Central Sierra Nevada

Problèmes avalancheux

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Likely

  Taille

  2

  While winds decreased some yesterday, they increased again overnight. They have also shifted directions from SW to more E and back to SW. These winds have remained strong enough to transport snow in many places and fresh slabs of wind-drifted snow likely exist on top of other wind slabs formed earlier in the storm. The largest of these wind slabs should exist on wind-loaded N-NE-E aspects and cross-loaded NW and SE aspects, but wind slabs could exist on any aspect due to the shifting winds. These wind slabs will remain fragile today. Large human-triggered wind slab avalanches that could bury a person or a car (D2-D3) remain likely. 

  Identify cornices and pillows of wind drifted snow. Wind slabs are on the slopes below these features. Route find around these areas and do not linger below them. Avoidance is a better tactic today than trying to determine if wind slabs are stable or unstable.

   

• 

  Loose Dry

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Likely

  Taille

  1

  Changes during the storm like fluctuating temperatures and snowfall rates have created weaker layers under more slab-like layers within the storm snow. These weaknesses will likely remain sensitive to the additional weight of a person on top of the snowpack today. Human-triggered storm slab avalanches are still likely on steep slopes in sheltered areas. These avalanches could easily involve enough snow to bury a person. 

  Use snow surface cracking triggered while breaking trail as a clue to the presence of this avalanche problem. Specifically choose terrain and routes that give you options for avoiding avalanche terrain if storm slab instability is present.

• 

  Storm Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpin
  • Limite forestière
  • Sous la limite forestière

  Probabilité

  Possible

  Taille

  2

  Prior to this storm weak sugary snow existed just below the snow surface on many near and below treeline NW-N-NE aspects. The storm has deposited 3 to 5+ feet of new snow on top of this old weak snow. The old weak snow may not be able to support this sudden large load of new snow resulting in the possibility of large deep slab avalanches. While they may be more difficult to trigger than the avalanche problems mentioned above,  deep slab avalanches would involve all of the accumulated new snow, plus the additional depth of wind drifted snow if occurring near treeline. Very large avalanches may occur in below treeline areas on slopes without a history frequent avalanches.

  Becoming caught in a deep slab avalanche near or below treeline may not be survivable. If you hear a whumph or experience snowpack collapse, unstable snow associated with this problem is present in the immediate area and an avalanche would have just occurred if the slope were steeper. Practical ways to avoid this avalanche problem are aspect avoidance or conservative terrain selection.