Natural Avalanche Activity on Andesite Ridge

Location Name: 
Andesite Ridge
Region: 
Donner Summit Area
Date and time of avalanche (best estimate if unknown): 
Sun, 02/09/2014 - 02:00
Location Map: 
United States
39° 21' 0.9072" N, 120° 21' 48.2976" W
See map: Google Maps
US


Red Flags: 
Recent avalanche activity
Recent loading by new snow, wind, or rain
Rapid warming
Obvious avalanche path
Terrain Trap
Observation made by: Forecaster
Avalanche Observations
Avalanche Type: 
Dry
Slab
Slope: 
37degrees
Trigger type: 
Natural
Crown Height: 
3 ft
Aspect: 
Northeast
Weak Layer: 
Storm Snow
Avalanche Width: 
100ft.
Terrain: 
Near Treeline
Elevation: 
7 880ft.
Bed Surface: 
Storm Snow
Avalanche Length: 
200ft.
Number of similar avalanches: 
2
More detailed information about the avalanche: 

Two large natural avalanches were observed on near treeline NE facing slopes on Andesite Peak. Judging by the amount of snow on the debris and bed surfaces of these avalanches they occurred sometime last night. The low angle slopes connected to these avalanches were also fractured but on these lower angle slopes the snow remained in place. A snowpit showed that the failure layer existed about 70 to 100 cm (2 to 3 ft) below the surface. In the pit profile is showed up as a change from harder snow to much softer snow. Based on the widespread fractures near the avalanches and their location some kind of persistent weak layer was likely the actual failure layer. Buried surface hoar, facets near the thin rain crust seen in this area a few days earlier, or maybe near surface facets? due to the wet and rainy conditions we were not able to determine if these kind of layer existed. Tests at the crown of one of these avalanches showed that whatever the failure layer was, fractures could still propagate along that layer.

Some other loose wet snow instabilities also occurred at all elevations in this area today due to rain falling on the snow up to at least 8200 ft.

Photo 1 and 2: The larger of the two avalanches. The snowpit is from this crown. The crown measured about 3 ft deep.

Photo 3: A smaller avalanche on an adjacent slope.

Snowpit or crown profile photo or graph: 
Avalanche Photos: 
Weather Observations
Blowing Snow: 
No
Cloud Cover: 
100% of the sky covered by clouds
Air temperature: 
Above Freezing
Wind Speed: 
Light
Precipitation: 
Mixed rain and snow
Air temperature trend: 
Warming
Wind Direction: 
Southwest
Accumulation rate: 
Less than 1 in. per hour
More detailed information about the weather: 

Wet snow and rain continued to fall today. Rain was falling at pass level at 10am and snow levels climbed to at least 8200 ft by noon with rain falling on the summit of Andesite Peak.