Scott and I toured on the south side of Stouts Mountain in the Big Hole’s. On our ascent we experienced wide-spread collapsing at all elevations; along ridge crests and then on open slopes in the top 1,000′ of the mountain. Maximum snow depth was 66cm with 10cm of new snow from the 1/10/12 snow storm. This new snow is very light in density and rests upon a hard and slick sun crust.
We performed a few hand shear tests on the way up in which all easily failed at an ice layer over top of facets. Our test pit at 8,250′ revealed a snow pack composed of sun crusts, ice layers and various layers of faceted snow in-between. Stability test results were ECTP 11 Q1 @ 30cm; failing on small-grained facets below an ice crust. 32 degree slope angle on a south aspect.
Although we did not gather snow pit information on other aspects, given the bull’s-eye data of collapsing snow and some shooting cracks combined with low stability test results, we chose to ski lower angle terrain back down the south side instead of dropping into the steeper and more complex terrain on the northeast face. Trustworthy stability data seemed difficult for us to come by in our evaluations so we enjoyed a nice long sunny descent down the south face.
The Gallatin National Forest Avalanche Center reported many similarities to what Scott and I experienced on our tour and digging into the snowpack; providing a nice reinforcement to our evaluations and decisions. Although they mention only two collapses in a long tour near Cooke City, we experienced a dozen or more on a short four-hour tour. Many of these collapses were initiated near rocks and shallow sections of snowpack as well as one large collapse below the summit when Scott broke through the sun crust.
GNFAC Friday Jan 13, 2012
Unfortunately there is never a point when conditions switch from unstable to stable. Instead, the odds of triggering an avalanche simply decrease. Almost 2 weeks ago, following the New Year’s Eve storm, all slopes were unstable and obvious signs on instability, like recent avalanches and collapsing and cracking, were widespread for at least a week after this storm. Signs of instability have become more isolated. Yesterday near Cooke City, skiers experienced two collapses during a long tour.
The reason signs of instability continued for so long is that snowpack structure is bad. In most cases a slab 1-2 feet thick rest on a variety of weak layers of faceted snow. This slab is supportable and if you don’t get off your sled or step out of your skis, it will be hard to tell what’s under it. Conditions might “feel good” when they are not. In some places like Mt. Ellis there is a layer of buried surface hoar (photo). On some S facing slopes, there are weak facets next to an ice crust. Two experienced skiers found this layer in Hyalite Canyon just as Doug did earlier this week (video). I found a similar layer near Cooke City on Wednesday. On other slopes the snowpack is relatively shallow and has big, obvious depth hoar crystals under the supportable slab.
While the odds of triggering an avalanche have slowly decreased, variability has increased. This variability means some slopes are more prone to producing avalanches than others. For this reason each slope deserves a careful evaluation of snow and terrain.