Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

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2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
2 · Moderato
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In sintesi

MODERATE avalanche danger exists due to possible persistent slabs on high elevation NW-N-NE aspects on slopes steeper than 32 degrees and some unlikely but not impossible wind slabs on wind loaded upper elevation slopes. While avalanches may have become harder to trigger, large destructive human-triggered avalanches remain possible today. In addition to the avalanche danger, firm icy slopes may exist on all aspects up to 9000 ft. due to a frozen rain crust.

Problemi valanghivi

  • Deep Persistent Slabs

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

    Data indicates that human-triggered, large, destructive persistent slab avalanches remain possible on upper elevation NW-N-NE aspects. These kind of avalanches have become more difficult to trigger, but a person's weight above a trigger point where a shallower snowpack exists or a larger or deeper trigger could cause a persistent slab avalanche. Trigger points could include areas near rocks or cliffs, areas where a shallow snowpack exists, buried stumps, trees, convex rollovers, and areas where the weak layer is just weaker. Large/deep triggers could include the isolated wind slab mentioned below, cornice failures, multiple people on a slope, stuck snowmobiles, or even bootpacking.  Any persistent slab avalanches that occur would have very serious consequences and would likely be unsurvivable. The persistent weak layer now has anywhere from 2 to 4 ft. of snow on top of it that can act as a slab layer. While data and observations have shown this layer to be most prevalent above 9000 ft. in the Mt. Rose area and above 8300 ft. along the Sierra Crest some observations have recently found it at lower elevations. These kind of avalanches will be harder to trigger in areas where a thick and supportable rain crust exists, but they may be possible in those areas as well.

    Due to the consequences associated with the large destructive avalanches that could occur if the persistent weak layer fails and the high degree of uncertainty associated with it, avoiding areas where this layer might exist is recommended. These avalanches could connect multiple start zones, could be triggered remotely, and could run farther than expected. Avoid North aspects above 32 degrees in steepness.  Have a travel plan.  Pay close attention to run out zones, up tracks, and regrouping areas. 

  • Wind Slabs

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

    While triggering a wind slab avalanche should have become unlikely, some unstable wind slabs may linger on isolated terrain especially in complex or extreme terrain like cliffy areas, couliors, gullys, etc. These wind slabs would exist on wind loaded near and above treeline N-NE-E aspects and some of the cross-loaded NW and SE aspects at upper elevations where more snow fell during the storm. In some of these areas the wind slabs could measure several feet in depth. In areas where most of the precipitation fell as rain, wind slabs should remain smaller and more isolated. In the event that a wind slab avalanche occurs it could step down to the persistent weak layer mentioned above especially on upper elevation north aspects where the PWL is most likely to exist. 

    Clues like cornices, blowing snow, wind drifted snow, wind pillows, snow surface scouring, ripples, and other wind created textures can help determine where wind slabs may exist.

Avalanche Discussion

Yesterday numerous obs from the Mt. Rose backcountry found that facets still exist near the bottom of the snowpack on NW-N-NE aspects above 8800 ft. on Incline Lake Peak, Chickadee Ridge, and on Slide Mt. Most of the compression tests on this weak layer on Incline Lake Peak resulted in sudden collapses as the layer broke during the test with a few showing less dramatic results. On Slide Mountain near the crown of the avalanche incident that occurred on 12/10, Extended Column Tests and Propagation Saw Tests indicated that human-triggering of this layer remains possible and that if it breaks the resulting fractures could travel a long distance (video here).  Below 9000 ft. in the Rose area a rain crust existed. This crust was breakable near 9000 ft. and became supportable at the lower elevations. 

Across the Lake on the Sierra Crest where the snow levels remained higher, a thick supportable rain crust existed up to 9000 ft. with firm icy slopes on all aspects up to that elevation. Above 9000 ft. the crust turned to snow. Observations near a large avalanche that occurred on Elephant's Back during Friday night or early Saturday morning found that the slide had stepped down to the weak facet layer near the base of the snowpack. On a N aspect on Red Lake Peak at 8900 ft, a thick supportable rain crust capped the snowpack, but snowpit tests still showed that the facets layers near the base could be problematic. A similar snowpack existed on Andesite Peak near 8000 ft. on a NW aspect with a thick supportable rain crust on the surface and weak snow near the base that collapsed during compression tests.    

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