Collapse of faceted layer at Elephant's Back

Location Name: 
Elephant's Back
Region: 
Carson Pass Area
Date and time of avalanche (best estimate if unknown): 
Wed, 12/12/2012 - 12:00
Location Map: 
United States
38° 40' 48.4176" N, 119° 58' 59.5524" W
See map: Google Maps
US


Red Flags: 
Whumphing noises, shooting cracks, or collapsing
Recent loading by new snow, wind, or rain
Obvious avalanche path
Terrain Trap
Observation made by: Professional Observer
Avalanche Observations
Avalanche Type: 
dry,slab,
Slope: 
35degrees
Trigger type: 
Skier
Crown Height: 
1 ft
Aspect: 
Northeast
Weak Layer: 
Storm Snow
Avalanche Width: 
30ft.
Terrain: 
Above Treeline
Elevation: 
9 200ft.
Bed Surface: 
Storm Snow
Avalanche Length: 
15ft.
Number of similar avalanches: 
1
More detailed information about the avalanche: 

While two of us stood at edge of Elephant's Back windloaded zone (disturbance of snow in foreground of pic 1), the faceted layer beneath recent surface crust collapsed at least halfway across upper bowl of Elephant's Back. This loud collapse dropped the slope by ~3cm and triggered a small slab avalanche that failed on a density change within the storm snow (just behind disturbance in foreground), and a larger but shallower slide that displayed both slab and loose dry characteristics (a shallow crown, but no visible flanks or stauchwall and loose debris). The debris is obvious in full sized pic, but the crown is not (click on full sized image and then enlarge). Crown zippered between the first two rock formations and was ~15cm deep by 70m wide. Neither of these avalanches was large enough to harm a person. The mid-slop bench at Elephant's Back is still quite prominent and may have prevented slope-wide release at the faceted layer.

The most windloaded portion of Elephant's Back, where the small slab occured, held 50cm of 1F hard storm snow bonding well to the old crust, with another 20-30cm of upside down Fist to 4F hard snow at the surface. Just 20m downslope of this area, no wind slab was observed, with 15cm of storm snow and instabilities limited to loose dry - again failing on a density change within the storm snow and not at the crust/storm snow interface. This was consistent with other observations on non-wind loaded test slopes along Frog Lake Ridge.

Pic 1 was altered to better show details.

Pic 2: Shovel blade rests on previous surface crust, ski pole rests on bed surface of the small slab in pic 1.

 

Avalanche Photos: 
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: 
Static
Wind Direction: 
Southwest
Accumulation rate: 
Less than 1 in. per hour
More detailed information about the weather: 

Consistent snowfall throughout day, approaching 1" per hour but very low density.