Test slope failure on Silver Peak

Location Name: 
Silver Peak
Region: 
Cabin Creek, Deep Creek, or Pole Creek Area
Date and time of avalanche (best estimate if unknown): 
Thu, 12/23/2010 - 13:15
Location Map: 
United States
39° 13' 13.5048" N, 120° 14' 33.8748" W
See map: Google Maps
US


Red Flags: 
Recent loading by new snow, wind, or rain
Obvious avalanche path
Observation made by: Forecaster
Avalanche Observations
Avalanche Type: 
dry,slab,
Slope: 
40degrees
Trigger type: 
Other - explain below
Crown Height: 
Less than 1 ft
Aspect: 
Northeast
Weak Layer: 
Storm Snow
Avalanche Width: 
30ft.
Terrain: 
Near Treeline
Elevation: 
8 000ft.
Bed Surface: 
Storm Snow
Avalanche Length: 
100ft.
More detailed information about the avalanche: 

4-10 inches of soft snow existed on the snow surface in the Silver Peak area today. Hand pits and ski cuts on non-wind-loaded test slopes did not result in any failures above or below treeline. Layer bonding tests indicated that the new snow is bonding well to the older snow. Ski cuts and dropping oven to refrigerator sized pieces of cornices onto wind-loaded slopes less than 38 degrees in steepness also did not result in any failures above or below treeline.

In wind-loaded near treeline areas, oven-sized cornice pieces dropped onto the slopes below them resulted in surface sluffs that entrained the new snow. Larger pieces of cornices (refrigerator-sized) dropped onto more wind-loaded, steeper (38-40 degrees) portions of the same slope resulted in a small 4-6 inch soft slab avalanche described in the first video.

On wind loaded 40-44 degree test slopes above treeline, similar sized cornice pieces triggered slightly deeper soft slab avalanches (up to 12 inches). The cornices in the above treeline terrain were much more fragile and broke farther away from their edges. (Video 2).

A sun crust had started to develop on the southerly aspects below 7600 ft by mid afternoon.

Photo: Evidence of rain up to 8300 ft in the Silver Peak area during the last storm.

 

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

20101223silver 011

20101223silver 014

Weather Observations
Blowing Snow: 
Yes
Air temperature: 
Above Freezing
Wind Speed: 
Light
Precipitation: 
Air temperature trend: 
Warming
Wind Direction: 
Southwest
Accumulation rate: 
More detailed information about the weather: 

The wind was transporting small amounts of snow today near ridgelines.