Turnagain Pass

The last known human-triggered avalanche was Saturday on Eddie’s Ridge, where a skier remotely triggered a small but deep pocket failing on the Thanksgiving crust. The avalanche was around 2 feet deep on average and 50 feet wide. Nobody was caught or carried. It is very likely there was natural avalanche activity last night; we will hopefully be able to get a look at some of the activity today.

A fast-moving storm brought around 1.5 feet of snow to the mountains near Girdwood and Turnagain Pass over the past 12 hours, with only 4-6″ near Summit and Seward. The heavy snowfall was accompanied by very strong easterly winds, with average speeds of 50-78 mph and gusts as high as 117 mph recorded on Sunburst last night. While this is less snow than we were predicting, it is still a major load that will likely produce big avalanches. Light precipitation should pass quickly this morning, but the avalanche conditions will remain very dangerous through the day today. 

This period right after the storm is the most common time for people to get in trouble. New storm and wind slabs will be very tender and reactive, making large to very large human-triggered avalanches very likely, even if we aren’t seeing widespread natural avalanches during the day. A few things that make this storm particularly scary are the way the snow came in and the snowpack it was falling on. The storm started cold and finished warm, which means even in areas that weren’t affected by the winds (which may be few and far between), there will be a layer of denser snow sitting on top of a layer of softer snow. That will be more likely to produce avalanches. In addition to the high likelihood of avalanches failing within the new snow, this storm also just added a lot of stress to a buried weak layer, which may have the potential to produce destructive avalanches (see Problem 2 below for more on this).

Large avalanches failing in upper elevations will have the potential to run far distances into low-angle runout zones. If you are planning on getting out to check out all of this new snow, be sure to avoid traveling on or below steep terrain today. Even places like the motorized uptrack or the south face of Sunburst that are often relatively benign have the potential to produce big avalanches today.

Last night was intense- stations recorded wind speeds of up to 117 mph, with almost 1.5 feet of snow falling in just 12 hours. Data from the Sunburst weather station (top) and Center Ridge Snotel site (bottom).

The layer of faceted snow on top of the Thanksgiving crust has been a big question mark for us over the past week, and this loading event will be the first big test of that weak layer. Immediately prior to the storm, a skier triggered a small but deeper avalanche failing on this layer on Eddie’s Ridge. We also received a report of a large natural avalanche that likely failed on this layer about a week ago on the back side of Seattle Ridge. Based on what we’ve been seeing in snowpits, the layer is most concerning in middle and low elevation bands, which will see the most traffic today. There is a lot of uncertainty with just how reactive this weak layer will be today. Given the depth (it’s now 3-4′ deep on slopes that haven’t been wind-loaded) and the fact that the crust/facet layer is very well connected across terrain features, if the layer does awaken it has the potential to produce massive avalanches. These can be triggered remotely- which means you could trigger an avalanche from flat terrain that is connected to steeper terrain.

The high uncertainty and high consequences of triggering one of these avalanches are very concerning. Conditions will be dangerous today, and travel on or below steep terrain is not recommended.

This skier-triggered avalanche on Eddie’s from Saturday may be a precursor of things to come, or it may be an outlier. Time will tell, but with 1.5″ water adding stress to the snowpack today is not the day to test the waters. Photo: Andy Moderow, 12.09.2023