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

MODERATE avalanche danger today will rise to CONSIDERABLE avalanche danger this afternoon and during the night as heavy rain impacts the forecast area. Loose wet avalanche problems will go from possible to likely today and some of these could involve large amounts of snow. In addtion, deep slab avalanches that would be very large and destructive and would have severe consequences may become possible tonight and tomorrow.

Avalanche problems

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1

    Significant amounts of rain on snow during this storm will saturate the snowpack. The rain will melt the bonds holding the snowpack together and deep loose wet snow will exist on all aspects and at all elevations. Loose wet avalanches will become likely today and remain likely throughout the storm on any steep slopes. Northerly aspects and higher elevation slopes that have experienced less robust melting during the warm weather will be more likely to hold larger loose wet instabilities. Wet snow instabilities could include pinwheels, rollerballs, and loose wet avalanches. Some of the loose wet avalanches could involve large amounts of wet snow and could be large enough to bury injure a person.

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    3

    Large deep slab avalanche activity may become possible tonight or tomorrow due to the significant loading expected during this storm. The rain will add load to the snowpack, weaken and melt the bonds holding the snowpack together, and could help lubricate bed surfaces. This added liquid could possibly be enough to overload the deeply buried Feb. facet weak layer and allow some deep slab avalanche activity to become possible on NW-N-NE aspects in areas that did not avalanche during the previous deep slab avalanche cycle. Some deep wet slab avalanches may also become possible tonight or tomorrow on other aspects especially in areas where the snowpack rests on surfaces like granite slabs. Any slab avalanches that occur during this storm will involve wet snow.

    This kind of avalanche problem is difficult to predict and a high amount of uncertainty is associated with it. What is certain is that if one of these avalanches does occur and a person is invovled, the consquences would be very serious. Due to the combination of severe consequences and high uncertainty backcountry travel in avalanche terrain or in the runout zones of steep slopes is not recommended during this storm.

Avalanche Discussion

* Spring snow conditions with a few inches of wet corn snow resting on top of a supportable melt-freeze crust existed on S-SE aspects of Red Lake Peak by 11 am yesterday. At the lower elevations (6400 to 7400 ft) of Johnson Canyon (Donner Summit area) punchy, unsupportable boot top deep wet snow existed on NW-N-NE-E-SE-S aspects by noon yesterday. 

* Observations targetting the Feb. facet layer on Silver Peak, Jakes Peak, and on Elephants Hump this week have shown that this layer remains loose and that fractures could still travel along the layer if it breaks. Data has also indicated that it would take a very large trigger (like maybe 2.5 to 4 inches of rain?) to get enough force through the strong snow above it to break this layer. This layer was responsible for the widespread deep slab avalanche cycles in March.

Regions covered