Central Sierra Nevada

Avalanche problems

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  2–3

  Weak faceted snow buried 1 to 2+ feet below the surface exists at all elevations, with the weakest snow on NW-N-NE-E aspects. Some weak snow is also buried on the southerly aspects in isolated areas. This snowpack structure has created a widespread persistent slab avalanche problem. People triggered several persistent slab avalanches yesterday and caused whumpfing almost everywhere they went. Additional rain and snow will make it even easier to trigger persistent slab avalanches today. Large human-triggered avalanches remain LIKELY today, and some natural avalanches may be possible. Once a fracture starts in the facet layer, it can propagate long distances like a falling row of dominos. Avalanches can break wider than expected, wrap around terrain features, and travel farther downhill than usual. People can also trigger these avalanches remotely from nearby slopes or flat areas at the bottom of a hill. In areas where rain falls on the snow, these avalanches could also have some wet snow characteristics. 

  Shooting cracks, whumpfing, and collapsing represent clear signs of instability. Digging into the snow and probing can help identify the buried weak layer. Increase safety margins by steering clear of any terrain steeper than 30° or terrain connected to/below steep terrain if you see signs of instability or feel weak snow. If there is any uncertainty about whether or not the weak layer exists, play it safe and assume the snowpack is unstable. 

  

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Likely

  Size

  2–3

  New snow and wind today will create new wind slabs on top of old wind slabs that have not fully bonded to the snowpack below them. In places where the persistent weak layer mentioned above exists below the wind slabs, they would behave like deeper versions of the persistent slab problem. In other areas where the snow surface was scoured and firm or bare ground existed before the last storm, these wind slabs will behave more like typical wind slabs. Large human-triggered wind slab avalanches remain likely today. They could run longer than usual or trigger additional avalanches as they move downhill. The largest and most unstable wind slabs will exist on NW-N-NE-E-SE aspects. 

  Use clues like blowing snow, cornices above a slope, drifted snow, or other wind-created surface textures to help evaluate which slopes hold wind slabs. Choose travel routes that avoid these wind-loaded areas. Cautious route-finding focusing on lower-angle terrain not connected to steeper terrain can help avoid both avalanche problems today.