Central Sierra Nevada

Avalanche problems

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  3

  Data and observations keep finding layers of surface hoar under a 2-3 ft. slab of consolidated snow in some open areas on near and below treeline N-NE-E aspects. This layer could still exist on some isolated NW and SE aspects as well. Triggering an avalanche on these buried surface hoar layers has become more difficult, but human triggered persistent slab avalanches that fail on a buried surface hoar layer remain possible on some slopes. As more snow accumulates across the forecast area over the next 36 hours this persistent slab problem should become larger and easier to trigger due to the added weight of the new snow. For today larger triggers like large cornice collapses or multiple people on a slope or smaller triggers on the right trigger point such as an area where a shallower snowpack exists or in areas near rocks have better chances of triggering a persistent slab avalanche. If any persistent slab avalanches do occur, they would be large and have severe consequences. Avalanches that fail on buried surface hoar can propagate long distances, can occur in places traditionally considered safe, and can be triggered from lower angle terrain.

  Due to the consequences of these types of avalanches, avoiding steep slopes where reactive surface hoar still exists represents the best way to manage this avalanche problem. Unfortunately, determining where the buried surface hoar still exists and where it is still reactive is difficult. Recent avalanche activity, shooting cracks, collapsing, whumpfing, and snowpit tests can provide some clues. However, a persistent slab triggered by a party on the slope sometimes represents the first and only clue to avalanche instability. 

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2

  The wind slabs that existed on wind loaded slopes yesterday have become increasingly difficult to trigger. However, some human triggered wind slabs may still be possible today on wind loaded near and above treeeline NW-N-NE-E-SE aspects in couliors, gullies, on hanging snowfields, or in other complex or extreme terrain. New human triggerable wind slabs will also start to form today on wind loaded W-NW-N-NE-E aspects in near and above treeline terrain. These new wind slabs should remain shallow and not extend very far down slope since the forecast only calls for 3 inches of new snow today. These new wind slabs will become larger, more widespread, and easier to trigger during the night and tomorrow as more wind and snow arrive across the forecast area. Some natural avalanche activity involving these new wind slabs could start to occur tonight.