Turnagain Pass

Poor visibility for most of the day yesterday made spotting avalanches difficult, but we did see very reactive wind slabs at tree line on Pete’s North. We triggered one on a test roll at 2100′ on a wind loaded aspect. As we travelled across the slope to find a pit location we triggered widespread shooting cracks on several wind loaded features that were just not quite steep enough to slide. Freshly built cornices were also quite touchy near tree line.

Small but very touchy wind slab on Pete’s N at about 2100′ on a W aspect. Photo 2.6.23

The latest storm on Sunday through Monday produced quite a bit of snowfall in Turnagain Pass, with up to 16″ of new snow reported from the north end of the pass and closer to 6″ on the south end of the pass near Johnson Pass Trailhead. All this new snow combined with strong winds made for quite a touchy snowpack yesterday. We saw very touchy wind slabs at tree line and also had reactive test results in our snow pits with propagation on several different potential weak layers.

Wind slabs and storm slabs should become less reactive today as the snowpack has some time to adjust to the new load. However, human triggered avalanches 1-2’+ deep are still likely at upper elevations on steeper slopes in areas that saw wind loading yesterday. Using hand pits and small test slopes are great ways to test how well the new surface snow is bonding to the old snow surface. Given the recent loading and the existing weak layers in our current snowpack we are recommending a cautious mindset today. Remember that a layer of buried surface hoar still exists in the upper snowpack and could produce large avalanches (see problem 2).

At lower elevations in areas sheltered from the wind we saw evidence that it is possible to trigger a storm slab on top of the 1/25 melt freeze crust that formed from sea level up to about 1800-2000′. The new snow at these elevations should start to bond with the existing snowpack pretty quickly but it is worth having this on your radar so you are not surprised by the potential for a low elevation storm slab avalanche.

With this amount of low density new snow dry loose avalanches (aka sluffs) and cornice fall are also likely on steeper terrain.

For the first time in over a week in Turnagain Pass we saw reactive snow pit test results yesterday on the 1/10 buried surface hoar layer. The area we tested had very large and well preserved surface hoar grains that were quite evident in the snowpack. It took more force to initiate the failure than we saw a few weeks ago when this weak layer was active, but when the layer failed it still had a lot of energy and the block slid right into our pit. This serves as a reminder that large avalanches 2-4′ deep on this layer of buried surface hoar are still possible. With the added load of the last storm this weak layer could be more reactive now than it has been during the recent spell of calm weather. To avoid this problem we recommend sticking to low angle terrain and remaining aware of any steep slopes overhead, especially if other groups are travelling above you.

Snowpit structure from 2100′ on Pete’s N with very intact layer of buried surface hoar from 1/10 that propagation in multiple extended column tests. Photo 2.6.23

A weak facet/melt freeze crust layer from Thanksgiving exists at the base of the snowpack 4-8′ deep. Triggering an avalanche on this layer is unlikely, but it would produce a very large avalanche so we are keeping it on our radar as a worst case scenario. The most likely place to trigger an avalanche on this layer is from a thin spot in the snowpack, like where wind has scoured the snowpack down or where rocks are penetrating through the snowpack. We hope to put this layer in our rear view mirror soon, but with the recent loading event it is possible that this layer could be reactivated temporarily.