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
2 · Moderato
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In sintesi

On slopes steeper than 32 degrees, CONSIDERABLE avalanche danger still persists on NW-N-NE-E-SE aspects in near and above treeline terrain due to a combination of wind slabs and persistent slabs. Even though the window for natural avalanches may have passed, human triggered avalanches remain likely. Unusual avalanche conditions exist. Avalanches on steeper slopes could be remotely triggered by a person standing in lower angle terrain. Careful snowpack evaluation, cautious route finding and conservative decision making are essential.

Problemi valanghivi

  • Wind Slabs

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

    Several human triggered and natural wind slab avalanches occurred yesterday morning. Additional snow since then and winds that have remained strong will mean that fragile wind slabs will remain widespread on wind loaded slopes today. Some of these wind slabs have accumulated above buried surface hoar making remote triggering and extensive propagation of some wind slabs a possibility. Human triggering of wind slabs large enough to bury a person will remain likely today especially in areas where the wind slabs rest above a buried surface hoar layer. Wind loaded near treeline NW-N-NE-E aspects represent the best places to find wind slabs on top of surface hoar, but this combination of wind slabs and surface hoar could also exist on isolated slopes below treeline or even in a few above treeline areas as well. Even in places where wind slabs did not accumulate above surface hoar, human triggered wind slab avalanches could still occur today. Any wind loaded N-NE-E aspect or cross loaded NW or SE aspect in near and above treeline could hold human trigger-able wind slabs. 

  • Deep Persistent Slabs

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

    Persistent slabs will remain a problem today due to the weak surface hoar layers buried in the snowpack on some slopes. Slopes where the wind slabs mentioned above have formed above this fragile persistent weak layer will pose the greatest threat to backcountry travelers, but smaller, shallower persistent slabs could still exist in non wind affected areas in near and below treeline terrain as well. Avalanches that fail on buried surface hoar can propagate long distances connecting trees or rocks, and they can wrap around corners. People can also trigger failures in the surface hoar layer on lower angle terrain that propagate into steeper avalanche terrain and cause avalanches. Most of the persistent slabs avalanches that result from the failure of a surface hoar layer in non wind loaded terrain today should remain relatively shallow, but deeper avalanches that fail on a more deeply buried surface hoar layer remain possible on isolated slopes. 

    Buried surface hoar does not exist on every slope so this avalanche problem will not exist on all slopes. Unfortunately, determining which slopes the buried surface hoar does exist on is difficult. Recent avalanche activity, shooting cracks, collapsing, whumfing, and snowpit tests can provide some clues, but sometimes the first clue is an avalanche triggered by the party on the slope. So far observations have shown buried surface hoar layers on N-NE-E aspects and a few NW and SE aspects.

Avalanche Discussion

Yesterday wind slabs that formed above buried surface hoar layers in near treeline and some above treeline terrain failed naturally near Bard's Bowl in Bear Valley and were remotely triggered by skiers standing in lower angle terrain well away from the slopes on Andesite Ridge on Donner Summit and in the Bard's Bowl area. The slides that resulted from the surface hoar failure propagated long distances along that layer. They measured 1-2 ft in depth. In below treeline, non wind affected areas in both of these zones, snowpit tests indicated instability due to the buried surface hoar layers, but no additional avalanches occurred. Snowpit tests near Tinker's Knob along the Sierra Crest between Squaw and Sugarbowl also found the buried surface hoar and yielded unstable results on it. On Becker Ridge near Echo Summit, some minor skier triggered cracking occurred, but observations did not reveal any signs of a buried surface hoar layer. 

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