Observation Date: 12/23/2020
Central Bitterroots, from 5300′ to 7900′. Almost no snow at the start to about 40″ total depth at 7900′.
Beautiful day. No wind.
none–it appeared windier down lower in the valley.
New Snow: 12+”
Large crown on north aspect beneath cliffs at 8000′. See attached photo. It probably ran on top of old faceted snow and took out the storm slab (12-20 through 12-22 snow). See comments below.
We did not experience collapses or cracking, but were not on any steep or wind-affected slopes.
Above 6500′, more than 12″ of snow that fell 12/20 was topped by a rain crust formed on 12/21. 10″ of new snow fell 12/22 atop the crust. Interestingly, below 7500′, we were punching through the crust, making for tricky skiing, but it was thick and supportable above 7500′. Rain crust characteristics and thickness probably depend on what type of grains the rain falls on, and whether or not they allow percolation or trap the droplets within pore spaces. What a rain event: The buried rain crust was still present at 7900′ where we topped out. Right now the deeply buried faceted old snow is probably the biggest avalanche concern, but this crust will be something to watch in the future, as weak snow might develop above and/or below it.
The deep avalanche crown (see photo) we observed probably ran on old faceted snow and took out the 20″+ new storm slab. It looked even deeper than that, though. I suspect slough debris coming off the cliffs above had probably thickened the slab.
Observer: Jeff Lonn