Bollettino valanghe

Central Sierra Nevada

Sierra Avalanche Center
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Pericolo valanghe

Sat
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
3 · Marcato
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Non aggiornato — questo bollettino è scaduto 3 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Shifting winds continue to drift snow into cohesive slabs and there is persistent weakness in the existing snowpack. The avalanche hazard is complicated by the potential to trigger avalanches low on the slope, well below traditional avalanche start zones. There is a high likelihood of serious trauma for anyone caught in an avalanche in the currently rocky, shallow snowpack. If in doubt or faced with uncertainty, choose not to travel in or below avalanche terrain.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Strong winds over the last 36 hours have drifted and deposited recent new snow into cohesive wind slabs. Winds are in the process of shifting from SW to NE which will redistribute snow on the ground. Expect to encounter a mix of wind-loaded and wind-scoured areas above treeline and near treeline across all aspects.

    Look for clues indicating the presence of wind slabs such as visible drifting snow, cornice formations, wind drifts, wind textured snow surfaces, and scouring vs deposition patterns on the ground. Consider avoiding areas where wind drifted snow has been deposited, especially if weak faceted (sugary) snow exists in the snowpack below.

    (12/07/2023) Visible wind slab clues that include drifting snow, rougher wind scoured areas and smoother wind slab deposit areas. Elephants Back, Carson Pass, CA Photo: SAC

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    Recent new snow deposited on top of a weak, faceted snowpack on NW-N-NE aspects. The structure and degree of weakness of the snowpack varies widely by location and by elevation. Regardless, numerous problematic weak layers exist. This avalanche problem is expected to remain with us for some time. Signs of instability may become more prominent today and tomorrow as the recent new snow settles and becomes more of a cohesive slab.

    While wind slab is expected to remain the most widespread avalanche problem today, persistent slab is the more difficult problem to manage. Signs of instability such as recent avalanches, snow surface cracking, snowpack collapse, or whumpfing sounds are indications to avoid NW-N-NE aspect avalanche terrain in the area. Keep in mind that previous NE wind events may have scoured weak snow out of the upper portion of a slope, laying a trap of weak snow on the lower half of the slope that is just waiting for a trigger. Anticipate wide avalanches that might be triggered low on the slope.

    (12/7/2023)

    New snow has accumulated above the old snow which is weakest on sheltered NW-N-NE aspects. Tamarack Peak, NV Photo: SAC

Avalanche Discussion

 

More on the persistent slab problem: In most areas, the weak faceted (sugary) snowpack exists above about 8,000', but could be encountered as low as 7,200' in isolated locations. Look for an easy to miss thin layer of faceted snow on top of the uppermost rain crust in the snowpack (photo below). As elevation increases expect to find layers of faceted snow in between rain crusts. Above about 8,500' or so, rain crusts disappear and the snowpack becomes nearly all weak faceted snow. Generally speaking, the higher up you go on wind protected NW-N-NE aspects, the weaker the snowpack.

There is still the potential to hit many obstacles (rocks, logs, open water, bushes, etc.) that are barely covered (or not) by the shallow snowpack. Areas with a supportable mid snowpack rain crust in the 7,500’ to 8,500’ elevation range provide the most usable snowpack. Anticipate encountering a different snowpack structure and a different set of travel conditions if you are moving from one place to another around the forecast area.

Regioni coperte