Central Sierra Nevada

Avalanche problems

• 

  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  2

  Over the last few days, data has indicated that the Dec 11 facets have started to adjust to the load above them and that the likelihood of triggering an avalanche has decreased. The 1 to 3 inches of snow that fell overnight on 12/30 should not provide enough new weight to overload this weak layer in most places. That being said, this layer is not strong by any measure. Persistent slab avalanches will remain possible, they just may not be as widespread. Signs of instability will become more sporadic with whumpfing, cracking, and unstable snowpack test results in some areas and few or no signs of instabilities in other areas. In some cases, a large avalanche may be the first and only clue that the slope is unstable. Persistent slabs behave in unpredictable ways. Wide propagation, large avalanches, mid-slope avalanches, and avalanches on slopes that already have tracks could occur. 

  Terrain choices represent a great way to manage persistent slab problems. Travel plans that target more conservative terrain or terrain without the weak layer can provide fun recreation opportunities with significantly higher safety margins. In many cases, the snow conditions on terrain less steep than 30 degrees are better than on the steeper terrain right now. Be aware of connected terrain above, below, or to the side of steeper slopes since remote triggering remains possible. Be patient, the bigger, steeper terrain will still be here after the persistent slab problem goes away. After all, it is a persistent slab problem, not a forever slab problem.

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alpine
  • Treeline
  • Below treeline

  Likelihood

  Possible

  Size

  1

  Winds shifted back to the SW on 12/30 and remained strong during the night. While 1 to 3 inches of new snow may not sound like much, wind transport can form deeper slabs of drifted snow on wind-loaded slopes. In addition to this new snow, some old snow also remains available for wind slab building. With the winds forecasted to shift to the N and E 12/31/2020 and 1/1/2021, wind slabs will migrate back across the ridges and reform on previously scoured slopes. These shifting winds and blowing snow mean that wind slabs could exist on any wind-exposed aspect 12/31/2020 and 1/1/2021. For the most part, these wind slabs should remain small, but some larger ones could form on the most heavily wind loaded slopes. Human triggered wind slabs will be possible. 

  Newly developed cornices, wind pillows, and wind created surface textures can all help to identify which slopes may hold wind slabs. Identifying where these wind slabs may exist only represents part of the strategy right now since the persistent slabs mentioned above may exist in the more sheltered terrain.