Skier triggered storm slab on Grouse

Location Name: 
Lowest open pitch lookers right of traditional skin track
Region: 
Blackwood Canyon or Ward Canyon Area
Date and time of avalanche (best estimate if unknown): 
Sat, 12/24/2016 - 11:15
Location Map: 
United States
39° 8' 22.5348" N, 120° 13' 58.3428" 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
Terrain Trap
Observation made by: Public
Avalanche Observations
Avalanche Type: 
Dry
Slab
Slope: 
35degrees
Trigger type: 
Skier
Crown Height: 
1 ft
Aspect: 
North
Weak Layer: 
Storm Snow
Avalanche Width: 
250ft.
Terrain: 
Below Treeline
Elevation: 
7 200ft.
Bed Surface: 
Old Snow
Avalanche Length: 
100ft.
More detailed information about the avalanche: 

Though visibility was limited this morning, we observed one natural slab avalanche along the ridge north of Grouse right below a cornice. There was also a small natural avalanche south of the Grouse skin track on a convex rocky rollover in a open area at about 8000 ft elevation. Snow was reactive and seemed poorly bonded at the old snow new snow interface throughout the morning. In the terrain north of the skin track observed whomping and collapsing, particularly in more open areas. I remotely triggered a very small avalanche on a convexity at about 7600 ft elevation north of the skin track. This failed on a density change within the storm snow. 

The avalanche in the photos and description was triggered by another party. According to the person who triggered it, he was 25-40 yards above and to the skiers left of the start zone when he triggered the slide, in the flatter terrain on top of the ridge. Visibility was mediocre as it was snowing fairly hard as we traversed below the slide. From what I could see, it seemed most of the slope had slid and the slide connected over rocky areas. The bed surface appeared to be mostly the new snow old snow interface, with a few smaller sections failing on a density change within the now snow. 

Avalanche Photos: 
Weather Observations
Blowing Snow: 
No
Cloud Cover: 
100% of the sky covered by clouds
Air temperature: 
Below Freezing
Wind Speed: 
Light
Precipitation: 
Snow
Air temperature trend: 
Static
Wind Direction: 
Accumulation rate: 
Greater than 1 in. per hour
More detailed information about the weather: 

Weather alternated between light flurries and fairly heavy snow throughout the morning. Winds were light and temps were 20 when we returned to the car.