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
1 · Debole
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In sintesi

MODERATE avalanche danger continues near and above treeline due to an ongoing wind slab avalanche problem. Human triggered wind slab avalanches remain possible today. Below treeline, avalanche danger is LOW.

Problemi valanghivi

  • Wind Slabs

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

    Moderate to strong SW winds combined with periods of snowfall have created several layers of wind slab near and above treeline on NW-N-NE-E-SE aspects. These wind slabs have been difficult to trigger but exist in widespread areas. Avalanche size up to D2 is possible (large enough to bury or injure a person).

    With triggering difficult, determining areas of instability may be challenging. In order to identify areas of concern, look for recent wind slab avalanches, cornices above slopes, firm hollow sounding snow, blowing snow, and wind pillow type snow drifts. Use these clues to identify the areas to avoid, or use low angle slopes. More desirable conditions for recreation in the form of softer snow exist in wind sheltered terrain away from areas of wind slab.

  • Loose Dry

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

    Storm slab avalanches are unlikely today, but have a level of concern for the isolated areas below treeline that received 10 to 14 inches of new snow over the past 24 hours. Periods of snowfall in the form of graupel (pellet snow) were observed yesterday and may provide a short lived weak layer within the top foot of the snowpack. Avalanche size is expected at D1 (not large enough to bury a person). The elevated consequences would come from being carried into or over down slope hazards such as cliffs, rocks, trees, or terrain traps.

    Look for human triggered snow surface cracking in wind protected areas below treeline. Quick hand pits may be helpful in identifying a problematic weak layer within the top foot of the snowpack. Evaluate terrain carefully and avoid specific slopes with increased consequences in any areas where signs of potentially unstable storm slabs are found.

Avalanche Discussion

* Observations made and received yesterday in the Mount Rose and Donner summit areas noted widespread areas of blowing snow and wind slab formation near and above treeline. Wind slabs were difficult to trigger (stubborn). Some tests indicated that if slab failure could be triggered, propagation would occur.

* Wind slab avalanches were noted on March 23 in the Mount Rose area and West Shore Tahoe area, triggered by natural cornice collapses.

* Numerous storm slab crowns from March 22 have been observed on NE-E aspects below treeline on Incline Lake Peak (Mount Rose area). These avalanches failed on a density change within the storm snow. On March 24, data from snowpit tests in one of the crowns combined with several ski cuts on terrain of similar aspect and elevation indicated that this weak layer is no longer reactive.

* More crowns from deep slab avalanche activity occurring during the March 20 - 22 storm cycle have been discovered on N-NE-E aspects both above and below treeline in the 7,000' to 10,000' elevation range. No reports of post storm deep slab avalanches have been received.

To the best of SAC's knowledge, triggering a deep slab avalanche has become very unlikely. For this reason it is no longer included as a specific avalanche problem. If you have unreported information relating to signs of deep slab instability occurring March 23 through the present, please submit an observation.

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