Avalanche forecast

Central Sierra Nevada

Sierra Avalanche Center
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Avalanche danger

Alpine
4 · High
Treeline
4 · High
Below
3 · Considerable
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Highlights

MODERATE avalanche danger will exist during the day due to wind slabs formed in the last 24 hours. The avalanche danger will quickly increase as the next storm impacts the forecast area starting this afternoon. The avalanche danger should rise to HIGH tonight night as wind slab and storm slab avalanche problems become widespread. Human triggered wind slabs will become very likely and storm slabs will become likely during the night. Travel in or near avalanche terrain is not recommended.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Certain
    Size
    2

    The wind slabs that have formed in the last 24 hours will remain fragile today and human-triggered wind slab avalanches will remain possible.  As more snow and wind start to impact the forecast area this afternoon, wind-loading will add wind slabs on top of the existing ones and wind slabs will quickly grow much larger and more widespread. Wind slab avalanches will also become much easier to trigger as the new snow and wind load slopes faster than the wind slabs can bond to the existing snowpack. Natural wind slab avalanches will become likely tonight. Human triggered wind slab avalanches will become very likely tonight and into tomorrow.  Expect wind slabs on wind-loaded near and above treeline NW-N-NE-E-SE aspects. Some of these wind slabs avalanches could be large and destructive. 

    Look for signs of recent wind slab avalanches, blowing snow, cornices, cornice formation, and wind pillows.  Avoid steep wind-loaded terrain and run out zones below corniced slopes.

  • Loose Dry

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    2

    Storm slab avalanches will not become an issue until later this afternoon or during the night tonight. Changing conditions during the storm will create weak layers within the storm snow. Snowfall rates that exceed 2 in/hr during the storm will cause the new snow to accumulate too fast for those weak layers to adjust and storm slabs will become an avalanche problem. Expect human triggered storm slab avalanche to become likely during the night. 

    Look for signs of cracking or cohesion of the new storm snow.  Hand pits could provide a good tool for looking at the bonding of the new storm snow.  Avoid slopes with signs of unstable snow.

Avalanche Discussion

* Fragile wind slabs existed on wind-loaded slopes near the ridgelines yesterday. 2 natural wind slab avalanches were reported on the far east ridge of Tamarack Peak in the Mt. Rose area. On Castle Peak (Donner Summit area) and on Powderhouse Peak (Luther Pass area) ski kicks could trigger wind slabs from lower angle terrain above or close to the steeper wind-loaded slopes. On Castle Peak, they measured up to 2 ft in depth and up to 16 inches in depth on Powderhouse Peak.

* Skier-triggered shooting cracks occurred in these areas on lower angle wind-loaded slopes as well.

* Below yesterday's new snow a thin rain crust existed on Tamarack Peak and a layer of rain-soaked snow existed on Castle Peak and Powderhouse Peak. Observations indicated good bonding between these layers.  

* Snowpit tests and data on Powderhouse Peak targetting the old persistent weak layer buried about 3 ft below the surface showed significant gains in strength and did not reveal evidence of continuing instability associated with this layer. 

Observations and data targetting the persistent weak layer responsible for the deep slab avalanche cycle have shown significant increases in that layer's strength and in the strength of the snow above that layer. Tests on that layer across the forecast area including several comparison tests in places where this layer failed and had clearly unstable test results now do not indicate signs of continued instability.

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