Central Sierra Nevada

Lawinenprobleme

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  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Likely

  Größe

  1.5–3

  New snow combined with intense gale-force winds have built large slabs of wind-blown snow on the leeward sides of ridges, gullies, and open slopes. The largest wind slabs exist in near and above treeline areas that have filled in with wind-drifted snow. Still, open areas below treeline may also hold some wind slabs due to the strength of the winds. These wind slabs will continue growing as strong winds keep moving snow around today. Due to the changing conditions during the storm, these dense, heavy wind slabs could fail on weaknesses within the storm snow. Large and fragile cornices loom over many wind-loaded slopes. In some cases, large cornice collapses or wind slab avalanches may have enough weight to break old, weak layers several feet down, resulting in very large avalanches that could break wider and deeper than expected.

  Wind slab avalanches remain likely today. Look out for places where blowing snow has filled in a slope. Ripples on the snow surface, cornices above a slope, drifted snow, and other wind-created textures can also help identify where wind slabs have formed. Due to the slightly different wind direction (S and SW) and the strength of the wind, these wind slabs may exist in some unusual places. Recent avalanche activity and cracking in the snow represent obvious clues that unstable wind slabs lurk nearby. Travel plans prioritizing lower-angle, lower-consequence terrain that gives steep wind-loaded slopes, areas below large cornices, and avalanche run-out zones a wide berth make sense today. 

  

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  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Hochgebirge
  • Waldgrenze
  • Unterhalb der Waldgrenze

  Wahrscheinlichkeit

  Possible

  Größe

  1.5–3

  Old, weak snow exists below several feet of storm snow in parts of the forecast area. Signs that these weak layers have reached their tipping point over the last few days include long shooting cracks with remotely triggered cornice collapses, a large snowmobile-triggered collapse/whumpf in which the snowpack dropped 1/4-1/2 an inch, and numerous unstable snowpit test results. So far these signs of instability have been most prevalent in the eastern and southern parts of the forecast area on NW-N-NE-E facing slopes at elevations above about 8000 ft. While we have not had an avalanche reported on these layers yet, the evidence suggests that these persistent slab avalanches are possible. In addition to the persistent slabs, changing conditions during the storm may allow some weaknesses within the storm snow that could produce smaller slab avalanches in similar areas. 

  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. Due to this uncertainty and lack of feedback, dealing with persistent slabs is a challenge even for professionals. Stepping back from terrain where the problem might exist represents the most reliable way to reduce the potential consequences of an unexpected, persistent slab avalanche.