Turnagain Pass

Seattle Ridge

Despite pretty good visibility yesterday we only observed 1 slab avalanche in the recent storm snow. This was located along Seattle Ridge between Main Bowl and Pyramid Peak at about 3000′ on a W aspect. From afar the avalanche appeared to be a wind slab that released along a steep convexity on a wind loaded aspect. It was a soft slab that ran about 150′ vertical feet.

Soft slab avalanche from wind transported new snow along Seattle Ridge. Approximate location of crown shown with red line and avalanche debris shown with blue line. Photo 11.23.22

Thanks to the 12-14″ of new snow yesterday we have lots of loose snow at the surface available for wind transport. At higher elevations the new snow is dryer and we saw some evidence yesterday that it had been transported by the wind along exposed ridgelines. However, the lack of avalanche activity observed yesterday indicates that the new snow is bonding well to the old snow surface and wind slabs are isolated to more exposed features at higher elevations. Winds are expected to remain light today, but new slab avalanches could develop very quickly if you find yourself travelling in an area where wind speeds are strong enough to transport snow.

To assess the potential for wind slabs you can use small test slopes to check whether the surface snow is cracking or feels like it is not bonded well to the snow underneath. Wind slabs could fail at the interface with the old snow (1 – 2′ deep) or you could see wind slabs failing within the new snow (0.5 – 1′ deep). You are most likely to find a wind slab on slopes below ridgelines, on steep convex terrain features, or in cross loaded gullies. Look for signs of wind transport on the snow surface to show you which aspects are likely to be wind loaded in the area you are travelling.

At the very bottom of the snowpack we have been tracking a layer of facets that produced large avalanches during multiple storm cycles in early November. Now that the snowpack is quite wet and includes multiple thick melt freeze crusts below 3000′ the likelihood of triggering an avalanche on this weak layer is pretty low. In the alpine above 3000′ those melt freeze crusts are much thinner or don’t exists at all and it would still be possible to trigger an avalanche on this persistent weak layer. Your ability to impact this layer and release a large avalanche is more likely in areas with a thinner snowpack, like the Johnson Pass area or Crow Pass area. Outside our forecast zone, Summit Lake is another area that is more likely to harbor this week layer longer due to their thinner snowpack.

Since the snowpack is only about 3 feet deep right now it is not too hard to dig down to the ground and take a look at this weak layer before you commit to big terrain features in the alpine. An extended column test or compression test are good ways to check whether this weak layer is reactive in the location you are travelling. On Tuesday on Magnum at 3200′ Andrew had this layer fail in his snowpit tests (see ob here), so it is probably still reactive in some areas.

You can see the video from Seattle Ridge here if it does not load in your web browser.