Snowmobiler triggered avalanche Sheep Mtn.

Yesterday afternoon (12/30) we rode to Henderson Bench and dug a pit at 9,600' on a NE facing slope. HS was 188cm and we had ECTN22 and ECTN23 at 146 cm above the ground. New snow from the last week was over a foot deep and it was snowing steadily, an inch an hour at times while we were out. Snowfall tapered off in the evening, then this morning dropped 4" in town and more in the mountains.

Today we were rode north of Cooke City. We rode over Lulu Pass and dug a pit at Goose Creek at 9,500' elevation on a NE facing slope. HS was 216 cm with 40cm of new snow from the last couple days. The top 14" of the new snow weighed 0.7" snow water equivalent. We got ECTN 25 and ECTN 27 at 150cm above ground on facets sized 0.5-1mm.

Later we rode over the saddle between Sheep and Scotch Bonnet and saw an avalanche that appeared very recent, and likely triggered by a snowmobiler. It was on a west facing slope at 10,000', and was a hard slab 1-4 feet deep and 150' wide, ran about 200' vertical. AMu-HS-R3-D2-O. This was a clear sign that weak layers exist, especially where the snowpack is relatively shallower, and recent snow has been drifted into thick slabs that are unstable over this weak snow.