Central Sierra Nevada

Problèmes avalancheux

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Possible

  Taille

  2

  The wind slabs that formed yesterday will remain on the near and above treeline wind loaded N-NE-E aspects and some of the cross-loaded NW and SE aspects. These wind slabs should have become slightly more difficult to trigger today due to the end of new precipitation, decreasing winds, and cooling temperatures. However, human-triggered wind slab avalanches will remain possible where wind slabs exist. The largest and most fragile wind slabs will exist above yesterday's snow line where most of the precipitation fell as snow like the Mt. Rose area. In some of these areas the wind slabs could measure several feet in depth. In areas where most of the precipitation fell as rain, wind slabs should remain smaller and more isolated. In the event that a wind slab avalanche occurs it could step down to the persistent weak layer mentioned below especially on upper elevation north aspects where the PWL is most likely to exist. 

  Clues like cornices, blowing snow, wind drifted snow, wind pillows, snow surface scouring, ripples, and other wind created textures can help determine where wind slabs may exist.

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Possible

  Taille

  2

  Like the wind slabs mentioned above, persistent slab avalanches should have become more difficult to trigger today, but human-triggering of these large destructive avalanches will remain possible. Any persistent slab avalanches that occur would have very serious consequences and would likely be unsurvivable. The persistent weak layer now has anywhere from 2 to 4 ft. of snow on top of it that can act as a slab layer. In some areas where the warm temperatures and rain may have penetrated to this layer, the PWL will gain strength once the snowpack refreezes. That refreeze will take a few days and the PWL could remain weak during that time period. Data and observations have shown this layer to be most prevalent on N aspects above 9000 ft. in the Mt. Rose area and above 8300 ft. along the Sierra Crest with some isolated observations showing it on a few NE aspects as well. This layer could have changed dramatically during the storm and a significant amount of uncertainty exists as to where it may have gained enough strength to support the snow above it and where it will remain weak.

  Due to the consequences associated with the large destructive avalanches that could occur if the persistent weak layer fails and the high degree of uncertainty associated with it, avoiding areas where this layer might exist is recommended. Larger triggers like multiple people on a slope, other avalanches, or cornice failures could easily cause a failure on the persistent weak layer. Areas where a shallower snowpack exists or where trigger points are closer to the snow surface are especially suspect.  These avalanches could connect multiple start zones, could be triggered remotely, and could run farther than expected.  Avoid North aspects above 32 degrees in steepness.  Have a travel plan.  Pay close attention to run out zones, up tracks, and regrouping areas.