Turnagain Pass

On Friday a skier triggered avalanche from the Crow Creek area was reported. The avalanche occurred at 1900′ on a SSW aspect and was about 20″ deep. The weak layer was reported to be a layer of surface hoar or facets above the 1/25 melt freeze crust. The skier also observed shooting cracks in the area prior to triggering this avalanche. Otherwise, during a brief clear period we did not see any recent avalanches in Turnagain Pass yesterday but plenty of signs of recent wind transport at upper elevations.

Skier triggered avalanche about 20″ deep by 60′ wide on a weak layer above the 1/25 crust in the Crow Creek area. Photo 2.17.23 from Malcolm H

We are expecting another storm system today with pretty low snowfall amounts but moderate to strong winds that will be able to transport the soft surface snow into fresh wind slabs 1-2′ deep. Snowfall totals will vary from 1-3″ in Turnagain Pass, Girdwood, and Summit Lake to 6-12″ near Portage. Cloud cover is expected to remain throughout the day so accessing higher elevation terrain where wind slabs are most likely to form will likely be challenging due to visibility.

To identify areas harboring wind slabs look for shooting cracks, signs of recent wind transport, and hollow feeling snow on the surface. Jumping on small, steep, wind loaded terrain features can be a very effective way to get a sense of how touchy wind slabs are before committing to larger slopes. In areas receiving active wind loading today, like along upper elevation ridgelines and cross loaded gullies, human triggered avalanches are likely.

NWS snowfall estimates from Saturday morning through Sunday morning. Graphic courtesy of NWS Anchorage 2.18.23

In addition to the wind slab problem on the surface we also have several buried weak layers in the snowpack that could produce larger avalanches. The most likely buried weak layers to trigger an avalanche on are in the upper 1-2′ of the snowpack and include a new layer of surface hoar that was buried on 2/15 and the storm snow interface from 2/5. Yesterday several groups reported propagation in snow pit tests on the newest layer of buried surface hoar from 2/15 about 4-6″ deep. This layer could now have fresh wind slabs on top which might cause surface avalanches to be larger and more reactive than typical. The 2/5 storm interface looks like a layer of weaker faceted snow 1.5-2.5′ deep at upper elevations but below 2000′ it is a layer of facets on top of the 1/25 melt freeze crust. The recent avalanche from Crow Creek Area likely failed on this layer of facets above the 1/25 melt freeze crust.

Deeper down in the snowpack the 1/10 buried surface hoar continues to show occasional signs of instability in snowpit tests. Over the past few weeks we have been able to find unstable results in snowpits in areas near treeline where the layer is well preserved and there has not been much skier traffic. The depth to this weak layer varies depending on location. In areas like Portage, where there has been a lot more snowfall in recent weeks, it is buried up to 5’+ deep and is unlikely to be triggered by a person. In thinner areas like near Johnson Pass trailhead (Lynx Creek, Pete’s, Silvertip) this layer is closer to 2-3′ deep and it is still possible for a person to trigger an avalanche in isolated locations.

The layer of facets at the 2/5 interface showed signs of instability in some but not all of our snowpit tests yesterday. Photo 2.17.23

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.