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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Non aggiornato — questo bollettino è scaduto 11 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Avalanche danger is considerable on NW-N-NE-E-SE aspects at all elevations on slopes steeper than 32 degrees.  Moderate avalanche danger exists on S-SW-W aspects at all elevations.  Natural avalanches are possible and human triggered avalanches are likely due to winds slabs, storm slabs and persistent slabs.  We have received 2 to 3' of wet, dense, heavy snow over the last 24 hours.  Dangerous avalanche conditions exist.  Careful snowpack evaluation, cautious route finding, and conservative decision making are essential today.

Problemi valanghivi

  • Wind Slabs

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

    2 to 3 feet of new snow has been deposited in the last 24 hours above 7500' along the Sierra Crest.  Strong to gale force SW winds have moved this new snow to NW-N-NE-E-SE aspects and created large wind slabs in excess of 4-5' deep.  With this heavy load of new snow last night expect to see natural avalanche activity related to these wind slabs last night and through the morning hours.  Winds in the 60 to 70mph range with gusts over 100mph will make for wind slabs possible below treeline and much further down slope than normal.  Gale force winds will continue throughout today continuing wind slab development.  Look for obvious clues such as blowing snow, cornice formation, snow surface scouring, and wind pillows to determine where these wind slabs exist.

  • Loose Dry

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

    2 to 3 feet of new snow has fallen across the Sierra Crest in the last 24 hours.  Temperatures have trended warmer throughout the storm and are expected to peak as the storm exits mid day today.  This warming trend has caused a density change with the heavier (more dense) snow on the top and the lighter (less dense) snow on the bottom.  This density change will allow storm slab avalanches to be a problem for today within the new snow that has fallen over the last 24 hours.  Storm slabs can exist anywhere the recent storm has deposited snow which has not been blown away or scoured by the strong to gale force SW winds.  Signs of avalanche instability for storm slabs could be recent avalanche activity, whumping, shooting cracks and collapsing.

  • Deep Persistent Slabs

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

    Through the forecast area where the persistent weak layer of collapsed surface hoar exists capped by the 12/13 rain crust, large and destructive natural and human triggered avalanches are possible today.  This layer has been observed in the Ebbetts Pass, Echo Peak, West Shore, Ward Canyon, Deep Creek, Donner Summit and potentially in other locations as well.  Snowpack tests targeting this persistent weak layer over the last week have shown consistent results that indicate propagation is likely if failure of the weak layer could occur.

    Near crust facets have developed around the 12/10 rain crust in some areas of the forecast region.  Test results indicate that human triggering is unlikely but with a large load of snow, as is currently occurring, it could become possible.

    Either of these above situations could cause large destructive avalanches involving in excess of 4-6' of snow.  These persistent slab avalanche problems are not common in our maritime snow climate-the consequences involved would be severe.

Avalanche Discussion

Recent observations yesterday from Tamarack Peak (Mt. Rose area) and Powderhouse Peak (Luther Pass area) showed 8 to 12'' of new heavy snow fell through the daytime hours. Wind slabs were developing throughout the day with snowpack tests and informal observations pointing to these wind slabs getting larger and weaker.  Gale force winds and blowing snow made for almost zero visibility.  Targeting the 12/10 rain crust on Tamarack Peak showed that weak faceted snow does still exist around this crust and propagation is possible.  While skier triggering may have become unlikely, additional large loads may make it possible to fail this layer.

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