Turnagain Pass

With pretty good visibility yesterday we were able to take a closer look for recent avalanches after the last big storm and found many large avalanches. The most notable was on the NW shoulder of Magnum on the skiers side of Turnagain Pass, where an avalanche 3-6′ deep and 500′ wide released naturally during a period of warm temperatures and sunshine on Thursday afternoon. The weak layer is a layer of facets that has been lingering underneath all the new storm snow from the past 10 days. This avalanche sympathetically triggered many smaller avalanches along the creek between Magnum and Sunburst that extended for over half a mile along the gully wall.

Very large avalanche on the NW shoulder of Magnum at 2200′ that released on a layer of facets underneath all the new storm snow from the past 10 days. Photo 3.24.23

There was also a deep avalanche that released on the NW aspect of Eddies which appears to be roughly the same depth and likely includes all the storm snow from the past week. Another large avalanche on the west face of Pyramid was observed from across Turnagain arm along the Seward highway. Finally, we saw two fairly large avalanches that released on a SW aspect at about 3000′ along the rideline that extends west from Peak 4940′ (see ob here for more details). Another group touring in the Portage area a very large avalanche on Explorer Glacier that covered most of the lower glacier with debris. These are all likely associated with a persistent weak layer that was created during the dry spell in early March prior to the very active storm period over the past 10 days.

Deep avalanche crown on NW aspect of Eddies that propagated around terrain features, which makes a persistent weak layer the likely culprit. Photo 3.24.23

Another deep avalanche released on the W face of Pyramid and ran down to the valley bottom. Photo 3.24.23

Debris all the way across Explorer Glacier from a large avalanche that released on a E aspect around 2000′. Photo 3.24.23 from Alex Palombo and Matt Sturgess

Lots of active wind transport in the treeline and alpine elevation bands were forming fresh wind slabs yesterday. Wind speeds have decreased slightly overnight and 1-2″ of new snow fell which might make it harder to pick out areas with recent wind loading based on the snow surface texture. These wind slabs should be around a foot deep and are likely for human triggering today and possible for natural avalanches, especially in the morning while the wind speeds are expected to be 10-20 mph. To identify areas with lingering or fresh wind slabs look for hollow feeling snow on the surface or any signs of shooting cracks on small test features. The most likely place to find sensitive wind slabs is along upper elevation ridgelines and cross loaded gully features. In the big picture this avalanche problem is kind of like a remora, tagging along on the surface with the big shark that is our deep persistent weak layer (see problem 2).

With sunny skies expected today it is possible that loose snow avalanches could release naturally on solar aspects. Cornices could also be more touchy than normal today thanks to the active wind loading over the past 24 hours. Finally, with the warm temperatures and sunshine glide cracks are opening up and could release naturally, so it is best to avoid spending time underneath them.

Skier triggering a small wind slab on Eddies. Photo from Alaska Guide Collective 3.24.23

Now that the dust has settled after the last 10 days of active storms we are seeing clear signs of unstable avalanche conditions and the potential for very large human triggered avalanches in our core forecast zone. The roughly 7′ of new snow that fell over the past 10 days has settled to about a 4′ deep slab on top of a weak layer of facets or a sun crust that formed on the surface during the dry and warm spell of weather in early March. So far most of the large avalanche activity we have observed in Turnagain Pass has been on aspects that were shaded during that March dry spell and have a layer of facets underneath all the new storm snow. However, in Summit Lake there was a very large avalanche triggered this week on a southern aspect (see ob here) where the weak layer was facets underneath the sun crust that formed on solar aspects during that same period. We don’t really know yet if this setup of facets underneath the crust exists on steep solar aspects in Turnagain Pass, but regardless we recommend approaching the mountains very cautiously because the consequences of triggering an avalanche on this weak layer could be very severe.

Due to the depth of the weak layer it is not really feasible to assess the snowpack using standard stability tests. This is a case where the recent avalanche activity combined with observations of poor snowpack structure are enough to justify avoiding large slopes over 30 degrees and remaining very aware of the potential for avalanches releasing overhead and running out to valley bottoms. Human triggered avalanches 3-6′ deep with potential for very wide propagation are possible today. Natural avalanches on slopes getting warmed up by active solar input are also possible today. Even seasoned avalanche professionals have been surprised by the ability for this weak layer to propagate across terrain features and the ability to remote trigger avalanches from lower angle terrain onto connected steeper slopes. The most likely place to trigger an avalanche on this layer is from a thin spot in the slab due to wind scouring or rocky terrain.

Snowpack structure adjacent to the avalanche on the NW shoulder of Magnum, with a 4′ deep slab on top of a weak layer of facets. Photo 3.24.23