Central Sierra Nevada

Problèmes avalancheux

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Possible

  Taille

  1–2

  Wind slabs still exist on slopes where wind-blown snow piled up over the last few days. Some of these wind slabs may remain unstable, especially if they exist on top of old, weak snow. While the NW-N-NE-E-SE aspects should still hold more widespread wind slabs, some wind slabs could also exist on other aspects due to yesterday's NE winds. These winds may have helped make existing wind slabs more dense as well. Expect a mix of wind-loaded and wind-scoured conditions in near and above treeline terrain.

  Cornices above a slope, wind drifts, wind-sculpted surface snow, and wind-scoured areas can all help identify where potentially unstable wind slabs may exist. Using this information to plan a route around the wind slabs can help limit your exposure and may lead to more consistent snow conditions. 

  

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Possible

  Taille

  1–2

  Old, weak, sugary snow (facets) exists below the recent snow on many NW-N-NE aspects. The exact structure and weakness of the snowpack varies widely by location and elevation. Wind-affected snow or consolidated new snow could provide enough of a slab to complete the ingredients for a persistent slab avalanche. People could still trigger persistent slab avalanches that fail on any of the numerous weak layers today, especially in areas where more cohesive snow rests on the weak layer. 

  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 clue that a slope remains unstable. 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. If a persistent slab avalanche does occur, it could break wider than expected or be triggered low on the slope.

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