Human Triggered Persistent Slab

CB Avalanche Center2019-20 Observations

Location: Crested Butte Area
Date of Observation: 01/16/2020
Name: Evan Ross
Subject: Human Triggered Persistent Slab
Aspect: East, South East, South
Elevation: 9,000-11,000

Avalanches: Human Triggered a large Persistent Slab Avalanche on a cross-loaded east facing slope at 10,800ft. The slab average 1.5ft thick, was F to 4F hard, and failed in 1.5 to 2mm NSF on the 1/8 interface. A few parts of the avalanche gouged deeper into the snowpack. AMc-SS-R1-D2-O

Weather: Increasing clouds in the morning becoming overcast. Calm winds. Weather stations showed temperatures rising to near 30 at 11,000ft.

Snowpack: Headed to Evan’s Basin to check on recent avalanche activity. I would estimate that many of these slabs ran in the storm snow on non-persistent grains given their size and propagation. Digging near one of those crowns on an east-facing slope, produced a clean shovel shear tests within the recent snow on non-persistent grains. At that test profile site, the elevations was 11,150, east aspect, slope angle was about 34 degrees, and the HS was 120cm. The 1/8 interface consisted of 1mm rounding faceted grains. CT18 RP on the 1/8 interface. The rest of the snowpack in this location wasn’t concerning without a big loading event that could break deeply into that snowpack.

The triggered avalanche was on the same aspect as the above test slope. The differences was a slightly lower elevation, but mainly a cross-loaded slope. The slab above the 1/8 interface was more cohesive, and the interface was noticeably weaker. I’m not 100% sure why the interface was so different between the two locations. My best guess is that the snowpack were the avalanche was triggered, had a below average snowpack height previous to the last wind event.