Turnagain Pass

Avalanche danger is going to be increasing throughout the day today as snowfall starts to pile up and strong winds impact the area.  Snow totals are expected to be in the 4-6″ range during daylight hours for Turnagain Pass and Girdwood, but favored areas like Portage and Placer could see closer to 16″ of snowfall today. This is just the opening salvo of a storm system that is expected to drop 2-4′ of snow in Turnagain and Girdwood over the next 48 hours, with the latest forecasts preferring Girdwood for snowfall. Favored areas like Portage and Placer could see up to 6′ of snow over the next 48 hours. Snow line should fluctuate between 500 – 1200′ during the storm. Strong winds will accompany the snowfall, and we have already seen averages of 20-35 mph and gusts of 40-60 mph at ridgetops over the past 12 hours. Winds speeds are expected to remain strong today and then increase overnight during the peak of the precipitation.

Human triggered avalanches up to 2-3′ deep will be trending towards very likely and natural avalanches will be likely in areas with rapid loading of new snow and wind. Areas with a weak snowpack structure could see very large avalanches releasing in deeper weak layers (see problem 2 for more details). We recommend conservative decision-making and careful terrain and snowpack evaluation today as the storm starts to impact the area. In areas getting heavy snowfall today, like Portage and Placer, we recommend avoiding avalanche terrain as the conditions could deteriorate quickly in these areas.

Wind Slabs: Strong winds over the past 12 hours have redistributed the surface snow into fresh wind slabs up to 3′ deep, especially at upper elevations. Likely areas to find wind slabs are along ridgelines, in cross loaded gullies, and convex terrain features. Look for shooting cracks, hollow feeling snow, and signs of wind transport on the snow surface to identify areas with fresh wind slabs.

Storm Slabs: As snowfall totals start to increase this evening and overnight the potential for avalanches in protected areas at the interface of the old snow surface will be increasing. If heavy snowfall arrives earlier than expected this could start to be an issue today, otherwise we expect that storm slabs in the new snow will be a bigger issue overnight and tomorrow.

Snowfall totals from Wednesday evening through Saturday morning. Courtesy of NWS Anchorage 2.23.22

With the added weight of new snow and wind loading over the next 48 hours we could see buried weak layers reactivating and producing large avalanches. We investigated a large avalanche in Crow Creek on Goat Shoulder yesterday that was 3-4′ deep, 700′ wide, and ran 2000′ to the valley bottom (see ob here). This avalanche released naturally on a layer of buried facets from November that are a major concern in areas with a shallow overall snowpack throughout the region, such as Crown Pass, Lynx Creek, Silvertip Creek, and the southern end of Seattle Ridge. In addition, this weak layer is a major concern outside our forecast zone in the Summit Lake area and interior Kenai Peninsula in general. The combination of weak November facets on top of a firm and consistent crust from the Halloween storm give this layer the potential to produce avalanches that propagate very widely and run out all the way to valley bottoms. It is likely that storm snow or wind slab avalanches releasing in the new snow could step down to these deeper weak layers and produce a much larger avalanche than expected.

Several thin crusts in the upper snowpack formed by rain, rime, or warm temperatures also have the potential to harbor weak layers either above or below the crust and produce wider propagating avalanches than a normal wind slab or storm slab. The best way to evaluate these layers is using instability tests in a snow pit to assess the potential for propagation in the area you are travelling.

Checking out the crown of the large avalanche from Monday evening on Goat Shoulder, see ob here for details. Photo 2.23.22