Central Sierra Nevada

Problemi valanghivi

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Likely

  Dimensione

  2–3

  Several large persistent slab avalanches occurred yesterday, including:

  • A remotely triggered a persistent slab in old-growth trees on the West Shore (Hidden Peak). The party stated, "[the avalanche] propagated so far I could not see the other side of the path." This slope had seven previous tracks on it.
  • Another persistent slab on Hidden Peak (West Shore) in the trees at 7500 ft.
  • Two persistent slabs on Jakes Peak (West Shore), one of which was likely remotely triggered.
  • A group of snowmobilers reported seeing three very large, recent natural avalanches on N-NE aspects in the Castle Peak area (Donner Pass). 
  • Widespread whumpfing and collapsing from the West Shore, Trimmer Peak (South Lake Tahoe), Blue Lakes (Carson Pass), and more isolated snowpack collapses on Castle Peak (Donner Pass). 

  The persistent weak layers like surface hoar (weak feathery snow crystals) and facets (weak sugary snow) have not adjusted to the heavy storm snow above them. Triggering large persistent slab avalanches will remain likely today.

  People can trigger persistent slab avalanches from a distance while traveling below or next to a slope in lower-angle terrain. They also may not release until a person has committed to a slope or after several people have already traveled on the slope. These avalanches can break much wider than expected and may occur in sheltered, tree-covered terrain that people typically consider "safe." The persistent weak layers may also lurk in more exposed terrain where wind-blown snow has created a much thicker slab. Expect large to very large avalanches several feet deep when these old, weak layers of snow fail. 

  Uncertainty does exist concerning this problem, and many people may not see any evidence of a persistent slab. Unfortunately, a large avalanche on a seemingly stable slope might be the first and only clue you get. Stepping back from terrain where the problem might exist or using lower angle slopes not connected to steeper terrain represents the most reliable way to reduce exposure and the potential consequences of an unexpected, persistent slab avalanche.

   

  

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Possible

  Dimensione

  1–2

  Some wind slabs may linger where wind-blown snow has filled in areas near ridges, in gully features, and on open slopes. Large, fragile cornices have also formed above many wind-loaded slopes. The NE winds may stiffen or scour the snow in some of these areas, but wind slab avalanches may remain possible, especially in complex or heavily wind-loaded terrain. The possibility also exists that these could step down and fail on the persistent weak layers mentioned above.