Large Cornice Triggered Wind Slab Avalanche on Judah

Location Name: 
Mt. Judah
Region: 
Donner Summit Area
Date and time of avalanche (best estimate if unknown): 
Tue, 03/15/2016 - 10:00
Location Map: 
United States
39° 17' 49.488" N, 120° 18' 39.204" W
See map: Google Maps
US


Red Flags: 
Recent avalanche activity
Recent loading by new snow, wind, or rain
Obvious avalanche path
Observation made by: Forecaster
Avalanche Observations
Avalanche Type: 
Dry
Slab
Slope: 
38degrees
Trigger type: 
Other - explain below
Crown Height: 
3 ft
Aspect: 
Northeast
Weak Layer: 
Other
Avalanche Width: 
500ft.
Terrain: 
Near Treeline
Elevation: 
7 700ft.
Bed Surface: 
Other - explain below
Avalanche Length: 
300ft.
Number of similar avalanches: 
5-10
More detailed information about the avalanche: 

This wind slab avalanche occurred on a NE-E facing 38 degree wind loaded slope at 7700 ft. The trigger looked to be a large piece (mini van sized) of cornice that had broken off and fallen onto the wind slab below. The slab broke on a storm snow weakness about 65 cm below the surface and then stepped down to a run on a crust at the base of the recent snow about 85 cm below the surface. This slide was a D3 and could have easily buried a car. The debris accumulated in a trees below the slope, and the slide did knock down several 15 to 25 cm diameter trees. This slide likely occurred sometime between Monday (March 14) afternoon and Tuesday (March 15) based on the sharpness of the debris and the fact that there was very little evidence of storm snow covering the debris. Numerous other wind slab avalanches had occurred during or just after the storm on Mt. Judah and on the terrain along the Sierra Crest south towards Tinker's Knob. Tests and observations on this avalanche and other areas around Mt. Judah during the day on March 17 indicated that the snowpack had gained strength and that the weaknesses present at the time of this slide had been assimilated into the snowpack by the 17th. See more info from the 17th here. 

Photo 1:  Overview of the avalanche showing the two-tiered crown

Photo 2 and 3: Overview of the debris pile

Photo 4: Looking up from the debris in the trees towards the likely spot where the large cornice broke to trigger the slide.

Photo 5: ECTN 30 at the crown of the avalanche on March 17th indicated that fractures propagating through this snowpack have become unlikely. 

Avalanche Photos: 
Weather Observations
Air temperature: 
Precipitation: 
Air temperature trend: 
Wind Direction: 
Accumulation rate: 
More detailed information about the weather: