Central Sierra Nevada

Lawinenprobleme

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  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Likely

  Größe

  2

  Persistent slabs are back. Prior to this storm, it took several days for the persistent weak layers buried in the snowpack to adjust to the snow above them. Old weak sugary snow (the Dec 11 facets) existed about a foot below the surface on most near and below treeline W-NW-N-NE-E aspects. Observers have even found it in a few isolated areas above treeline. Observers have also reported another thin layer of weak snow closer to the surface. Now 6  to 10 inches of new, dense, heavy snow has accumulated on top of the snowpack in less than 12 hours. This new load will likely push the old weak snow to the breaking point again. Adding a person on top of that new snow would likely provide enough of a trigger to cause a persistent slab avalanche today. These avalanches could break partway down a slope, propagate farther than expected, and people could trigger them remotely from less steep terrain. These avalanches could also have serious consequences as many obstacles still exist in the shallow 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. The best travel advice for this kind of avalanche problem: stick to slopes less steep than 30 degrees that are not connected to steeper slopes for fun snow and better safety margins.

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  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Likely

  Größe

  1

  The strong winds will move the new snow around and pile it up on the leeward slopes. These new slabs of wind drifted snow will form on top of a variety of old snow surfaces including firm wind-scoured and wind-packed snow. The new wind slabs will be fragile today since they have just formed and have not had time to bond to the existing snow surfaces. In some cases, these wind slabs may form on slopes where the persistent weak layers mentioned above exist. Human-triggered wind slab avalanches will be likely on the most heavily wind-loaded slopes. Larger avalanches that could propagate much farther than expected and be remotely triggered could occur where the wind slabs exist above the persistent weak layers. 

  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. Avoiding these fragile wind slabs only represents part of the strategy today since the persistent slabs mentioned above are likely to exist in many of the more sheltered terrain. Managing multiple avalanche problems at all elevations will be required today.