John and B

Salt Lake » Mill Creek Canyon » Wilson Fork » Wilson Glade

Wilson Glades

9,400'

Northeast

35°

Skier

Unintentionally Triggered

Hard Slab

Persistent Weak Layer

Density Change

3'

100'

400'

Skier triggered deep slab avalanche (Picture 1) on the mid-pack facets, it was the third skier on the slope. Slide was triggered by small air onto slope (Picture 2) from the skiers left rib into the left center of the second bowl (more northerly) in Wilson Glades. First skier was not part of group and had center punched steepest section of the bowl. Second skier was at the base of the slope and experience a loud settling and whump. Slide looks to have been triggered by the energy of third skier landing on the wind slab over 1/2 way down the 400' slope. The slab fractured approx. 100' across the slope to the south-east across the bowl. We reason that the 150' of slope above the trigger point then slid sympathetically. Bed surface was early December snow pack that had decomposed, but was supportable to a persons weight. A bit of stomping on the base surface showed that it was poorly bonded. Approximately 2.5-3' of the snow pack slid, while the total snowpack depth was 4' (Picture 3). The slab was the recent wet storm (Dec . 22nd) and was not particularly firm (4F) and roughly 15-18" thick (Picture 4). The rime layer was apparent within the slab and was approx. 3 mm thick. Weak layer was December near surface faceted snow (Picture 5), but showed some cohesion and was not typical "sugar snow". Majority of the slope was between 32-33 degrees with isolated pockets of 35 degree slopes. A number of the slide blocks remained on the slope attesting to the general low-angle of the slope. Slide initiated in a steep pocket showing that a little extra energy even in an isolated steep section among a lower angle slope can wake up the deep slab monster. Debris pile was 6-10' thick in the deepest parts and would have drug a caught skier through trees. Would have not been a pleasant rescue. Whole thing was a bit sobering and in retrospect we still feel good about the decision to ski in that area. This just brings to light how unstable the pack really is right now. Moving forward it is time to reassess what we consider "low angle".