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

Even though avalanches have become more difficult to trigger, human triggered avalanches remain possible on some NW-N-NE-E-SE aspects at all elevations due to lingering persistent slabs and wind slabs. The persistent slabs could occur on slopes steeper than 32 degrees, while the wind slabs would be more problematic on slopes steeper than 35 degrees. The avalanche danger is MODERATE today, but the consequences of a human triggered avalanche would be serious. Evaluate snow and terrain carefully; identify features of concern.

Problemi valanghivi

  • Deep Persistent Slabs

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

    Despite the fact that persistent slabs have become more isolated and difficult to trigger, human triggered persistent slab avalanches that fail on a buried surface hoar layer remain possible on some slopes where buried surface hoar still exists under a 2-3 ft. slab of consolidated snow. Open areas in near and below treeline terrain on N-NE-E aspects represent the most likely places to find buried surface hoar, but it may also still exist on some isolated NW and SE aspects. Larger triggers like large cornice collapses or multiple people on a slope or smaller triggers on the right trigger point such as an area where a shallower snowpack exists or in areas near rocks have better chances of triggering a persistent slab avalanche today. If any persistent slab avalanches do occur, they would be large and have severe consequences. Avalanches that fail on buried surface hoar can propagate long distances, can occur in places traditionally considered safe, and can be triggered from lower angle terrain.

    Due to the consequences of these types of avalanches, avoiding steep slopes where reactive surface hoar still exists represents the best way to manage this avalanche problem. Unfortunately, determining where the buried surface hoar still exists and where it is still reactive is difficult. Recent avalanche activity, shooting cracks, collapsing, whumpfing, and snowpit tests can provide some clues. However, a persistent slab triggered by a party on the slope sometimes represents the first and only clue to avalanche instability. 

     

  • Wind Slabs

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

    Wind slabs have also become more difficult to trigger, but like the persistent slabs mentioned above some human triggered wind slabs may still be possible today. Wind loaded slopes on near and above treeeline NW-N-NE-E-SE aspects in couliors, gullies, on hanging snowfields, or in other complex or extreme terrain represent the best places to find lingering wind slabs. Some fragile wind slabs may still linger in other isolated heavily wind loaded areas as well.  

Avalanche Discussion

Yesterday observations on Rubicon Peak revealed frozen rain crusts ranging from supportable to breakable on the snow surface below 8000 ft. and a layer of dense heavy snow (also sometimes supportable and sometimes breakable) above 8000 ft. Hard wind slabs existed on the near and above treeline slopes exposed to wind loading. While ski cuts on small wind loaded test slopes did not produce results, snowpit tests on the wind slabs did still yield some unstable results. Below these heavier surface layers the most recent snow remained soft and unconsolidated. Snowpit observations also found at least one of the buried surface hoar layers lurking about 70 cm down in the snowpack underneath a firm slab layer. This surface hoar layer consisted of large feathery surface hoar that had not collapsed yet. Snowpit tests on the buried surface hoar layer indicated that triggering this layer has become more difficult. Tests also indicated that if it breaks fractures can still travel along that layer, and failure of this layer could still produce large avalanches in areas where the surface hoar layer exists. 

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