Central Sierra Nevada

Problemi valanghivi

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  Deep Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Possible

  Dimensione

  2

  The likelihood of triggering an avalanche has decreased in many areas. However, persistent slab avalanches do remain possible in some places especially in areas where the weak layer sits closer to the surface. A person hitting just the right spot (trigger point) on a slope or larger triggers like multiple people on a slope of a large cornice failure could also be more problematic. Persistent weak layers do not gain strength uniformly. Most data indicates that the old weak snow buried in the snowpack continues to adjust to the weight of the snow above it, but some data like the whumpfing and test slope failure reported yesterday from Brockway Summit indicate that the Dec 11 facets remain active.

  Persistent slabs behave in unpredictable ways. Signs of instability like whumpfing, cracking, and unstable snowpack test results may or may not exist. Sometimes triggering a large avalanche represents the first and only clue that the slope is unstable. Travel plans built around terrain less steep than 30 degrees not connected to steeper slopes or terrain without the weak layer can provide fun recreation opportunities with significantly higher safety margins. In many cases, those slopes hold great snow conditions right now.  While this problem will be here for a while longer, it won't last forever. Be patient, the bigger, steeper terrain will still be here after the persistent slab problem goes away. 

  Test Slope Failure on a N facing below treeline slope on Brockway Summit 1/1/2021.

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

  • Alta quota
  • Limite del bosco
  • Sotto il limite del bosco

  Probabilità

  Possible

  Dimensione

  1

  Another bout of strong SW winds over the last 24 hours should have moved snow back across the ridges and built slabs of drifted snow on wind-loaded slopes. Some wind transport may continue today. For the most part, these wind slabs should remain small, but some larger ones could exist on the most heavily wind-loaded slopes. Human-triggered wind slabs will be possible today. 

  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.