Wind Slabs and Deep Persistent Weak Layers on Andesite Ridge

Location Name: 
Andesite Ridge
Donner Summit Area
Date and time of observation: 
Tue, 12/22/2015 - 11:00
Location Map: 
United States
39° 20' 59.3628" N, 120° 21' 55.656" W

Red Flags: 
Recent avalanche activity
Whumphing noises, shooting cracks, or collapsing
Recent loading by new snow, wind, or rain
Obvious avalanche path

Observation made by: Forecaster
Snowpit Observations
More detailed information about the snowpack: 

70-85 cm of new heavy wet snow had accumulated above the old snow surface in the Andesite Peak area between 7000 and 8200 ft. A layer of heavier wetter snow that fell towards the end of the storm existed on the top of the snowpack. This layer was also becoming saturated as rain continued to fall on the snowpack throughout the day. 30 to 35cm below this heavy layer a softer less dense layer of storm snow existed. This upside down snowpack structure was widespread in this area. Below the new snow the 12/13 rain crust still exists and snowpit tests indicated that fractures could still propagate along this deeply buried persistent weak layer. 

In wind loaded terrain large cornices had formed above the leeward slopes. These cornices would easily fail with ski kicks and would fail several feet away from the edge of the cornice. One refrigerator sized cornice piece dropped onto a wind loaded test slope did trigger a wind slab failure that broke on the layer of softer snow mentioned above. 

Photos 1-3: 1-2ft deep wind slab failure on a NE facing, 39 degree, wind loaded test slope in near/above treeline terrain. The trigger was a refrigerator sized cornice piece dropped onto the slope.

Photo 4: Rain runnels in a meadow at 7500 ft. 


Snowpit or crown profile photo or graph: 
Snowpack photos: 
Weather Observations
Blowing Snow: 
Cloud Cover: 
100% of the sky covered by clouds
Air temperature: 
Above Freezing
Wind Speed: 
Air temperature trend: 
Wind Direction: 
Accumulation rate: 
Less than 1 in. per hour
More detailed information about the weather: 

Snow level was hovering between 7900 and 8200ft. Rain was heavy at times.