Central Sierra Nevada

Problemi valanghivi

• 

  Persistent Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

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

  Probabilità

  Likely

  Dimensione

  2–2.5

  We are seeing our third avalanche cycle that is failing on the same Jan 2 persistent weak layer (PWL). Depending on location, this layer is composed of buried surface hoar, faceted snow, and/or a crust/facet combo.  This layer is buried 1 to 3 feet below the snow surface in wind sheltered areas and up to 6 feet deep in wind loaded areas. Seven persistent slab avalanches were reported to occur yesterday, all of which were triggered by backcountry travelers moving through low angle non-avalanche terrain adjacent to steeper avalanche terrain. This type of remote triggering could easily occur again today.

  There are also signs that the weak faceted snow and depth hoar that exist near ground level at the bottom of the snowpack and has been dormant since late December is waking up due to the amount of new snow load received over the past week. There is a slight chance today that an avalanche that fails higher up in the snowpack on the Jan 2 PWL could step down to this deeper weak layer. Additional new snow loading tomorrow will increase the likelihood of avalanches failing near ground level in the snowpack.

  Recent avalanches, snow surface cracking, snowpack collapse, or whumpfing sounds will indicate unstable persistent slabs in the immediate area.  These avalanches could be remotely triggered from low angle terrain above, below, or off to the side of a steeper slope.  Conventional wisdom would suggest that there would be less instability today than yesterday. This particular weak layer combined with wind drifting of snow leading to continued post-storm loading will challenge that convention. Choose backcountry terrain and route selection very carefully today. Very conservative terrain choices that maintain extra wide margins of safety are recommended.

  

• 

  Wind Slabs

  N

  NE

  E

  SE

  S

  SW

  W

  NW

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

  Probabilità

  Possible

  Dimensione

  1–2

  Visible drifting of recent new snow was reported yesterday from throughout the forecast area. Drifting is expected to continue today as SW winds increase ahead of tomorrow's storm. This drifting snow will continue to deposit into cohesive slabs along the downwind side of ridgetops.  Where the Jan 2 PWL exists, that will be the expected avalanche size and type in these areas. In areas where the Jan 2 PWL does not exist, comparatively smaller avalanches of just wind drifted snow could be triggered by backcountry travelers today.

  Look for areas where blowing snow is depositing and for any areas of snow surface cracking.  Clues such as recent avalanches, built out cornices, wind drifts, and loaded vs scoured snow surface conditions can help identify the areas of greatest concern. Exercise increased caution today around and below steep wind loaded areas and consider avoiding them if you think the Jan 2 PWL might be present.