Turnagain Pass

Girdwood: 4′ deep slab at ~1700′ triggered by Alyeska ski patrol during avalanche mitigation work. This avalanche released on a layer of buried facets on top of the halloween crust.

4′ deep avalanche at Alyeska triggered during avalanche mitigation using explosives. Photo 12.12.21 Alyeska Snow Safety

Turnagain Pass: A few small natural avalanches were observed on the south side of the pass on the Seattle Ridge side. This area had higher winds and active wind transport in the afternoon so it is likely that these are small wind slabs. It is possible that one or both of these avalanches were remote triggered from below on the deeper faceted weak layer.

Small natural avalanche on S face of Seattle Ridge across from the Sunburst parking lot. Photo: Peter Wadsworth 12.12.21

Second small natural avalanche on S face of Seattle Ridge (center of photo in Alders). Photo: Peter Wadsworth 12.12.21

The winds were fairly light in the northern portion of Turnagain pass yesterday (Eddies, Tincan, Sunburst, Cornbiscuit, Lipps), but there was a lot of snow being moved around on the southern end of the pass, Seattle Creek drainage, and other areas of the forecast zone. A few small natural avalanches released at lower elevations on the south face of Seattle Ridge which was receiving heavy downslope winds and active wind loading in the afternoon. In areas that saw wind transport yesterday it will be possible to trigger a wind slab today ranging from 1-3′ deep that could be composed of either soft or hard snow depending on how strong the winds were. Look for shooting cracks in recently wind loaded areas, such as gullies and just below ridgelines, to determine if wind slabs are reactive. A small wind slab has the potential to step down into deeper layers and create a very large avalanche 3-6′ deep on a layer of buried facets (see problem 2 for more details).

We have received winds from multiple directions since the last snowfall which makes it tricky to determine wind loading patterns just by looking at the snow surface texture. Travelling outside the beaten track will help you determine whether an area has been wind affected and you can use small test slopes to see how reactive those slabs are. It will be important to identify wind loaded features and consider the consequences of triggering a deeper avalanche on a layer of faceted snow 3-6′ deep before deciding whether to step into avalanche terrain today.

Wind transport evident in snow surface texture in the foreground and active downslope wind loading in the background on south face of Seattle Ridge. Photo: Peter Wadsworth 12.12.21

You might be getting tired of hearing about it, but there is still the possibility of triggering a very large avalanche on a layer of weak sugary facets over a stout crust buried 3-6′ deep. This layer exists across the forecast zone and has produced many large avalanches in the past two weeks. As we get further away from the latest loading event (12-24″ new snow last Thursday/Friday) and the layer gets more deeply buried the likelihood of triggering an avalanche is decreasing while the consequences remain very high. The likelihood of triggering an avalanche on this layer is higher in thin areas of the snowpack or in regions that have a lower overall snowpack depth, such as the south side of Turnagain Pass and Summit Lake.

Alyeska snow safety has been consistently triggering avalanches on this layer during their mitigation work, which is a great reminder that the layer remains reactive but that may require a larger load (explosives) to release. This is the type of scenario where the tenth or hundredth skier/rider on a slope could find the sweet spot and trigger a large avalanche. Conservative decision-making and terrain selection is paramount for managing this type of avalanche problem. Digging snowpits can be helpful to track the progression of this faceted layer and determine the structure in the area you are travelling, but it is buried deep enough that the results of stability tests are inconsistent and do not accurately evaluate the potential for triggering or propagation. The avalanche activity we have seen on this layer so far has been between 1500 to 3000′ in elevation.

Snowpack structure from a thin area on the south side of Turnagain Pass on lower slopes of Seattle Ridge. The total snowpack depth was variable but ranged from 2.5 to 3.5′. The faceted layer is partially scooped out at the bottom below the ‘CT 6’ line. Photo: Peter Wadsworth 12.12.21