Turnagain Pass

During a brief period of decent visibility yesterday we saw several new avalanches from the past 2 days in Turnagain Pass. Most of these look like they released on a layer of buried surface hoar about 1-3′ deep and were at low and mid elevations (1400-2400′). Visibility was poor at upper elevations so if there were recent avalanches up high we might not have been able to see them.

Several crowns along mid elevations of Seattle Ridge at 1500 – 2000′ that had wide propagation that indicates buried surface hoar as the weak layer. Photo 1.18.23

Zoomed in view of smaller avalanches that released on pretty low angle slopes at about 1500′ on the SW aspect of Seattle Ridge. Photo 1.18.23

Another avalanche on SW aspect of Seattle Ridge at 2300′ that was closer to the motorized uptrack with similar wide propagation indicative of surface hoar as the weak layer. Photo 1.18.23

We got a lot less snowfall over the past 24 hours than had been previously predicted, with 2-4″ falling in Turnagain Pass and 3-6″ in Girdwood. The winds are the bigger story today with averages of 15-25 mph and gusts of 50+ mph over the past 24 hours. There will be a lot of snow being transported at upper elevations thanks to the 12+” of soft snow on the surface from the surprise storm on Monday night. This rapid loading is piling onto a layer of buried surface hoar about 1-3′ deep that is widespread across the forecast area. Large natural avalanches are likely and human triggered avalanches are very likely 1-3′ deep in areas where this buried surface boar is being loaded by wind transported snow.  Travel in avalanche terrain is not recommended.

Surface hoar tends to produce avalanches on surprisingly low angle slopes with very wide propagation, as we have seen in recent avalanches on the SW aspect of Seattle Ridge. This can easily catch people off guard because avalanches can occur in areas that are typically safe. In the past few days there have been several remote triggered avalanches on this layer, which means you could trigger an avalanche from low angle terrain onto steeper slopes above, below, or across from where you are travelling. It is important to be aware of other groups in your area and to avoid spending time underneath steeper terrain features and avalanche paths.

Wind slabs 1-2′ deep are also likely today and could trigger a larger avalanche on either the buried surface hoar or the Thanksgiving facet/crust combination, which has produced a lot of very large avalanches in the forecast area in the past two weeks (see problem 2).

Snowpack structure at low elevations on Tincan in an area where we triggered several collapses near the normal skin track. Photo 1.18.23

The last large natural avalanche cycle on the Thanksgiving facet/crust layer occurred after a short but intense snow storm followed by strong winds on Jan 4th. These are similar conditions to what we have seen in the past 24-48 hours, so we think the potential for very large avalanches 3-6’+ deep is higher today. There is also the possibility of an avalanche releasing on a shallower layer, like a wind slab or persistent slab on buried surface hoar (see problem 1), then stepping down to this deeply buried weak layer and creating a much larger avalanche. With the stormy weather so far this week it has been difficult to get into higher elevation terrain to look for recent avalanche activity on this deeper weak layer, so we are uncertain whether the recent storm has caused natural avalanche activity. This layer is most concerning above 2500′ and is most likely to be triggered from a thin spot in the snowpack, like near a wind scoured ridgeline or where rocks penetrate part way through the snowpack.