Central Sierra Nevada

Avalanche problems

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Very Likely

  Size

  3

  Strong winds and significant new snow accumulation over the last 24 to 72 hours have created large unstable wind slabs on near and above treeline slopes exposed to wind-loading. These wind slabs will continue to grow in size and become more unstable as more snow and wind impact the forecast area today. Large natural and human-triggered wind slab avalanches are very likely to certain today. Expect wind slabs on wind-loaded N-NE-E and cross-loaded NW and SE aspects. Some wind slabs may even form in more traditionally sheltered spots or extend into below treeline areas. Wind slab avalanches could entrain enough snow to bury cars (D3) or even trains (D4) or destroy a swath of forest.  

  Look for signs of recent wind slab avalanches, blowing snow, cornices, cornice formation, and wind pillows.  Avoid steep wind-loaded terrain and run out zones below corniced or wind-loaded slopes.

• 

  Loose Dry

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Very Likely

  Size

  2

  Rapid snow accumulation will overload weaknesses that have formed in the storm snow. Human-triggered storm slab avalanches are very likely today and natural storm slab avalanches are likely. Due to the amount of new snow storm slab avalanches could certainly involve enough snow to bury or injure a person (D2) and some of these storm slab avalanches could be large enough to bury a car (D3).

  Recent avalanche activity and shooting cracks, as well as hand pits to feel for weak layers and slab layers in the new snow, can help identify where storm slabs may exist. This avalanche problem could exist in areas traditionally thought of as safe like tree-covered slopes. Avoid slopes with signs of unstable snow.

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  4

  Most observations and data targetting the persistent weak layer responsible for the previous deep slab avalanche cycle have shown significant increases in that layer's strength and in the strength of the snow above that layer. However, a few pieces of data (some of which were collected just yesterday) still indicate that with enough new loading or a large enough trigger (like the other avalanches mentioned above) this layer could possibly reactivate. A significant amount of uncertainty exists as to whether or not this storm could provide enough loading to reactivate this layer or if the old persistent weak layer has gained enough strength to support the new load. In many areas, this layer can support the new load, but it is possible that in some places it cannot. If it does break the resulting deep slab avalanches would be huge, unsurvivable, and could run very long distances (size D4 to D5).

  Due to this combination of uncertainty, variability in the data, and the catastrophic consequences of a deep slab, avoiding slopes where even a remote possibility of a deep slab may exist is the best choice for today.