From: Erich Peitzsch
Date: December 26, 2012
Location: Jewel Basin/ Crown Bowl
Seth Carbonari and I rode and skied into the Jewel Basin in the northern Swan Range to investigate the snowpack after a report of an unintentional skier-triggered avalanche in Crown Bowl. The initial report sums the incident up well (http://www.flatheadavalanche.org/2012/12/skier-triggered-avalanche-jewel-). On our tour visibility hampered our ability to see much of the crown. Thus, we dug a snowpit on an adjacent slope with the same aspect. We also performed a ski cut and intentionally triggered a small avalanche (SS-ASi-R1-D1.5-O). This avalanche was about 20 ft. wide and the crown was 6-10 inches deep. We found the slab to be a wind slab just off the ridge with the bed surface/failure layer as a layer of small facets (0.5-1.0 mm) about 20 cm from the surface.
Our Extended Column Tests resulted in partial propagation on two layers; one layer about 20-25 cm from the surface and the other about 65-70 cm from the surface (see attached pit profile). We experienced localized cracking just off the ridge where fresh wind slabs have formed. Otherwise, we experienced no collapsing (whumpfing) of the snowpack. These fresh wind slabs sitting on top of small faceted crystals were our main concern. We also experienced notable sluffing of the most recent low density snow. These new snow sluffs would run a decent distance with a sufficiently steep slope angle.
We also dug a pit on an easterly aspect just below the ridge where there was a much shallower snowpack and our ECTs resulted in no propagation. However, we experienced wind loading on this aspect. Happy and safe riding and skiing!