Lawinenlagebericht

Central Sierra Nevada

Sierra Avalanche Center
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Lawinengefahr

Tue
Hochgebirge
2 · Mässig
Waldgrenze
2 · Mässig
Unterhalb
2 · Mässig
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Kernpunkte

People may still be able to trigger avalanches where slabs of wind drifted have piled up near ridges or where shallow but heavy new snow rests on top of old, weak snow in sheltered areas. Move carefully around slopes near wind drifts or below cornices. Keep an eye out for any signs of unstable snow like snow surface cracking, snowpack collapse, or whumpfing sounds, and watch out for those places as well.

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–2

    While some of the slabs of wind-drifted snow that have formed over the last few days may have gained strength, others could remain unstable today, especially in areas where these wind slabs exist on top of weak, sugary snow (facets). In these places, a person could trigger a wind slab lower on the slope, and wider-than-expected avalanches could still occur. In many places, the wind slabs also rest on top of old crusts, which can provide an efficient sliding surface.

    Look for signs of wind-blown snow, like cornice formations, wind pillows, or other signs of snow distribution on the ground to identify areas with wind slabs. Exercise increased caution around places where wind-drifted snow has been deposited.

    (12/2/2023)

    Skier-triggered shooting cracks in a wind slab on Castle Peak.

    Photo: SAC

  • Persistent Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1–2

    Weak snow existed in many areas before this storm, and now, a thin layer of heavy, cohesive new snow rests on top of these old, weak layers in some places. In some areas, the weak layers seem to support the new snow, but other areas exhibit signs of instability like snow surface cracking. Despite some observations indicating that these layers may be gaining strength and no reported persistent slab avalanches, it may still be possible to trigger a persistent slab avalanche, especially on NW-N-NE aspects at upper elevations. In more isolated areas, weak faceted snow also exists on E and W aspects. There is a high amount of uncertainty here with highly variable weak layer distribution and strength. This uncertainty makes predicting exactly where or if persistent slab avalanches may occur difficult at best.

    Any snowpack collapses, whumpfing sounds, or shooting cracks represent signs of unstable snow. If you encounter any of these signs of instability, avoid NW-N-NE aspect avalanche terrain in the area.

Avalanche Discussion

Throughout the forecast region, 1 to 2 feet of snow can be found on NW-N-NE aspects above 8000'. Continuous snow cover starts around 7,200' to 8,000' depending on location. There is a lot of variability in the structure of the existing snowpack depending on elevation and location. Looking beneath the recent new snow, the snowpack on NW-N-NE aspects holds concerning weak layers in some areas. Some locations are thick rain crust either with or without an easily missed thin layer of facets and sometimes surface hoar on top of the crust. Some locations are a mix of layers of rain crusts and faceted snow. While other areas are all faceted snow and depth hoar without any crusts. There is generally little to no rain crust within the snowpack above 8,800' to 9,000'. Recent warm weather may start to help these weak layers slowly gain strength.

Previous NE wind events scoured the upper portion of many wind exposed slopes down to rock, dirt, and/or ice before the recent snowfall. Despite this scouring, weaker faceted snow persisted on the lower portions of many slopes. A much thinner snowpack or bare ground exists across E-SE-S-SW-W aspects.

While the snowpack has become more usable for over the snow travel, there is still the potential to hit many obstacles (rocks, logs, etc.) that are barely covered. Areas with a supportable rain crust below the recent new snow are the most usable. If you are traveling on snow for recreation, you should come up with a daily travel plan based on the current conditions and associated hazards. 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.

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