices or drifted pillows of snow along the upper portion of the slope are likely to hold unstable wind slabs. Snow surface cracking is a sign of unstable snow in the immediate area. Use low angle or wind protected terrain to avoid the areas where wind drifted snow is depositing as a wind slab. Take care not to venture onto slopes with a persistent slabs problem while trying to avoid the wind slab problem.

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

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

  Probabilidad

  Likely

  Tamaño

  2

  The existing snowpack is in poor condition to handle new snow loading. The long dry spell from Nov 18 to Dec 11 caused the existing snowpack to weaken and turn sugary from the faceting process. This weak snow is widespread below treeline and near treeline on W-NW-N-NE-E aspects. It has been noted in isolated areas above treeline as well. The forecast snowfall amounts today and tonight are expected to bring this weak snow either very close or to the point of failure. Persistent slab avalanches will break in the snowpack below the old/new snow interface and could have wide propagation, wrapping around corners and other terrain features that often confine wind slabs and storm slabs. Persistent slab avalanches can be triggered in low angle terrain from a distance to the side, above, or from below (remote triggering).

  Staying on wind protected slopes in the trees is not a tactic for avoiding persistent slabs. It is a tactic for avoiding wind slabs. You have to route plan to manage both today. The situation is expected to suddenly become complex and difficult to manage as today's storm progresses. The easy answer is to pull back into low angle, conservative terrain that is less than 30 degrees in slope angle and without steeper slopes adjacent or above.