Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
3 · Considerable
Treeline
3 · Considerable
Below
3 · Considerable
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Highlights

CONSIDERABLE danger exists on slopes 35 degrees and steeper at all elevations on NW-N-NE aspects and on near and above treeline, wind loaded, W, E, and SE aspects. MODERATE avalanche danger exists in all other areas. Human triggered wind slab and storm slab avalanches are likely today.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    3

    Natural wind slab avalanches will become possible today and human triggered wind slab avalanches will be likely. More strong winds and additional snowfall today will cause the fragile wind slabs that already exist on leeward aspects to increase in size and extent. Wind slabs 2-5 ft in depth could exist on wind loaded slopes by the end of the day. These wind slabs rest on top of a mix of old crusts, layers of weak sugary snow, and layers of lighter storm snow. These wind slab avalanches could fail at or above the old/new snow interface. Due to the shifting south and southwest winds the wind slabs could exist on W-NW-N-NE-E-SE aspects in near and above treeline terrain.

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    Changing temperatures, winds, and snowfall rates will cause a variety of layers to form within the new snow. Some of these layers will represent small slab layers and some will represent weak layers. As a result of this stratigraphy unstable storm slabs could exist on all aspects in near and below treeline terrain. The most fragile storm slabs will lurk on the below treeline north and northeast aspects where thin layers of soft sugary snow exist near the old/new snow interface. Snowpack failure is expected to occur just below, at, or above the old/new snow interface with slab thickness expected at around 1 to 2 ft. Human triggered avalanches will be likely below treeline today on the northerly aspects and possible on all other aspects.

Avalanche Discussion

Yesterday, ski cuts on a below treeline steep NE facing slope near Grouse Rocks resulted in a small (6-8 inch crown, 75 ft. wide) storm slab avalanche that ran down hill for 150 ft. This slide failed on the layer of weak sugary snow at the base of the storm snow. On Tamarack Peak stomping on a ridgeline triggered a wind slab avalanche 300 ft. away in steeper wind loaded terrain. This avalanche had a 2-3 ft crown and measured about 300 ft. wide. It ran down to the bottom of a 350 ft. slope where the debris piled up. This wind slab avalanche failed on a slightly less dense layer of storm snow at the base of the new snow on top of a firm melt freeze crust.

Farther south on Powderhouse Peak, some data indicated that the new snow had bonded well to the old snow surfaces, while other data indicated weak and fragile bonding. Some tests indicated that fractures could easily travel along this interface and other tests indicated that they should not. Ski cuts on test slopes in this area did not produce any results.

In all three areas, strong winds continued to move snow and wind load leeward slopes yesterday.

Regions covered