Turnagain Pass

Seattle Ridge – Several recent avalanches along Seattle ridge yesterday which appeared to be from cross loaded wind slabs. We saw two debris piles that were big enough to bury a person along the far southern end of Seattle ridge across from the Johnson Pass trailhead and one fresh debris pile just north of the Seattle Ridge uptrack (photo below).

Natural avalanche just north of Seattle Ridge uptrack that released due to wind loading across the face of Seattle Ridge. This was a D2, which means it was large enough to fully bury a person. Photo 2.25.22

The major snowfall event that was supposed to take place over the past 48 hours was a bit of a dud, producing less than half the expected snowfall. We ended up with roughly 1″ of water at Turnagain Pass and Girdwood over the past 2 days, which translates to 6-12″ of wet to moist snow. In areas closer to the coast, like Portage and Placer, snowfall totals from the past 48 hours are likely double or triple what we have observed at Turnagain Pass and snowfall is expected to continue to trickle in today. The storm did produce some impressive winds with averages of 50-70 mph on ridgetops during the first half of the day yesterday and gusts up to 120 mph.

The primary avalanche problem today is wind slabs 1-3′ deep in Turnagain Pass and Girdwood and 2-6′ deep in Portage and Placer. We saw evidence of wind slabs releasing naturally yesterday and creating avalanches that are big enough to bury a person (see recent avalanches). Today the potential for natural avalanches is much lower as the wind and snowfall have decreased significantly, but human triggered avalanches are still likely in wind loaded areas. We recommend easing into steeper terrain today and evaluating how well the new snow is bonding with the old snow surface. Looking for shooting cracks, hollow feeling snow, and signs of recent wind transport on the snow surface are all good ways to identify areas with lingering wind slabs.

In areas with a shallow snowpack, such as Crow Creek, Lynx Creek, Silvertip, and the south end of Seattle Ridge, there are buried weak layers that could produce larger avalanches and need to be evaluated carefully (see problem 2 for more details).

Cornices: The new snow and strong winds the past two days will have added some mass to our already large cornices throughout the area. Be aware that cornices could be more reactive to human triggers today so try to avoid spending time underneath them and give them a wide berth along ridgelines.

Loose Snow Avalanches: The surface snow was moist and sticky up to 2000′ yesterday so we did not have any loose snow avalanches, but at higher elevations where the snow is drier it is possible that sluffs on steeper terrain will be an issue today. In addition, warm temperatures at lower elevations or solar warming could cause wet loose avalanches in the new snow.

Shooting cracks on wind loaded features in Tincan trees yesterday. We also had propagation on an ECT with 9 taps (ECT P 9) indicating that the fresh wind slabs were reactive. Photo 2.25.22

The sun poked through yesterday afternoon, and we were somewhat surprised that we did not see much avalanche activity. Plenty of snow being transported along ridgelines though! Photo 2.25.22

There is a dramatic difference in the snowpack structure between deeper versus shallower snowpack areas across the forecast zone. In deeper snowpack areas, like near Alyeska in Girdwood and Turnagain Pass, recent avalanche activity has been limited to storm snow instabilities which have been relatively quick to heal after the past few snowfall events. In shallower snowpack areas, like Crow Creek in Girdwood and Lynx Creek on the far southern end of Turnagain Pass, we have seen wide propagation and deeper crown heights during recent storms which indicate that buried weak layers are able to be reactivated by new snow loading. Similar weak snowpack structure exists in the Summit Lake area, which is outside our forecast zone.

There are several potential buried persistent weak layers in these areas, but the most concerning are facets from November which are the likely culprit for large natural avalanches along Penguin Ridge and Crow Creek in Girdwood and potentially the cause of large avalanches in Lynx creek. We don’t know if it is possible for a person to trigger an avalanche on these deeper weak layers but the consequences of being involved with an avalanche of this size are severe, so we recommend careful evaluation of the snowpack and terrain as well as conservative decision-making in areas harboring these persistent weak layers.