22-23

Observer reported "A number of Avalanche Crowns on an east aspect at 9500 ft." in Tepee Basin.

An observer reported "A number of Avalanche Crowns on an east aspect at 9500 ft." in Tepee Basin

Surface hoar forming near grotto falls parking lot, on a tree but still indicative of area surface hoar. Photo: E. Jeannette

We dug three snowpits over the course of two days from 11/12 to 11/13 around Cooke City and found a weak layer 20-30cm above the ground at various aspects and similar elevations. The weak layer was composed of small rounding facets and was less than 1cm thick. The weak layer was at the interface between old and new snow. The old snow below the weak layer was composed of coarse, rounded/rounding faceted grains. The new snow above the weak layer ranged from 60-80cm in height and was right-side-up, progressing from fist hardness at the surface to one-finger immediately above the weak layer. Test results were as follows:

Goose Lake – NE aspect, 10,100 ft (45.11627, -109.92057): ECTP25, 80cm deep. HS 110cm

Scotch Bonnet – SW aspect, 9,800 ft (45.07038, -109.94489): ECTN (*did propagate 60cm deep on "31st" tap, a very hard whack after no formal result). HS 80cm

East Henderson – E aspect, 9,700 ft (45.05855, -109.94966): ECTP29, 65cm deep. HS 90cm

Additional notes: 11/12 had clear skies, no wind, and warm temps. A 5 cm thick sun crust formed on SE, S, and SW aspects. No natural nor human-triggered avalanches were observed on 11/12-11/13. A continuous cornice had formed along the entire N-S ridgeline on the east side of Henderson by 11/13. 

A number of Avalanche Crowns on an east aspect at 9500 ft.

We observed lots of well developed surface hoar on Chestnut Mountain on top of approximately 12” - 15” of snow. 

When walking around near grotto falls and below fat and thin chance there was significant surface hoar forming. The crystals near fat and thin chance appeared to be 1mm in size and near the grotto falls parking lot specific areas had surface hoar up to 3 or 4mm in size. 

Our group of four went to Bridger Bowl to evaluate the snowpack and get some decent turns. We dug a pit at the bottom of the pitch of hidden gully at 7460 ft and found a graupel layer sitting on an ice crust that was a layer of concern. Since the snow above it wasn't cohesive enough to form a slab we found Q3 fracture quality at ECTP 23. As we transitioned to ski after skinning further up towards hidden gully there was a much more reactive wind slab that formed near the surface. We were at 7990 ft on a ENE aspect and found 5 cm of fresh soft storm snow on top of a 13 cm wind slab that was 4F+ in firmness. The CT results were a Q1 fracture on 1 tap and this was alarming. Our group was ready to ski and for this reason an ECT test was not performed, but this alarming CT result made us rethink our original plan of skiing directly along a wind loaded gully where we found the weak wind slab layer to be. The quick pit that showed this wind slab was dug below a rock outcropping between hidden and northwest passage. We believed this was a concern specific to this terrain feature and that that reactive of a wind slab was unlikely to be found away from this outcropping. We felt that in sheltered and treed areas that aren't getting sluff and wind deposited snow dumped on them because of their terrain features we would be safer.