Natural Avalanche on Upper Sherwin/The Perch headwall

AVALANCHE OBSERVATION
The Perch

Drainage:

Submission Info
Member
Friday, February 10, 2017 - 3:30pm
Red Flags: 
Recent avalanche activity
Recent loading by new snow, wind, or rain
Avalanche Type: 
Slab
Slope: 
40degrees
Trigger type: 
Natural
Crown Height: 
1 ft
Aspect: 
Northeast
Weak Layer: 
Storm Snow
Avalanche Width: 
300ft.
Terrain: 
Above Treeline
Elevation: 
10 100ft.
Bed Surface: 
Old Snow
Number of similar avalanches: 
1
More detailed information about the avalanche: 

Natural avalanche of the new storm snow across the majority of the small upper headwall on the ridge at the top of the Perch in the Sherwins. I spotted the crown from afar as I skinned up the ridge solo. Once there, I was able to safely approach on the bed surface, which was the harder old snow surface from before Thursday Night/Friday's storm with ~5 cm of new snow from daytime Friday on top. I did a few handpit tests on analogous unslid snow on the ridgeline and got a very easy shear about halfway (~25 cm) down in the ~50cm of new storm snow where there is an density interface - the first pit slid on isolation, the second with almost no force. This mid-slab interface matches with what we saw in a pit profile on Punta Bardini earlier in the day (see Josh's pit observation).

The slide itself released all the way to old snow (both layers of new snow), but looking at the point in the crown, it looks like a point release directly by the ridgline at the cornice that likely propagated across the slope to either side (my guess is that it started in the more sensitive top layer and stepped down to entrain all of the new snow).

Based on the amount of new snow on the bed surface, I'm guessing that this went naturally sometime in the late night hours or early morning Friday. I saw one similar-looking slide in the bowl further southeast, but it was too far away and in out of the clouds, so I wasn't able to get much detail on it.

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

Mostly calm except right up on the ridgeline. Winds were still light enough that there was not significant wind loading, even right up at the ridge - just enough wind to increase density of the top layer and form a bit of a soft slab.

By the late afternoon it was quite warm below 8500 - I didn't have a thermometer with me, but it was certainly close if not above freezing. Snow quality was correspondingly wet at these elevations.

The Perch 37° 36' 14.4108" N, 118° 58' 44.3388" W

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