Cawley

Salt Lake » Big Cottonwood Canyon » Days Fork » Banana Days

Banana Days

10,300'

North

38°

Skier

Unintentionally Triggered

Soft Slab

Persistent Weak Layer

Facets

2'

60'

600'

Toward the end of a long day of spelunking I triggered what initially seemed like a storm snow avalanche at the top of a short, steep, rock-lined gully in upper Banana Days. I was deliberately ski cutting the top of this slope when a 10' wide, 1' deep, very soft slab released at my feet, running into the choke of the gully below me. Below the narrowest portion of this slope, the weight of the debris released a wider avalanche below cliff bands on either side of the narrows. On the skier's right side of this sympathetic release, the slab failed in pooled graupel that likely accumulated during one of the graupel events in the latter half of this week, and the crown in this portion of the slide was about 16 inches deep; on the skiers left side, a 20-foot wide, 2' soft slab failed on the faceted snow that sat atop the snowpack for several weeks in mid/late November. The debris from this slide entrained quite a bit of snow, ran until the slope angle decreased significantly, and could have buried a person. Although it was a crowded day in Silver/Days, nobody was in the area when this slide occurred. This slide occurred during what was likely the peak of instability during a day of moderate-heavy snowfall. Several density changes were noted throughout the day, with heavily rimed flakes and clean, angular stellars falling in consecutive periods. What initially seemed like a short, manageable portion of complex terrain worth navigating in order to harvest the pow-pow below, was in immediate hindsight a perfect spot to activate a couple of distinct buried weak layers. The "November Facets," which have lacked a sufficient load to propagate clean shears in snowpit tests, showed their potential with the help of significant shock-loading, and I would not be surprised if additional snow-n-blow overnight makes avalanches failing on this layer-as well as wind slabs and storm snow avalanches failing on buried graupel or various crusts-easier to trigger tommorow. I would consider midslope terrain features, on NW-N-NE facing slopes above about 10,000', which may have been sheltered during recent wind/rain events, to be most suspicious for possible persistent slab avalanches. I reported this slide to Alta Central, and it was in turn reported to the UAC, as a 1' deep, 50 foot wide avalanche. Drew's snowpit is an accurate representation of the snowpack I found in the crown of this slide.