Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Fri
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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In sintesi

Multiple avalanches occurred yesterday throughout the forecast region.  Weak snow at the old snow surface, below our recent storm snow, will continue to make triggering avalanches likely today.   Winds will also continue to form wind slabs in exposed areas.  Look for signs of unstable snow and avoid suspect slopes if venturing out into the backcountry.

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Our persistent slab avalanche problem has become much wider in scope.  Yesterday, multiple avalanches were reported failing on either near surface facets, buried surface hoar, or near crust facets that formed on the snow surface and are now buried under our most recent storm snow.  These avalanches varied in size from 8'' to 18'' deep, up to D2 in size, with some having very wide propagation.  Many of these avalanches occurred in wind loaded areas, but avalanches may be possible in below treeline terrain also.  These weak layers are not expected to go away quickly, with avalanche activity remaining likely for today.

    Look for recent avalanche activity, cracking in the new storm snow, and whumpfing sounds.  These avalanches can be remotely triggered with wide propagation.   Maintain wide safety margins away from avalanche terrain and out from below avalanche runout zones.

    (1/3/2024)

    Crown line. Avalanche propagated the full distance of the slope releasing wind slab over near crust facets.  Ward Canyon Area.

    Photo: Public

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–2

    Wind slab avalanches occurred yesterday.  Some of these wind slabs are thought to have failed on one of the persistent slab weak layers, and some were just normal wind slab avalanches.  Winds switched to the NE last night and increased in speed.  These winds will be redistributing snow and forming new winds slabs in different locations today.  Wind slabs that have formed on top of a persistent weak layer will be much easier to trigger and could have very wide propagation.

    Look for areas where blowing snow is actively occurring, where cornices or wind drifts have formed, and for any areas of snow surface cracking.  Snow surface scouring may also help to determine where previous blowing snow was occurring as the wind direction changed overnight.  Use these clues to identify areas of potentially unstable wind slab and move around these areas with caution.  

    (1/3/2024)

    Crown and snowboard track with terrain in background.  Carson Pass area.

Avalanche Discussion

More on our persistent avalanche problem:

Our snowpack is slowly building and getting deeper.  A weak snowpack structure still exists in many areas above 8500' in the forecast area where large faceted snow grains and depth hoar exist at the base of the snowpack.  There has been some avalanche activity in the last few weeks associated with this weak layer, mainly in areas where additional wind loading on top of this layer has caused failure.  This current storm is not likely enough additional load to cause this weak layer to fail, and would most likely need additional wind loading to make avalanches possible.  Larger future storms or areas that receive heavy wind loading are still the main concern with this weak layer moving forward.

In addition-multiple persistent weak layers formed on the snow surface last week and have been buried by the Jan 2-3 storm.  These weak layers include near surface facets, buried surface hoar, and near crust facets.  Avalanches have been reported on some of these weak layers and some of these layers may not have enough snow load to fail at this point.  The buried surface hoar and near crust facets have been found on NW-N-NE aspects at all elevations.  The near crust facets seemed to have formed on a melt freeze crust that developed on east aspects that received some sun last week.  Avalanches will remain likely today and could potentially increase this weekend as we receive more snow.  

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