Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Alta quota
2 · Moderato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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Non aggiornato — questo bollettino è scaduto 13 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

CONSIDERABLE avalanche danger exists in near and below treeline terrain on NW-N-NE aspects 30 degrees and steeper due to widespread persistent slabs. Human triggered persistent slabs remain likely today in near and below treeline terrain. Remotely triggered avalanches can occur under these conditions. MODERATE avalanche danger remains on the near and above treeline NW-N-NE-E-SE aspects due to lingering wind slabs. In addition to the avalanche danger a myriad of season-ending obstacles still lurks just below the surface. Conservative terrain selection, careful snowpack evaluation, and conservative decision-making are essential.

Problemi valanghivi

  • Deep Persistent Slabs

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

    Human triggered avalanches remain likely in near and below treeline terrain on NW-N-NE aspects where the recent snow rests on top of the Dec. 7th facet layer. Any areas that had snow cover prior to this storm remain suspect. The recent snow currently represents a soft slab, because it has slowly started to consolidate. At the same time the cold clear weather has started to weaken the new snow. Depending on how fast it consolidates or how fast it weakens the new snow may become more (faster consolidation) or less (faster weakening) slab-like. A more slab-like layer would make these persistent slabs easier to trigger.

    Since fractures can travel so easily through the persistent weak layer, people can trigger persistent slabs remotely while standing in low angle terrain or on ridgelines or anywhere that the snowpack under their feet connects to steeper and/or deeper terrain. Wide propagation and avalanche activity on lower angle slopes to around 30 degrees remains likely today below treeline.

  • Wind Slabs

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

    Some of the wind slabs that formed during the storm may still linger on the near and above treeline NW-N-NE-E-SE aspects. In many areas the strong E and NE winds that occurred last night and this morning have scoured snow away from these winds slabs making them smaller and more shallow. In areas where a mix of rocks, dirt, and a few inches of old snow existed below these slabs they have become more difficult to trigger. However, some of these wind slabs could sit on top of the persistent weak layers mentioned above on the near treeline NW-N-NE aspects. Human triggered avalanche activity resulting from the failure of these wind slabs remains likely on slopes where the persistent weak layers exist and possible on slopes that just hold wind slabs.

Avalanche Discussion

More widespread collapsing, whumphing, and cracking occurred yesterday in the Tamarack Peak area (photo, video, snowpit) and in the Silver Peak area (more info). These failures failures occurred as Dec. 7th facet layer below the recent snow collapsed in response to the additional weight of a person. These collapses propagated along the Dec. 7th facets for long distances across entire slopes. People could initiate the collapses in areas where less snow existed or in low angle terrain, and the fracture could travel into deeper and steeper terrain. In some areas enough anchors (stumps, bushes, logs, trees, rocks, etc.) existed to prevent the collapses from traveling too far. Near Silver Peak a party reported seeing a small snowboarder triggered slide on a north facing slope (more info).

In the Tamarack Peak area smaller, skier-triggered shooting cracks also occurred on wind loaded NE facing test slopes in near treeline terrain (photo). The wind slabs on these slopes remained small and did not extend very far down slope.

Barely covered rocks, stumps, logs, and other hard immobile objects still exist on all slopes especially on those facing the southern half of the compass. The shallow snowpack does not have enough structure or strength to keep a person above all these things. It only camouflages their existence.

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