Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

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

CONSIDERABLE avalanche danger exists at all elevations throughout the forecast region due to wind slab and storm slab avalanche problems.  Dangerous avalanche conditions exist with the potential for large destructive avalanches today. 

Problemi valanghivi

  • Wind Slabs

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

    Gale force S/SW winds throughout the last 2 days of this storm cycle have created large fragile wind slabs on NW-N-NE-E-SE aspects in near treeline and above treeline areas.  Some of these wind slabs sit on top of old weak snow layers that could allow remote triggering with wide propagation.  A warming trend overnight has created an "upside down" snowpack in some areas with heavier snow on top of less dense snow.  These wind slabs could involve multiple feet of snow and have severe consequences.

    Cautious route finding, decision making, and terrain management are critical today.  Large destructive avalanches are likely.  Look for blowing snow, cornice formation, and wind pillows to help identify where wind slabs exist. 

     

  • Loose Dry

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

    Storm slab avalanches will be likely today on all aspects in near and below treeline terrain.  2 to 4'+ of new snow has fallen in the last 2 days.  In many areas of the forecast region, this new snow sits on top of old weak faceted snow layers.  A warming trend overnight has made the recent storm snow "upside down" with heavier denser snow on top of less dense snow.  Large destructive avalanches that behave like persistent slab avalanches could occur today with remote triggering and wide propagation possible.  

    Look for cracking around skis or whumpfing sounds to indicate storm slab instabilities.  If these signs exist, move to different aspects and/or low angle terrain well below 30 degrees with no overhead hazard.

      

Avalanche Discussion

*  Observations from Andesite Peak (Donner Summit area) showed increasing instabilities throughout the day.  Several small skier triggered wind slab avalanches were observed along with storm slab instabilities that increased with the additional new snow.  Snowpack tests targeting the old weak faceted snow showed easy failure along with several whumpfing sounds throughout the day.

*  Tamarack Peak (Mt. Rose area) showed wind slabs failing easily and building throughout the day.  Storm slab instability was also observed at the new snow old snow interface.

*  Observation from Poison Peak (Bear Valley area) also showed weak faceted old snow layers below the new storm snow.

Most avalanche fatalities and accidents occur under the Considerable avalanche danger rating.  We have just received up to 4' of new snow with over 100mph winds within the last 48 hours.  Winds and snow are expected to wind down this morning.  Do not be surprised to see large natural avalanches out in the backcountry today that occurred during this storm.  On top of the wind slab and storm slab avalanche problems, uncertainty exist as to the strength and reactivity of the old weak faceted snow now buried under this storm snow.  Conservative decision making today is warranted.   

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