Turnagain Pass

No new avalanches were reported yesterday, other than some small wind slabs and loose snow avalanches releasing naturally in the upper Lynx Creek area. There was one larger slab in upper Lynx Creek but we are uncertain about the age due to all the recent wind activity. Otherwise more details have emerged from the Tincan Library avalanche that occurred on Sunday 2/19/23. Based on photos and investigation of the debris it looks like this avalanche failed on some kind of persistent weak layer within the upper 3′ of the snowpack, due to the wide propagation around several terrain features. Our best guess is that the strong solar input on Sunday afternoon was enough to make the surface snow act more like a slab which made whatever weak layer was lingering below the surface become more reactive despite not having any new snow added. This rapid solar warming is going to become more of a concern from here onward for the rest of the season. Check out the incident write up here for more information.

Mid-slope slab avalanche of unknown age from upper Lynx Creek area. Photo 2.20.23

Overview of Tincan Library avalanche with a person in the bottom right corner for scale. Photo 2.20.23 from Andy Moderow

Close up view of the crown from Tincan Library avalanche, based on width of propagation across multiple terrain features and the fact that it was the third skier on slope who triggered the avalanche we think some kind of persistent weak layer was involved. Photo 2.20.23 from Andy Moderow

The windy conditions that impacted the mountains yesterday have lingered overnight and as of 5 am the average wind speeds remain in the 5-15 mph range out of the NW with gusts up to 30 mph. We expect the winds to die down this morning and give way to a calmer day but wind slabs 1-2′ deep that formed overnight could remain touchy today. Human triggered avalanches are possible in areas that have seen recent wind loading, like along upper elevation ridgelines and cross loaded gullies. To identify areas with recent wind slabs you can feel for stiffer or hollow feeling snow on the surface and use small test slopes to check how reactive the surface snow is.

The sun should be back out this morning and may have an impact on the snow surface again, however clouds are expected to move into the area throughout the day which might limit the amount of solar warming today. Loose snow avalanches on solar aspects (east, south, west) could release naturally especially in areas where the sun is warming up rocky areas above steeper terrain. Cornice fall is also more likely with direct solar warming so try to be aware of any large cornices overhead that are baking in the sun.

Example of recently wind scoured surface texture from Sunburst, which indicates that the leeward slope where the snow was transported could be harboring a wind slab. Photo 2.19.23 from Megan Guinn

Based on the follow up investigation of the Tincan Library accident (details here) we think a weak layer of buried surface hoar or facets in the upper 3′ of the snowpack might have been involved in this avalanche. The area was experiencing rapid solar warming in the hours prior to the avalanche releasing which may have allowed the surface snow to act like more of a slab then it did when it was cold and dry. This transition to a stiffer slab on top on solar aspects may cause shallowly buried persistent weak layers to become more reactive, like the 2/15 buried surface hoar about 6-12″ deep or the 2/5 facets about 1.5-2.5′ down. Below 2000′ the 2/5 facet layer transitions to a melt freeze crust with facets around it. To check for these layers in the area you are travelling we recommend paying close attention to surface conditions by using small test slopes, hand pits, or snow pits. A snow pit is the best way to identify persistent weak layers in the snowpack and test their strength and propagation potential.

Deeper in the snowpack the 1/10 buried surface hoar is about 2-4′ down in Turnagain Pass. This layer has been largely unreactive in the past few weeks with the exception of a few concerning snow pit tests from isolated areas where the surface hoar grains were well preserved. On the southern end of the forecast area and thinner snowpack areas like Summit Lake very large avalanches releasing on this layer could still be a concern, especially if the snowpack has seen recent wind loading.

In areas with a thin snowpack (i.e. Silvertip and the southern end of the forecast region to Summit Lake) there are various weak layers near the base of the snowpack that remain a concern. For these areas, triggering a larger avalanche is not totally out of the question and a more cautious mindset is recommended.