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 due to possible persistent slabs on high elevation NW-N-NE aspects on slopes steeper than 32 degrees. While avalanches may have become harder to trigger, large destructive human-triggered avalanches may still be possible today. In addition to the avalanche danger, firm icy slopes may exist on all aspects up to 9000 ft. due to a frozen rain crust.

Avalanche problems

  • Deep Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    3

    Data indicates that the persistent weak layer of weak sugary snow (facets) still exists near the base of the snowpack on N aspects and some NW and NE aspects. Below 9000 ft. on the Sierra Crest triggering a failure on this layer has become unlikely due to the strong thick rain crust on the surface. Above 9000 ft. on the Sierra Crest and in the Mt. Rose area triggering an avalanche on this layer has become more difficult as the upper snowpack has become more supportable. However, a person's weight above a trigger point where a shallower snowpack exists or a larger/deeper trigger could possibly still release a persistent slab avalanche. Trigger points could include areas near rocks or cliffs, areas where a shallow snowpack exists, buried stumps, trees, convex rollovers, and areas where the weak layer is weakest. Large/deep triggers could include cornice failures, multiple people on a slope, stuck snowmobiles, or even boot-packing. Any persistent slab avalanches that occur would have very serious consequences and would likely be unsurvivable. The persistent weak layer now has anywhere from 2 to 4 ft. of snow on top of it that can act as a slab layer. Today's additional 1 to 2 inches of new snow should not add enough weight to change this avalanche problem, but as more snow accumulates tonight it could become easier to trigger.

    Due to the consequences associated with the large destructive avalanches that could occur if the persistent weak layer fails and the high degree of uncertainty associated with it, avoiding avalanche terrain where this layer might exist is recommended. These avalanches could connect multiple start zones, could be triggered remotely, and could run farther than expected. When a persistent weak layer like this exists, most informal observations will not provide reliable information about slope stability. Since these avalanches may not fail until someone hits the right spot on the slope, multiple tracks could exist on the slope before it slides. Snowpits can determine if the layer exists and tests can help to see how weak it is, but remember, snowpit work never proves stability, it shows potential signs of instability. Avoiding NW-N-NE aspects steeper than 32 degrees is the simplest strategy for dealing with this persistent weak layer, especially if one has an aversion to or insufficient knowledge of snowpit work.

Avalanche Discussion

Yesterday observations from Tamarack Peak and Chickadee Ridge both found that the persistent weak layer still exists near the bottom of the snowpack. The new snow above this layer continues to consolidate. Snowpit tests indicated that this layer has become more difficult to trigger especially where a deeper snowpack exists. However, if this layer does break the resulting fracture can still travel along the persistent weak layer. These observations both came from N aspects above 9000 ft. On Tamarack Peak a semi-supportable rain crust existed on the snow surface up to 9000 to 9200 ft. with cold soft consolidated snow on the surface above that elevation. 

Observations on Castle Peak along the Sierra Crest showed a 6 to 10-inch thick frozen rain crust on all aspects up to at least 8800 ft. and likely up to around 9000 ft. based on the sunlight glinting off the north facing slopes on Donner Peak, Anderson, and Tinkers Knob. Getting force through this ice crust would be as difficult as arresting a fall on it.  

Even though the 1 to 2 inches of new snow predicted to fall above 7000 ft. today should not be enough to change the avalanche danger, the avalanche danger will increase tonight as more snow and wind impact the forecast area. New wind slabs may start to form this afternoon as new snow and wind impact the forecast area. These wind slabs will continue to grow overnight as the bulk of the storm impacts the region and human-triggered wind slabs could become a problem during the night. These wind slabs will be resting on a mix of consolidated snow and frozen ice crusts. In more sheltered areas, new snow resting on icy surfaces could also represent a problem during the night or for tomorrow. Tonight's new snow may also push the persistent slab problem to a deep slab problem as the persistent weak layer gets buried deeper in the snowpacks. Check back for the updated avalanche advisory in the morning before heading out into the backcountry tomorrow. 

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