Turnagain Pass

Lots of people got the memo about soft surface conditions and clear weather yesterday and were venturing into big terrain. We have reports of two human triggered avalanches yesterday, one from the south side of Cornbiscuit and one from BBQ bowl on Seattle Ridge (which is between -2 and -3 bowl from our motorozed place names map). Both of these avalanches appear to be relatively shallow slabs where there might have been lingering wind slabs or just poor bonding between the new snow from this week and the old snow surface. On Seattle ridge the party reported that the slab depth was about 18″. We also had a report of a glide avalanche release on Shark’s Fin and widespread loose snow avalanches in steep terrain and in areas getting intense solar input.

Photo of the crown from the Seattle Ridge BBQ bowl avalanche that was reported to be about 18″ deep. Photo 2.9.23 from Julia Dramis

Marked up photo of human triggered avalanche on S aspect of Cornbiscuit at about 3200′ taken from the Lipps ridgeline. Photo 2.9.23 from Allen Dahl

Avalanche conditions will be changing dramatically today, from cold, clear, and calm yesterday to strong winds and snowfall today. Wind speeds should reach 50-60 mph out of the east with gusts of 75+ mph at upper elevations by noon today. Snowfall is expected throughout the daylight hours today with 3-6″ of accumulation by midnight tonight. The combination of strong winds, lots of soft snow on the surface, and new snowfall will RAPIDLY increase avalanche danger. Wind slab avalanches 1-3′ deep are likely to release naturally with this much loading and will be very likely for human triggering. We recommend avoiding avalanche terrain at upper elevations today, where the winds speeds and snowfall will be most intense.

At lower elevations we have seen evidence of poor bonding between the new snow from this week and a melt freeze crust that exists up to about 2000′. Wind transport and new snow today could be enough to make the slab on top of this crust a little stiffer and more likely to produce avalanches. Cautious route finding is recommended and careful evaluation of snowpack conditions even on low elevation terrain features that are typically safe. As always, keep an eye out for any red flags like shooting cracks or collapsing as a strong indicator that weak layers are becoming reactive under the stress of the new snow and wind.

This layer of wind loaded snow did not propagate in our extended column test but it did pop off pretty easily when we levered on back of the column. Photo 2.9.23

The addition of wind loading and new snow today could make our buried weak layers more reactive than we have seen in the past few days. The most concerning layer is the 1/10 buried surface hoar, which still exists across the forecast area 2-4′ deep but has been unreactive in snowpit tests recently. In isolated areas where this layer is well preserved it is possible to produce large human triggered avalanches 2-4′ deep with potentially wide propagation. The best way to evaluate whether it is possible to trigger an avalanche on this layer is to dig a snowpit and use a compression test or extended column test to check if the layer is reactive. However, with the active storm system today we are simply recommending avoiding avalanche terrain in the alpine and upper treeline elevations which is where this layer is most concerning.

Snowpack structure from Sunburst ridgeline with 1/10 buried surface hoar about 90 cm deep (3 ft). Photo 2.9.23

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.