Wind slabs, skier triggered cracking and small avalanche on Andesite Ridge

Location Name: 
Andesite Ridge
Region: 
Donner Summit Area
Date and time of avalanche (best estimate if unknown): 
Wed, 02/29/2012 - 15:00
Location Map: 
United States
39° 21' 0.4644" N, 120° 21' 55.4256" 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
Obvious avalanche path
Terrain Trap
Observation made by: Forecaster
Avalanche Observations
Avalanche Type: 
dry,slab,
Slope: 
38degrees
Trigger type: 
Skier
Crown Height: 
1 ft
Aspect: 
Northeast
Weak Layer: 
Storm Snow
Avalanche Width: 
50ft.
Terrain: 
Near Treeline
Elevation: 
8 100ft.
Bed Surface: 
Storm Snow
Avalanche Length: 
150ft.
Number of similar avalanches: 
1
More detailed information about the avalanche: 

This avalanche occurred due to a ski cut at the location noted in photo 1 above. It started as a shallow wind slab then stepped down to near the interface between today's new snow and the snow that fell on Monday. Other test slopes in the area cracked but did not slide in response to ski cuts.

15-20 inches of new snow has fallen in this area since last night. Hand pits, pole probes and test pit showed that the new snow is slightly more dense than the snow that fell on Monday. This data also showed that the interface between Monday's snow and today's snow remains weak. A snowpit near the skier triggered avalanche pictured above showed that the Jan. crust/facet combo also remains weak. Extended column tests showed that fractures can propagate through both of these weaknesses.

The strong winds had affected most open slopes both above and below treeline in this area. The thickest wind slabs existed on N-NE-E aspects in near and above treeline terrain.

Photo 1: The skier triggered avalanche described in this observation

Photo 2: A skier triggered crack on a slope similar to the one that slid.

Snowpit: From a near treeline location near the avalanche.

Video: Extended column test in the snowpit showing propagation at the interface between Monday's snow and today's snow.

Snowpit or crown profile photo or graph: 
Avalanche Photos: 
Avalanche observation video: 

Extended column test near skier triggered avalanche on Andesite Ridge

Weather Observations
Blowing Snow: 
Yes
Cloud Cover: 
100% of the sky covered by clouds
Air temperature: 
Below Freezing
Wind Speed: 
Moderate
Precipitation: 
Snow
Air temperature trend: 
Cooling
Wind Direction: 
Southwest
Accumulation rate: 
Less than 1 in. per hour
More detailed information about the weather: