Turnagain Pass

We received report of a wind slab releasing in steep terrain in main bowl on Seattle ridge yesterday. We think this avalanche likely released at the interface of the old snow surface and the Christmas storm snow. After the Christmas storm several large avalanches released on the south and east aspects of Seattle ridge and at lower elevation on the NW ridge of Magnum. Low visibility this week has made detecting recent avalanche activity challenging.

Wind slab avalanche triggered by snowmachiner on a west aspect at 3000′ in main bowl along Seattle Ridge. Photo Hunter McConnel 12.30.22

As a large storm approaches today, increasing wind speeds will be the first thing to contribute to increasing avalanche danger. The combination of existing dry snow on the surface plus 6″ of new snow expected today will provide plenty of surface snow available to be transported into fresh wind slabs 1-2′ deep. The likelihood of human triggered and natural avalanche activity will increase throughout the day as more new snow falls and the wind speeds continue to increase. This morning winds are averaging 10-15 mph with gusts to 30 mph and by this evening we expect winds to average 50-75 mph with gusts of 100+ mph possible.

Wind slabs are most likely to be found along upper elevation ridgelines and cross loaded gullies. However, with very strong winds like we are expecting this evening you could find wind slabs in areas that are typically sheltered. To identify areas with fresh wind slabs look for active wind loading, hollow and stiff feeling snow, and shooting cracks on the surface. With conditions deteriorating throughout the day we recommend a conservative mindset and a quick bail out plan if visibility and travel conditions change quicker than expected.

Expected snowfall totals from Saturday at 6am to Sunday at 6am. Heavy snowfall should start between 12 pm and 3 pm on Saturday with winds ramping up prior to that. Graphic courtesy of NWS Anchorage 12.31.22

With poor travel conditions and visibility this week we still have a lot of uncertainty about the status of our persistent weak layers at upper elevations. Prior to the Christmas storm, a layer of facets above the Thanksgiving melt freeze crust was causing collapses and concerning test results in portions of the forecast area with a generally thinner snowpack, like in Girdwood Valley and near the Johnson Pass trail. The rain and wet snow up to 2500-3000′ that came along with the Christmas storm has helped to stabilize these weak layers at middle elevations, but we have limited information on how the new load has impacted these persistent weak layers above 3000′. At this point we believe there is still a chance of triggering an avalanche on a persistent weak layer 2-4′ deep that could produce a very large avalanche.

In addition, we have seen some evidence of a weak interface between the old snow surface and the Christmas storm snow which could produce avalanches 1-2′ deep. With the addition of a new snow load and very strong winds today it is possible that these buried weak layers could become reactive again at upper elevations. Sticking to smaller terrain features and lower angle slopes is the best way to avoid buried persistent weak layers. Hopefully after the New Years storm we can get more conclusive information that these layers are no longer a concern, but for now they should stay on our radar.