Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
2 · Moderate
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

Moderate avalanche danger exists today due to deep slab and loose wet avalanche problems.  Triggering a deep slab avalanche is still possible on specific terrain features throughout most of the forecast area.  Evaluate terrain carefully and identify features of concern.

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    3

    A deep slab avalanche cycle has been occurring throughout the forecast area after starting during our last storm cycle.  Widespread, large, deep slab avalanches have occurred failing on a layer of faceted snow that formed in February.  Reports continue to come in from large avalanches that occurred during or just after the last storm cycle on NW-N-NE-E aspects at all elevations.  We have had at least 3 human triggered avalanche events with the last 2 happening on Sunday.  

    This layer of buried facets will eventually gain strength over time and become non reactive.  That process has already begun in some areas.  In other areas, especially places with a shallower snowpack, recent snowpack tests continue to show unstable test results.  In terrain that has a shallow snowpack or where the weak layer is closer to the surface would be places a backcountry user could effect the weak layer.  Since the weak layer is deep, tracks on a slope do not necessarily indicate stability.  Possible trigger points could be rocks, trees, cliff areas, extreme terrain, and convex rollovers.  Triggering a deep slab avalanche has become a low probability but high consequence event.

    Deep slabs are unpredictable and signs of instability may not be present.  The only effective management option is avoidance.  An avoidance strategy would mean staying off slopes steeper than 30 degrees and slopes connected to steeper terrain on NW-N-NE-E aspects.  Good recreation conditions currently exist on Northerly aspects that are below 30 degrees in steepness with no steeper avalanche terrain connected.  

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    As daytime warming occurs, loose wet avalanche activity will become possible on E-SE-S-SW-W aspects at all elevations.  Increasing cloud cover and winds this afternoon should limited the amount of loose wet activity.  Most loose wet activity should be in the form of roller balls and pinwheels, but some larger loose wet avalanches are also possible.

    Avoid steep sun exposed terrain as daytime warming occurs.

Avalanche Discussion

* 3 human triggered Deep Slab avalanches have occurred over the last 4 days.  On Saturday, a skier triggered deep slab occurred on Schallenberger Ridge (Donner Summit area).  On Sunday, 2 deep slab avalanches occurred.  One in Negro Canyon (Donner Summit area) while digging a snowpit in lower angle terrain, and one at Lost Lakes (Carson Pass area) triggered by a snowmobiler.  All of these deep slab avalanches were at least D3 or above, all had wide propagation, with crowns ranging from 4 to 8' with one reported up to 15 to 20' deep in spots.

* Snowpack tests yesterday on Elephants Hump (Carson Pass area) showed consistent unstable test results on the buried facet layer on a deeply fractured slope.  This slope fractured, but did not avalanche, with cracks that went 500' wide from trigger point thought to have occurred on Sunday.

* Check out the observation page for more details on the widespread deep slab avalanche cycle.

* Warming effects have been reported throughout the forecast area on E-SE-S-SW-W aspects.   A small loose wet avalanche was reported from Incline Lake Peak on a SE aspect at 8800'.

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