Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Strong SW winds will pile wind-blown snow onto the NW-N-NE-E-SE aspects today creating slabs of wind drifted snow much deeper than the new snow accumulations in sheltered areas. Wind slabs will start out small this morning, but they will grow quickly as new snow falls. These wind slabs will form fast and may not have time to bond to the snow below them. Human-triggered wind slab avalanches are likely today. Open slopes near ridges will hold the largest wind slabs. Wind slab avalanches could involve enough snow to bury or injure a person. 

  Watch for signs of wind slab formation like blowing snow, newly formed cornices, piles of wind drifted snow and wind-created surface textures. Recent avalanches and cracking in the wind-blown snow are sure signs that the wind slabs are unstable. 

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Rain and wet heavy snow means that wet snow will exist on all aspects today. Expect sluffs of wet snow on steep slopes. Wet loose avalanches gather more snow as they move downhill, so long steep slopes can produce larger wet loose avalanches. Terrain traps like gullies, cliffs, and creeks can magnify the consequences of these avalanches. Most of the wet loose problems should occur near or below the snow level, but some wet snow instability could also exist at higher elevations where the precipitation falls as heavy wet snow. 

  Rain, rollerballs, pinwheels, or other signs of wet snow all indicate that the wet snow could be unstable. Traveling on lower-angle terrain that is not below avalanche terrain represents a good strategy to avoid wet snow problems. 

• 

  Wet Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1–2

  Rain falling on the snow will weaken the snow and add weight to the snowpack. In areas where weak snow remains buried below the surface, the rain could weaken these old facet layers enough for them to fail. Some wet slab avalanches may be possible today on NW-N-NE aspects where these buried weak layers exist. These avalanches could involve enough snow to bury or injure a person and could propagate farther across the slope than expected. 

  Uncertainty exists concerning this problem. Cracking in the snow or recent avalanches are sure signs that the snowpack is unstable, but the first sign of this problem could be triggering an avalanche. One way to mitigate the uncertainty is to avoid traveling on slopes where the old weak layer may exist.