Wind and Storm slab avalanches on Granite Chief Peak

Location Name: 
National Geographic Bowl, Granite Chief Peak
Region: 
Cabin Creek, Deep Creek, or Pole Creek Area
Date and time of avalanche (best estimate if unknown): 
Sat, 02/18/2017 - 11:00
Location Map: 
United States
39° 11' 54.1392" N, 120° 17' 22.5852" 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
Observation made by: Guide at Alpenglow Expeditions
Avalanche Observations
Avalanche Type: 
Dry
Slab
Slope: 
38degrees
Trigger type: 
Skier
Crown Height: 
1 ft
Aspect: 
Northwest
Weak Layer: 
Storm Snow
Avalanche Width: 
150ft.
Terrain: 
Near Treeline
Elevation: 
8 800ft.
Bed Surface: 
Storm Snow
Avalanche Length: 
400ft.
Number of people caught: 
0
Number of partial burials: 
0
Number of full burials: 
0
More detailed information about the avalanche: 

Strong SW wind and large amounts of wind transported snow moving about on the upper reaches of Granite Chief peak today. Moving down the NW ridge, we were able to drop a piece of cornice onto the windslab below, which triggered a small slab avalanche (3-6" crown, 30' wide).  This small slide ran down into steeper terrain below and triggered a much larger slab avalanche (10-18" crown, 250' wide, running full track into the valley below.  The start zone of this larger slide averaged 38 degrees and has an obvious convex rollover where the failure initiated. The smaller slide travelled approx 200 before triggering the second slide. We were able to get closer for observations by skiing the bed surface of the slide. The debris was easily big enough to bury a person. 

(All photos tweaked due to very low vis conditions)

Photo #1- Cornice drop and resulting small windslab avalanche

Photo #2- Larger slab avalanche triggered by the small slide above. 

Photo #3- Shooting crack and crown near flank of slide. 

The failure layer appeared to be a density change in the new storm snow. The slab that failed was sitting on a noticeably lower density layer of new snow, and the bed surface was soft and skiable.  

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

Howling winds, intense blowing snow and low vis up high, calm and better vis below 8k.