Several natural avalanches and one skier triggered avalanche on Andesite Ridge

Location Name: 
Andesite Ridge
Region: 
Donner Summit Area
Date and time of avalanche (best estimate if unknown): 
Sun, 03/06/2011 - 11:00
Location Map: 
United States
39° 21' 1.8432" N, 120° 21' 57.8016" W
See map: Google Maps
US


Red Flags: 
Recent avalanche activity
Whumphing noises, shooting cracks, or collapsing
Recent loading by new snow, wind, or rain
Rapid warming
Obvious avalanche path
Observation made by: Forecaster
Avalanche Observations
Avalanche Type: 
wet,slab,
Slope: 
38degrees
Trigger type: 
Natural
Crown Height: 
1 ft
Aspect: 
Northeast
Avalanche Width: 
800ft.
Terrain: 
Near Treeline
Elevation: 
8 200ft.
Bed Surface: 
Storm Snow
Avalanche Length: 
300ft.
Number of similar avalanches: 
5
More detailed information about the avalanche: 

Several natural avalanches occurred within the new snow on Andesite Ridge during the day today. These slides all seemed to have failed due to rain falling on the new snow. They all entrained all of the snow from the last 24 hours. Most of these slides occurred on N-NE facing wind-loaded slopes. One of them occurred on a NE facing non-wind loaded slope. Their slope angles ranged from 35 degrees to 40 degrees and crown depths ranged from 8-14 inches. They occurred between 7500 ft and 8200 ft. The largest of the slides was about 800 ft wide while the smallest was about 40 ft wide.

Several skier triggered shooting cracks occurred while touring today especially on wind loaded slopes or near some of the natural avalanches.

I triggered one avalanche similar to the natural avalanches. I triggered this slide from about 10 ft away while standing on a ridge near a wind-loaded, NE facing test slope.

Photo 1: The largest natural avalanche I saw. It occurred at the coordinates listed on this observation.

Photo 2: Another natural avalanche at closer to 7600 ft on a NE facing, wind-loaded slope. This one is slightly smaller than the ones at higher elevations.

Photo 3: The natural avalanche that occurred on a non-wind-loaded slope

Photo 4: Skier triggered shooting cracks

Photo 5: The remotely triggered avalanche

Avalanche Photos: 
Weather Observations
Blowing Snow: 
No
Cloud Cover: 
100% of the sky covered by clouds
Air temperature: 
Above Freezing
Wind Speed: 
Light
Precipitation: 
Rain
Air temperature trend: 
Static
Wind Direction: 
Southwest
Accumulation rate: 
Less than 1 in. per hour
More detailed information about the weather: 

6-12 inches of new snow in this area. Continuous light rain during the afternoon up to 8200 ft.