Central Sierra Nevada

Problèmes avalancheux

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Likely

  Taille

  2

  Every day this week, backcountry travelers have reported collapsing, whumfing, and unstable test results on the weak sugary snow buried on Dec. 11. New snow accumulating across the region will add more weight on top of this persistent weak layer. Since it had only just begun to adjust to the previous storm snow, this new load will likely overload the weak layer again. This weak buried facet layer exists on most near and below treeline W-NW-N-NE-E aspects. Observers have even found it in a few isolated areas above treeline. The additional weight of a person on the snowpack would likely provide enough of a trigger to break this layer and cause an avalanche today. These avalanches could break partway down a slope, propagate farther than expected, and people could trigger them remotely from below, above, to the side, or from any connected terrain. These avalanches would also have serious consequences as many obstacles still exist in the early season snowpack.

  Persistent slabs represent one of the most difficult avalanche problems to manage. Sometimes cracking and whumpfing can provide clues to instability; sometimes triggering a large avalanche is the only clue to instability you see. Avoiding travel in avalanche terrain where persistent slabs might lurk is the best strategy for dealing with persistent slabs. Terrain less steep than 30 degrees that is not connected to steeper slopes can still provide enjoyable recreation. 

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Likely

  Taille

  1

  More snow and wind today will form slabs of drifted snow on leeward aspects in exposed areas. These wind slabs will be fragile today and human triggered wind slab avalanches will be likely on the most heavily wind-loaded slopes. In some areas, these wind slabs could build on slopes where the persistent weak layer mentioned above exists. In these cases, larger avalanches that could propagate much farther than expected and be remotely triggered could occur. Cornices that form along ridgelines could also break easily and provide triggers for avalanches on the slopes below. 

  Newly developed cornices, wind pillows, and wind created surface textures can all help to identify which slopes may hold wind slabs. Signs of wind slabs instability include recent avalanches, cracking, or signs of denser snow over less dense snow. Managing multiple avalanche problems at all elevations will be required today.