Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1.5–2.5

  SW winds will pile new snow onto leeward-facing slopes near ridges, open wind-exposed slopes, and gullies. New wind slabs will form faster than they can bond to the firm snow surfaces below. They may start small, but they will quickly grow in size and extent as new snow accumulates. People will likely trigger wind slab avalanches on steep slopes where wind-blown snow piles up, and natural avalanches may be possible in areas with intense wind loading. The most fragile wind slabs will form on near and above treeline NW-N-NE-E-SE aspects. New fragile cornices will also form along many ridges. 

  Conditions will change fast today and the same slope that did not show signs of instability early in the day could produce a wind slab avalanche large enough to cause problems a short while later. Recent avalanches, blowing snow, cracking in wind-blown snow, and new cornices indicate the presence of unstable wind slabs.  Blowing snow will also decrease visibility and make identifying avalanche terrain more difficult today. Traveling in terrain sheltered from the winds can help avoid the wind slabs and should also provide more consistent snow quality.

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  Rising snow levels and rapid accumulation will allow storm slab avalanches to become likely today. In addition to the weaknesses that form within the storm snow, the new snow may have trouble bonding to the existing snow surfaces, especially if the new snow accumulates on top of yesterday's surface hoar. Like the wind slabs mentioned above, these storm slabs should start out small and quickly increase in size and distribution as the storm intensifies. By midday, unstable storm slabs could exist on any steep slope including below treeline slopes and other typically sheltered areas.

  Look for cracking in the storm snow and feel for different layers of snow as you travel through the terrain.  If you see any signs of storm snow weakness or doubt a slope's stability, move to lower-angle, non-avalanche terrain. 

• 

  Loose Wet

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  1–2

  As snow levels rise this afternoon, rain will fall on fresh snow and cause wet loose avalanche activity on steep slopes. Many of these wet snow instabilities would remain small, but they could still cause problems, and terrain traps like gullies, cliffs, or creeks would amplify the potential consequences. Longer slopes could produce larger wet loose avalanches.