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 12 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Above 9000 ft. on slopes 32 degrees and steeper, human-triggered avalanches remain possible on NW-N-NE aspects due to possible persistent slabs. Near and above treeline wind-loaded NW-N-NE-E-SE aspects above 8000 ft. steeper than 35 degrees may still hold human-triggerable wind slabs especially on unsupported slopes, couliors and gullies, and in complex and/or extreme terrain. The avalanche danger rating for these areas remains MODERATE.

Problemi valanghivi

  • Deep Persistent Slabs

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

    Warmer weather and wet snow/rain has allowed the Dec. 2nd facets to bond together and gain strength in terrain below 9000 ft. However, this persistent weak layer exists and remains weak on NW-N-NE aspects above 9000 ft. Recent data indicates that as elevation increases the Dec. 2nd facets weaken and that fractures can still travel through this persistent weak layer. Even though the snow above the Dec. 2 facets seems strong and consolidated, one trigger point is all it takes for a person to start a fracture in the buried weak layer that could result in a large dangerous avalanche. Observations like slope cuts and previous tracks are not reliable indications of stability when dealing with persistent slabs. Avoiding areas where these layers might exist or sticking to lower angle slopes without steeper terrain above them are the only mitigation options for persistent slabs. Any other technique is just rolling the dice.

  • Wind Slabs

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

    Wind slabs and fragile cornices still exist on wind-loaded N-NE-E aspects and on the cross-loaded NW and SE aspects in near and above treeline terrain. While they have become more difficult to trigger due to settlement and consolidation in the new snow, human triggered wind slab avalanches large enough to bury or injure a person may remain possible today. Wind-loaded slopes above cliffs, in gullies or couloirs, in other complex or extreme terrain, or in heavily wind loaded areas at the higher elevations represent the most likely places to find lingering unstable wind slabs. Wind slabs avalanches on slopes above 9000 ft. where the Dec. 2nd facet layer still exists could step down into those layers as well.

Avalanche Discussion

Observations from Carson Pass on Red Lake Peak and near Frog Lake yesterday found consolidated new snow on top of strengthening weak layers on NW-N-NE aspects below 9000 ft. Snowpack depths between 8000 and 9000 ft. ranged from 40 to 80 cm.

Above 9000 ft. on Red Lake Peak the story changed. At these higher elevations where temperatures remained colder during the storm the snowpack measured 100 to 140 cm in non-wind-loaded areas and the old persistent weak layers (now known as the Dec. 2 facet layers since they were buried on Dec. 2) at the base of the snow remained loose, dry, and weak. Snowpit data from a NE facing slope at 9700 ft. where the Dec. 2nd facets existed indicated that if they start to break, the resulting fracture can travel through that layer (video, snowpit). Hand pits, probing, and quick shovel pits only revealed the old facet layers on the NW-N-NE aspects (places that held snow during the fall). At these upper elevations, snowpit tests also indicated that fractures could travel along a graupel layer buried about one foot deep.

On Red Lake Peak cornices above wind-loaded test slopes broke easily when weighted, but those wind-loaded test slopes did not fracture when the oven-sized cornice pieces fell onto them. Ski cuts also did not produce any results on wind-loaded test slopes between 8700 and 9700 ft. even though snowpit data indicated possible instabilities concerning the winds slabs resting on top of a thin graupel layer.

Below 8000 ft. snow cover remains limited.

Regioni coperte