Small Windslab Avalanche near Grouse Rock

Location Name: 
North east of Grouse Rock
Region: 
Blackwood Canyon or Ward Canyon Area
Date and time of avalanche (best estimate if unknown): 
Tue, 12/16/2014 - 13:23
Location Map: 
United States
39° 8' 31.884" N, 120° 14' 15.738" W
US


Red Flags: 
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: 
Less than 1 ft
Aspect: 
Northeast
Weak Layer: 
Other - explain below
Avalanche Width: 
300ft.
Terrain: 
Near Treeline
Elevation: 
7 500ft.
Bed Surface: 
Storm Snow
Avalanche Length: 
75ft.
Number of people caught: 
1
More detailed information about the avalanche: 

First the objective: our party skied a few laps on the protected NE aspects off of Grouse Rock.  We observed around 10-15cm of snow was deposited about 150 feet downslope from the ridgeline near Grouse around 8300', and closer to 15-25cm of snow was deposited in the more protected areas around 7600' based on observations on 12/15 and 12/16 both around 9am in the same 2 locations.  On our last descent, a skier on a 33-35 degree slope east aspect triggered a 6-12" avalanche that propagated above both above the skier and laterally along a small convex bench.  The skier was able to ski out of the slide uninjured.  Based on the depth and the location I would consider this a storm wind slab.  The bed surface was a storm layer in the Dec 15/16 storm snow.  Also of importance: the map geotag is very approximate - I do not have exact GPS coordinates.

Now the subjective: After a few laps in the NE tree aspects near Grouse Rock we traveled north and ascended in the trees a more E/SE aspect (my apologies for not knowing the names).  We found far more scoured wind affected snow on this aspect and decided to descend/traverse the E facing slope around towards a more protected E/NE aspect.  Also while ascending we noticed wind crusts and lots of snow movement off of this SE slope.  This top crust was very reactive and created little cracks while breaking trail.  Where snow was softer and without the crust our informal hand shear tests revealed poor snow bonding on a old snow new snow interface 4-6 inches from the surface.  Based on this we decided to ski a very conservative low angle contour heading south back to the NE/E aspect we had been skiing all day for another lap.  We found slightly softer snow as we traversed to a more E/NE aspect but when descending a minor notch in a subtle bench a skier triggered the aforementioned avalanche.  The avalanche was not very deep but could've easily knocked the skier off of their feet and pushed them in the trees below causing injury.  It was also noteworthy how reactive this layer was, and how far it propagated across the slope.  Our group was extremely leery of cornices and wind slabs near the ridgeline but failed to heed necessary caution of wind loaded exposed features in near treeline mid-slope terrain.  As 2-6 more inches are predicted tonight I figured this was worth submitting!  Thanks SAC for all you do!

Avalanche Photos: 
Weather Observations
Blowing Snow: 
Yes
Cloud Cover: 
75% of the sky covered by clouds
Air temperature: 
Below Freezing
Wind Speed: 
Moderate
Precipitation: 
Snow
Air temperature trend: 
Warming
More detailed information about the weather: 

Wind gusts below the ridgeline were swirling throughout the day from west to south with a generally constant southwest wind.