Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Fragile slabs of wind drifted snow existed on slopes loaded by the SW winds yesterday, and people triggered numerous wind slab avalanches (see discussion below). The winds shifted to the NE during the night and moved snow to the other side of the ridges. New wind slabs will continue to form today as the winds continue to blow snow around. New and previously formed wind slabs could exist on any aspect today in near and above treeline terrain. Human-triggered wind slab avalanches will be possible.

  Blowing snow, cornices above a slope, wind pillows/drifts, ripples in the snow surface, and other wind created textures can all help identify where wind slabs may exist. Paying attention to the wind can also help determine which slopes will be scoured and firm. Traveling in more sheltered terrain could provide better recreation conditions and help avoid the wind slab problem, but you will still need to assess the potential for the deep persistent slabs mentioned below. 

  

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2–3

  Human triggered deep persistent slab avalanches still remain possible on NW-N-NE aspects at all elevations. These avalanches could fail on deeply buried weak layers within the snowpack resulting in large and destructive avalanches. An avalanche associated with this problem was triggered on Sunday afternoon on Waterhouse Peak in the Luther Pass area. Another potential one was reported yesterday from near Mt. Watson (S of Northstar), but when this avalanche occurred remains unknown. Most of the areas of concern for this deep slab avalanche problem exist on the Eastern side of the forecast region where a weak and shallow snowpack existed before the recent storms. This includes portions of the Carson Range on the NE, E, and SE sides of Lake Tahoe and locations east of the Sierra Crest in the Luther Pass, Carson Pass, and Ebbetts Pass areas.

  Deep slabs represent a very unpredictable and difficult to assess avalanche problem. Sometimes digging deep holes or probing can help determine whether or not the buried weak layer exists, but in many cases triggering a large deep slab avalanche is the first and only sign of instability. Areas that had a shallow and weak snowpack structure before the last major storm are most suspect for this deep slab avalanche problem to occur. Until this avalanche problem goes away, good terrain management skills are critical to reducing exposure. Utilize terrain under 30 degrees in steepness and areas away from steeper connected terrain. Avoid complex terrain with multiple trigger points and unsupported slopes.