Turnagain Pass

• We received a third hand report of a natural storm slab avalanche on W face of Eddies yesterday during the day. The depth was estimated at 10″ deep with the avalanche failing on the 3/16 buried surface hoar. The party that witnessed the natural avalanche also said they triggered a similar depth storm slab on their next lap in the same area. See ob here for more details.

A low intensity storm has been residing over our area for the past 3 days and dropped anywhere from 6-24+” of snow across the region. Coastal areas like Portage and Placer have been heavily favored by snowfall and likely received multiple feet of new snow in the past 3 days. Girdwood and Turnagain have been consistently picking up anywhere from 1-6″ of snow each day for the past 3 days, which is falling on top of the 3.16 buried surface hoar layer. Yesterday, a natural avalanche reported on W face of Eddies indicates that enough snow has fallen throughout the storm to cause storm snow avalanches in protected areas, which are likely easier to trigger due to the 3/16 buried surface hoar underneath.

As the snowfall tapers off today further natural avalanches will be unlikely in protected areas but human triggered storm slab avalanches are possible in areas that have received 6″ or more of new snow on top of the 3.16 buried surface hoar. Yesterday in the Crow Creek area we observed multiple melt freeze layers in the upper 6″ of the snowpack on southern aspects that are caused by the combination of warm temperatures and solar heating. These layers will add some strength to the upper snowpack and will make triggering a storm slab avalanche less likely on southern aspects.

In addition, fresh wind slabs 1-2′ deep will be likely to cause human triggered avalanches and possible for natural avalanches today, especially at upper elevations where the winds have been averaging 10-20 mph with gusts of 30-40 mph since yesterday around noon. We are somewhat blind to the upper elevation winds in Turnagain Pass because the Sunburst Weather stations stopped reporting data on 3.13, which we hope to get fixed next week. Look for hollow feeling snow, shooting cracks, and active wind loading to identify areas harboring wind slabs. The presence of the 3.16 buried surface hoar could make them easier to trigger on lower angle slopes. We recommend conservative decision-making and gradually easing into steeper terrain if you decide to travel in avalanche terrain today.

Loose Snow Avalanches: The storm over the past 3 days has provided a needed refresh to our snow surfaces. However, the added soft surface snow could lead to wet loose avalanche on southern aspects and dry loose avalanches on northern aspects.

Cornices: The combination of moist new snow and winds is a recipe for cornice building, which could make cornices more sensitive to failure under the weight of a person. As always, give a wide berth to cornices when walking along ridgelines and try to minimize time spent underneath them.

Glide Avalanches: The warm temperatures recently have opened up many glide cracks, especially along the E face of Seattle Ridge. These can release anytime and cause large avalanches, so it is best to minimize time spent underneath them.

Example of 3.16 buried surface hoar underneath the storm snow from the past 3 days, which was about 10-12″ deep on Eddies. Photo from Brooke Edwards 3.18.22

Melt freeze layers in the upper snowpack on a E aspect at 2700′. In areas with these melt freeze crusts we did not find either layer of BSH. Photo from Andy Moderow 3.18.22

An older layer of buried surface hoar which was buried on 3.2 is a lingering concern for producing large avalanches with the potential for wide propagation and releasing on lower slope angles than typical. It has been a challenge to map the distribution of the 3.2 buried surface hoar layer because it is not always obvious in snowpits or hand pits. Prior observations have consistently shown the 3.2 BSH on Seattle Ridge and in some but not all areas on the skiers side of Turnagain Pass. The layer seems to be less widely distributed in the Girdwood area, but it is still worth looking for it before entering avalanche terrain. Instability tests like extended column tests and compression tests can help you locate the 3.2 BSH and assess how reactive it is in the area you are travelling.

Two layers of buried surface hoar from a snowpit on a N aspect on Eddies yesterday at around 2100′. Photo Brooke Edwards 3.18.22