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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In sintesi

MODERATE avalanche danger exists today at all elevations due to a combination of wind slab and loose wet avalanche problems. Backcountry travelers should not be surprised by the occurrence of a human triggered avalanche today. Areas of unstable snow will exist within surrounding greater areas of seemingly stable snow. Take active measures to avoid the specific terrain features where unstable snow is most likely to exist.

Problemi valanghivi

  • Wind Slabs

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

    Expect to find lingering wind slabs today near treeline and above treeline in wind loaded areas on NW-N-NE-E-SE aspects. Avalanche size could be very large with sufficient destructive potential to bury and destroy a car or break a few trees (up to size D3).

    These wind slabs may be difficult to trigger (stubborn) until fully committed to the slope. Signs of instability may be difficult to find today prior to an avalanche occurrence. Identify and avoid areas of potentially unstable wind slab. The most likely areas of unstable snow will exist below cornice features and on slopes with wind pillows across the upper portion of the slope.

    Rapid warming today may exacerbate the wind slab problem on sun exposed E-SE aspects.

  • Loose Wet

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

    Rapid warming is expected today. Mid March levels of incoming solar radiation combined with a higher spring-like sun angle will allow for surface wet snow formation and the possibility of loose wet avalanches. Loose wet instability is possible on all aspects today. Avalanche size could be large enough to bury or injure a person (up to size D2).

    Roller balls will occur naturally today as snow falls off of rocks and trees. This will be the first clue for potential loose wet instability. Human triggered roller balls are an additional sign of wet snow instability. Avoid steep slopes in areas where roller balls are occurring.  

Avalanche Discussion

* Observations made yesterday on Andesite Peak (Donner Summit area) revealed reactive wind slabs below cornice features in near treeline and above treeline terrain. Wind slab avalanches were intentionally skier triggered on test slopes with crowns up to 2 feet deep. Higher density wind transported snow on top of lower density storm snow was the problematic snowpack structure. Problematic storm slabs were not seen in below treeline terrain in this area. Snowpit tests indicated that non-persistent weak layers within the storm snow were gaining strength. Observations made Friday in this same area revealed widespread wind slab and storm slab avalanches.

* No additional field observations were received from Saturday to aid in the creation of today's advisory.

* The forecast area picked up 4 to 6 feet of snow at and above 7,000' this past week. The vast majority of that snow fell Tuesday afternoon through Saturday morning (March 13-17).

Triggering a Deep Slab avalanche has become very unlikely. The vast majority of observations and data targeting the once faceted persistent weak layer have indicated that strength gains are sufficient to handle the new snow load. There have been a few outlying pieces of data to suggest that while a deep slab avalanche is very unlikely, it is not impossible. If a deep slab avalanche were to occur it would be huge, likely unsurvivable, and could run very long distances into mature forest (size D4+). The only way to manage this type of avalanche problem is through avoidance of NW-N-NE aspect avalanche terrain.

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