Central Sierra Nevada

Problèmes avalancheux

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Possible

  Taille

  1–2

  Yesterday, a group skinning near a N-facing slope remotely triggered shooting cracks across the slope. They heard a loud sound like "thunder" as the old, weak snow collapsed (click here for the observation). Old, weak layers of rotten snow like that one exist below the recent snow on many NW-N-NE aspects. While the exact structure and weakness of the snowpack varies widely by location and elevation, the snowpack generally gets weaker as the elevation increases. People could still trigger persistent slab avalanches that fail on old, rotten snow today, especially in areas where wind-affected snow or more consolidated recent snow rests on the weak layer. If a persistent slab avalanche does occur, people could trigger it from a distance or low on the slope, and it could break wider than expected.

  Sometimes, clues like recent avalanches, snow surface cracking, snowpack collapse, or whumpfing sounds help alert people to unstable, persistent slabs. Other times, triggering an avalanche represents the first sign of an unstable slope. Some previously stable slopes may become unstable as the new snow consolidates above the still-weak old snow. Remember that previous NE wind events may have scoured weak snow out of the upper portion of a slope, laying a trap of weak snow on the lower half of the slope that is just waiting for a trigger.

  Use low-angle, non-avalanche terrain not connected to steeper areas above or beside it if you see signs of instability or feel weak snow below the surface. If you are uncertain whether the weak layer exists, play it safe and assume the snowpack is unstable, even if there are tracks on the slope. As an added benefit, lower angle slopes make controlling your speed and avoiding the barely covered obstacles easier.