Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 11 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Avalanche danger is moderate on all aspects and elevations on slopes steeper than 35 degrees due to wind slabs and deep persistent slabs.  Our snowpack has doubled in size in most areas with a heavy, wet, dense layer of new snow.  Human triggered avalanches are possible today with large destructive avalanches possible in some areas.  Evaluate snow and terrain carefully.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    3

    Large wind slabs were formed by the Monday/Tuesday storm up to 4-5' in depth.  These wind slabs were weak enough to cause avalanches yesterday with small cornice drops in the Donner Summit area and the problem will continue through today until these wind slabs can gain strength. Continued active windloading was reported throughout the day yesterday with SW winds strong enough to move snow.  SW/W winds today are in the 40-50mph range with gusts to 70mph along the Sierra Crest.  The amount of available snow for transport is decreasing and we should have limited additional wind slab development today.  There is the possibility that triggering a wind slab could step down to a deep persistent weak layer, the resulting avalanche would be large and destructive.

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    3

    The persistent slab problems we had before this last storm system are now deep persistent slab problems and are buried anywhere from 3' to 8' deep in the snowpack.  These deep persistent slabs exist on NW-N-NE aspects at all elevations and include the 12/13 rain crust and 12/10 rain crust.  Deep persistent slabs are unpredictable in behavior, they could be remotely triggered, and most of our tests and observations are not reliable due to the depth of these weak layers.  Large triggers (cornice failure, large group, etc.) may be required to cause an avalanche on these layers, or a wind slab avalanche could step down into these weaker layers.  An avalanche involving any of these deeper layers would be large, destructive and have severe consequences. 

Avalanche Discussion

Observations from Frog Lake Cliffs (Carson Pass area) showed many natural avalanches which appeared to be failing on the collapsed surface hoar layer below the 12/13 rain crust with crown lines greater than 3' deep.  Widespread cracking and active windloading occurred throughout the day.

On Andesite Ridge (Donner Summit Area) storm snow instabilities did exist with large density changes within the new snow.  Wind slabs failed with cornice drops in the 1-2' crown height range.  Rain was observed up to at least 8000' in the morning hours, much higher than forecasted for the day. 

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