Wind Slabs and Storm Slabs on Castle and Andesite

Location Name: 
Castle Peak and Andesite Ridge
Region: 
Donner Summit Area
Date and time of avalanche (best estimate if unknown): 
Sat, 12/22/2012 - 14:30
Location Map: 
United States
39° 21' 2.2608" N, 120° 21' 56.0736" 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: 
2 ft
Aspect: 
Northeast
Weak Layer: 
Storm Snow
Avalanche Width: 
350ft.
Terrain: 
Near Treeline
Elevation: 
8 100ft.
Bed Surface: 
Storm Snow
Avalanche Length: 
300ft.
Number of similar avalanches: 
4
More detailed information about the avalanche: 

2-3 feet of new snow fell since yesterday in the Castle Peak  area. The strong winds formed thick and fragile wind slabs on any wind loaded slopes. Skier triggered avalanches resulting from the failure of these wind slabs occurred on wind loaded SE-E-NE-N-NW facing test slopes in below and above treeline terrain today. Many of these slides were remotely triggered failed when skier cam within 20 ft of the slope. Long shooting cracks and collapsing also occurred in wind affected areas. The wind slabs measured between 6 inches (on the less wind loaded SE aspects) and 2-3 ft (on the heavily loaded NE aspects). They all failed on weaknesses within the storm snow or on the density change buried on Dec. 17th. None of them stepped down to the rain crust or to the Dec. 2nd facets.

Photos 1 & 2: The largest remotely triggered avalanche. This slide occurred on a 38 degree, NE facing slope in near treeline terrain. The GPS coordinates attached to this observation mark the location of this avalanche. It was triggered by two skiers standing next to each other on the the ridge above. They were about 10 ft away from the edge of the ridge when the slope released on the other side of the ridge.

Photo 3 & 4: Smaller but similar slides on Andesite Ridge remotely triggered by a skier on the ridge.

 

Photo 5: Skier triggered shooting cracks on a wind loaded test slope.

Photo 6: A small remotely triggered test slope on a SE aspect on Castle Peak at about 8200 ft. This test slope was triggered from where the pole is in the photo.

In sheltered below treeline terrain ski cuts on steep test slopes resulted in soft slab releases 6-12 inches deep that failed within the new snow. Widespread skier triggered shooting cracks occurred in these sheltered areas as well. Hand pits in sheltered below treeline terrain also indicated that both the storm snow weaknesses and the Dec 17th density change mentioned above could fail today.

Photo 7: A below treeline sheltered slope where a soft storm slab fractured and only moved about 2 ft down slope due to the shallow angle of the slope.

 

 

 

Snowpit or crown profile photo or graph: 
Avalanche Photos: 
Weather Observations
Blowing Snow: 
Yes
Cloud Cover: 
100% of the sky covered by clouds
Air temperature: 
Below Freezing
Wind Speed: 
Strong
Precipitation: 
Snow
Air temperature trend: 
Cooling
Wind Direction: 
Southwest
Accumulation rate: 
Greater than 1 in. per hour
More detailed information about the weather: