Turnagain Pass

After several weeks of nearly continuous snowfall, the last pulse of precipitation that fell overnight Thursday and into Friday morning was enough to push the snowpack over the edge and cause a widespread natural avalanche cycle. The most notable avalanche was the one that closed the Seward highway just north of Girdwood for the first half of yesterday but there were many others throughout the area which are clear indicators of dangerous conditions. We received too many avalanche observations yesterday to post them all, but here is a selection of the recent avalanche activity with links to the observations where you can find more detail.

• Girdwood area – A large natural avalanche released in the early morning hours yesterday from Penguin ridge just north of Girdwood and covered the highway in 10-15′ of debris. We are still sorting out the exact details of this avalanche activity but it appears that an avalanche initiated in the new wind loaded snow and was able to step down into deeper weak layers and create a wide propagating avalanche. It is not clear how large the natural avalanche was that covered the highway versus the additional avalanches that were released by hazard reduction efforts of the DOT during the day yesterday.
  Avalanches along the Seward highway just N of Girdwood with very wide propagation which stepped down to deeper weak layers in some areas (details here). Note that this photo includes the natural avalanche that struck the road as well as the avalanches that resulted from hazard reduction work of the DOT and it is difficult to tell which is which. Photo 2.18.22

Three more natural avalanches in California Creek that released mid-slope (roughly 2500-3000′) and had relatively wide propagation. Photo 2.18.22

• Turnagain Pass – Those lucky few who were on the right side of the road closure and made it up to Turnagain Pass yesterday reported several wind slab and storm slab avalanches. These include avalanches that were likely triggered by wind loading along ridgelines, some storm slabs that were reactive to human triggers in protected areas, and two avalanches on Lipps (photo below) that appear to have been related to the sun creating wet loose avalanches that triggered a storm or wind slab below. See observation here for photos of smaller avalanches on Tincan and Seattle Ridge.

Two large natural avalanches on the W face of Lipps that appear to have been triggered by the sun releasing wet loose snow or rollerballs coming down from above (details here). Photo 2.18.22

Today looks like a beautiful day to be in the mountains and after a few weeks of mostly stormy weather folks are probably chomping at the bit to take advantage of the new snow and visibility. However, we need to consider the widespread natural avalanche cycle that happened over the past 36 hours and the fact that dangerous conditions remain throughout the forecast area. A variety of different types of avalanches were reported yesterday and we recommend careful evaluation of the terrain and snowpack as well as conservative decision making if you get into the backcountry today.

The primary avalanche problem today will be wind slabs up to 2′ deep that were created overnight Thursday and early Friday morning. These are most likely to be found at upper elevations along ridgelines, cross loaded gullies, and convex rollovers. In addition, observations from yesterday showed that storm slabs are possible to trigger in wind sheltered areas (details here). With the warm temperatures since the storm ended we expect that the new snow is starting to bond to the old snow surface and the chances of triggering storm snow avalanche are decreasing, but the potential for persistent weak layers underneath the new snow could make these avalanches last longer than normal (see problem 2 for more details). Quick stability tests like hand pits and using small test slopes to check for shooting cracks are effective ways to assess how well the new snow is bonding to the old snow surface.

The sun is finally starting to play a role in the avalanche conditions, with the potential for solar triggered wet loose avalanches to release a larger storm or wind slabs on east, south, and west aspects. We saw an example of this on Lipps yesterday where two avalanches were probably triggered by loose wet snow coming down from above (details here). Be aware of the terrain above you if the sun comes out today because it is possible for natural avalanches to be triggered from solar warming.

Snowpit yesterday from Seattle Ridge at 2900′ showing potential for avalanches about 1-2′ deep at the interface between the new snow and old snow surface (ob here). Photo 2.18.22

The wide variety of avalanches observed yesterday have us scratching our heads a bit about exactly what weak layers were responsible for this widespread activity. In areas with a thinner snowpack we know that there are buried facets from November above a strong crust which has the potential to create very large avalanches and likely played a role in the avalanches on Penguin Ridge that struck the highway. These layers are less likely to trigger in areas with a deeper and stronger snowpack but the large avalanches yesterday are a scary reminder that this deeply buried weak layer exists throughout the area.

Other weak layers exist in the upper snowpack that seem to be highly variable in distribution across the forecast area and have the potential to create wide propagating avalanches, such as the natural activity in California Creek yesterday (see recent avalanches). Our uncertainty is high about the distribution of weak layers because avalanches appear to have released on different layers in Girdwood versus Turnagain Pass yesterday. We recommend carefully evaluating the snowpack structure before entering avalanche terrain and keeping an eye out for weak layers that could cause avalanches to propagate widely.