Observation Date: 12/21/2017
Stonewall Creek / Lincoln, MT. Popular skin route up to the ridge between Sunrise Bowl and Terrain Trap Bowl.
15 degree F, Mostly Cloudy, light to moderate winds, no precipitation during the day.
5-10 mph winds with gust up to apx. 15mph from the southwest
New Snow: 12+”
Natural slides had occurred during the storm cycle on southwest slopes including Sunrise bowl, Terrain Trap Bowl, and all gullies that lead into these bowls. The slides consisted of the new snow that had accumulated during the storm cycle and slid on the old and new snow interface. Crowns wrapped around all aspects of gullies and reached their entire lengths from top to bottom in some cases. The slides looked to have failed at typical start zones on the slope (roll overs, +35 degree slope) but also continued to propagate on lessor steep angle slopes as the avalanche made its way down gullies. Total new snow accumulation during the storm cycle equaled approximately 80cm.
We performed a compression test and two extended column test. The new snow interface seems to be bonding well with the old snow as we were unable to get the new snow layer to propagate during any of our test. However, anyone reading this should perform additional test since Stonewall is know to have variable test results depending on where you dig.
Deeper in the snowpack showed the most signs of instability. At about 35cm from the ground, we were able to get the snowpack to fail on a 10cm facet layer that failed on ice crust. (See test results below). This layer was very reactive during our compression text but we were unable to get to fail during the two extend column test we performed. It didn’t take much of a tug from behind to get this layer to react and shear cleanly after our ECT. It will be interesting to see how this layer reacts with more weight added to the slope overtime.
Stability Test Results:
Elevation: 6500ft, Aspect: SW, Slope: 25 degree, total snow depth: 145cm
CT: 14, SQ: 1 – Distance from ground 35cm. Failure on facet/ice crust interface.
Observer: Al Byrd