Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Mon
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Out of date — this forecast expired 3 years ago. You’re viewing past conditions, not the current bulletin.

Highlights

Wind drifted snow has formed into cohesive slabs near ridgelines and thin slabs of recent new snow exist on top of older, weaker sugary snow. This has created avalanche concerns for all snow covered areas. Move carefully around slopes that are below cornices or wind drifts and any areas where snow surface cracking, snowpack collapse, or whumpfing sounds are occurring.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    The first wind slab avalanche of the season was intentionally triggered on Dec 1 on the east face of Mt. Judah near Donner Summit (more info). Additional snowfall and SW wind over the past 24 hours has kept this avalanche problem ongoing with additional drifting and snow surface cracking observed yesterday.

    In some areas, wind slabs may exist on top of a very weak, sugary (faceted) snowpack. Triggering lower on the slope than is common for a wind slab avalanche could easily occur anywhere a slab exists on top of weak faceted snow and the avalanche could fail wider than expected. In some areas, an easily missed, thin layer of faceted snow exists sandwiched between the recent new snow and a firm rain crust below it.

    Use clues such as visible drifting snow, cornice formations, wind pillows, or other signs of snow distribution on the ground to identify areas that hold wind slabs. Exercise increased caution around areas where wind drifted snow has been deposited.

     

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Thin slabs, 4 to 8 inches thick composed of recent new snow have formed on top of a very weak snowpack in some areas, mainly on NW-N-NE aspects. In more isolated areas, weak faceted snow also exists on E and W aspects. There is a high amount of uncertainty here. Relatively small, incremental loads were added to the snowpack over the last few days. There is a high degree of variability in recent new snow amounts and the structure of the existing snowpack around the forecast area, making it difficult to highlight the locations where a persistent slab avalanche is most likely to occur.

    Any snowpack collapses, whumpfing sounds, or shooting cracks encountered are signs of an unstable snowpack. If you are encountering 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'.

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.

Regions covered