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

In sintesi

While avalanches remain unlikely in most areas, the avalanche danger has increased to MODERATE due to a weakening snowpack allowing persistent slab problems to exist in specific areas. Human-triggered avalanches may be possible today in terrain with the right combination of a slab layer resting on top of a persistent weak layer of loose weak snow. This combination does not exist everywhere. Evaluate snow and terrain carefully and identify where slabs may exist on top of a persistent weak layer.

Problemi valanghivi

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1

    Even though it has not snowed since Dec. 20th, the snowpack continues to change while it sits on the ground. The recent conditions have caused the layer of weak sugary snow (facets) below the surface to grow progressively weaker. In fact, it has grown so weak that in some specific areas where a slab layer exists on top of the persistent weak layer, the weak layer can barely support the snow above it. This combination of slab resting on top of a weak layer does not exist everywhere. In most places, avalanches remain unlikely because either little to no slab exists on top of the persistent weak layer or the weak layer still has enough strength to support the slab above it. In those areas where the combination of slab over persistent weak layer does exist, human triggered avalanches may be possible. These areas include slopes on NW-N-NE aspects where hard wind slabs exist in near and above treeline terrain and some areas in near and below treeline terrain where smaller softer slabs may exist. Persistent slab avalanches can be triggered from the bottom of a slope or other connected slopes. They often break above the person who triggers them and may fail after several people have used a slope. They may not fail until the trigger has fully committed to the slope. 

    Clues like whumphing and collapsing, shooting cracks, hollow sounding snow, or digging into the snowpack to find a loose weak layer of snow below a harder layer of snow can indicate that persistent slab avalanche conditions may exist. These clues may be difficult to find and the slope avalanching may be the first clue of a persistent slab problem. Despite the fact that these kinds of avalanches should remain small, they could still have serious consequences in areas with terrain traps or if they carried a person into/over any of the numerous shallowly buried or exposed obstacles. Avoid steep slopes where slabs may exist on top of loose weak snow. Choosing sheltered terrain where no slabs exist will avoid this problem and provide softer snow to recreate on.

Avalanche Discussion

* Whumphing has occurred on Black Butte (near Carson Pass), on Deep Creek Peak (north of Squaw), and on Silver Peak (also north of Squaw) in the last 3 days as the additional weight of a skier caused the persistent weak layer to collapse below a slab. Tests also yielded unstable results on Rubicon Peak in areas where hard wind slabs sat on top of a layer of weak sugary snow. The layer of weak sugar snow (persistent weak layer of facets) exists on many northerly aspects, but little to no slab layer exists above this weak layer in most places. The signs of instability have remained limited to areas where the right combination of a slab layer resting on top of the persistent weak layer exists.

* Variable snow surface conditions ranging from wind scoured snow and firm icy crusts on exposed slopes to some lingering areas of soft unconsolidated snow on sheltered below treeline northerly aspects exist.

* Northerly facing terrain above 8000 ft in the Mt. Rose area and along the Sierra Crest north of Emerald Bay holds the best snow coverage. Overall less snow cover exists south of Emerald Bay, but decent coverage exists above 8,500 ft. on northerly aspects. Coverage becomes patchy on other aspects and large areas of bare ground exist on many southerly aspects at all elevations.

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