Turnagain Pass

No new avalanches were reported yesterday. On Thursday there were two human triggered avalanches, one on Cornbiscuit and one on Seattle Ridge, that both seem to be related to lingering wind slabs that were not bonding well with the old snow surface.

Detailed view of the human triggered avalanche from Cornbiscuit on Thrusday. Photo 2.9.23 from Peter Wadsworth

The wind showed up right on cue yesterday, with average wind speeds of 20+ mph and gusts of 40+ mph from 10 am through 11 pm at upper elevations. Snowfall accompanied the strong winds and Turnagain Pass has picked up 5″ of new snow as of early Saturday morning. Girdwood was not favored for snowfall yesterday with just 1-2″ reported in the past 24 hours. More new snow is expected today with 2-4″ of additional snowfall possible in Turnagain Pass and Girdwood and higher totals near Portage. Winds will be lighter compared to yesterday with averages of 10-20 mph and gusts into the 30s, but still strong enough to transport new snow into wind slabs.

Natural avalanches 1-3′ deep in areas with recent wind loading are possible today and human triggered avalanches are likely. Identifying features that have been recently wind loaded will be important to avoid avalanches today. Looking for wind loading patterns on the snow surface and stepping off the beaten path to feel the snow surface with your machine or ski poles are good ways to get a sense of the distribution of wind loaded terrain features. Small tests slopes can also be useful to check how reactive surface wind slabs are before committing to larger terrain features.

In areas that might have seen higher snow totals yesterday, like on the northern end of Turnagain Pass, it may be possible to trigger a storm snow avalanche in sheltered areas if the new snow is not bonding well to the old snow surface. Below 2000′ there is a melt freeze crust from 1/25 about a foot deep in the snowpack that has shown signs of not bonding well with the storm snow from earlier this week. With a new snow load and wind potentially making the surface snow more cohesive this layer might be able to produce storm slabs 1-2′ deep today. This layer is a little unusual since it only exists at lower elevations and has the potential to catch people off guard. Keep an eye out for red flags like shooting cracks and whumphing or better yet dig a snowpit to evaluate whether this weak layer is problematic in the area you are travelling.

Light snowfall is expected across the region today, with highest snow totals near Seward and Portage. Graphic from NWS Anchorage 2.11.23

Example of a snowpack structure from earlier this week with the problematic 1/25 melt freeze crust buried about a foot deep. Photo 2.8.23

The added stress of new snowfall and wind loading might be enough to reactivate our buried weak layers today. The most concerning layer is the 1/10 buried surface hoar which could produce large 2-4′ deep avalanches in isolated areas where the weak layer is well preserved. Since this layer is now buried pretty deeply it is difficult to test without stopping to dig a snowpit and using a compression test or extended column test. Over the past two weeks we have seen this layer acting mostly unreactive, with the exception of one snowpit on Pete’s N (see video here) where the layer was well preserved and very reactive in snowpit tests during an active loading period.  To avoid this problem entirely you can simply stick to low angle terrain and give the snowpack some time to adjust to the new load.

Pesky layer of 1/10 buried surface hoar still widespread in the forecast area, but seems to only be reactive in isolated areas where it is well preserved. Photo 2.8.23

In areas with a thin overall snowpack, like near Silvertip and the southern end of the forecast region, there are deeper weak layers near the base of the snowpack that are concerning. The most widespread weak layer is the Thanksgiving facet/crust combo but in some areas there is also a weak layer of basal facets that is concerning. With active weather today it is possible that these deeper weak layer could become active again and produce very large avalanches.