Turnagain Pass

Tincan Trees – Wind slab and storm slab avalanches were very reactive on Tincan yesterday, with remote triggering and wide propagation on all terrain with steep enough slope angles. Several avalanches were triggered by different parties and they ranged from 12-24″ deep and 50-150′ wide. Most of the avalanches we saw were remote triggered by skiers on lower angle terrain adjacent to steeper roll overs. Visibility was very poor so we don’t know how reactive the new snow was above treeline, but it is a safe guess that the potential for wide propagation exists in alpine areas as well.

Wind slab with very wide propagation on a small steep terrain feature near treeline on Tincan. Photo from Anonymous 4.1.22

As of the publication of this forecast the SNOTEL station at Center Ridge in Turnagain Pass stopped transmitting data at 5 pm last night, so we are blind to the amount of new snow that fell overnight. The weather forecast was expecting 6-12″ of additional snowfall overnight. In the field yesterday we saw 6-10″ of  new snow had already fallen by 4 pm yesterday, which means anywhere from 12-22″ of new snow could have fallen in Turnagain Pass over the past 24 hours. Girdwood appears to have received less snowfall, with 3-8″ total over the past 24 hours. Portage and Placer have received about 12″ so far, with another 6″ expected in those coastal areas throughout the day today. Without knowing whether the Turnagain Pass overnight snowfall forecast verified and the fact that we saw very touchy wind slabs and storm slabs on Tincan yesterday I am taking a conservative mindset to the forecast this morning.

The new snow fell on top of a layer of surface hoar and facets which formed above a breakable melt freeze crust that exists on all aspects up to at least 3000′. These weak layers above a crust were causing widespread avalanches with wide propagation and easy remote triggering at treeline on Tincan yesterday (see ob here). Below 2000′ the new snow was more moist and we did not see the same kind of propagation and remote triggering, but the new snow was still not bonding well to the old surface and wet loose avalanches were easy to trigger. In addition, strong winds with averages of 20-40 mph for the past 24 hours and gusts of 40-60 mph are redistributing that snow into wind slabs in the alpine and treeline elevation bands.

The primary avalanche problems today are wind slabs and storm slabs that have formed over the past 24 hours and are expected to be up to 3′ deep in wind loaded areas. Human triggered avalanches are very likely and natural avalanches are likely. Winds are expected to decrease today and minimal new snow is expected to fall today, so the potential for natural avalanches will likely decrease throughout the day. We recommend conservative decision-making and terrain selection today as you start to evaluate how the new snow is bonding to the old snow surface. Using small test rolls to check for shooting cracks and small avalanches is a good idea before venturing into any bigger terrain. We also have deeper weak layers in the snowpack that have the potential to produce very large avalanches (see problem 2 for more details).

Two remote triggered avalanches at treeline on Tincan which released at the interface of the new snow and old snow. Photo 4.1.22

Glide Avalanches: Several reports of glide avalanches releasing this week mean that we need to be extra cautious around glide cracks. They have been growing over the past couple weeks across the forecast area and seem primed to start releasing based on the recent activity. These can create very large and destructive avalanches, and the best way to avoid them is just to stay out from underneath them.

Cornices: We have also seen several large natural cornice falls this week that have triggered avalanches on the slopes below. With the recent strong winds and moist snow cornices have been growing again and could be extra sensitive to breaking under the weight of a person. Give them a wide berth along ridgelines and be aware of actively building cornices if you are travelling underneath them.

With a new load added to the snowpack over the past 24 hours there is a higher chance of triggering a deeper avalanche on a layer of buried surface hoar or near surface facets that are now buried 4-6′ deep. Deep persistent slabs are very hard to evaluate with instability tests and unfortunately require patience to let them become unreactive before safely venturing back into bigger terrain. The easy way to avoid them is to select lower angle terrain, generally less than 30 degree slope angle. If you do venture into steeper terrain remember to follow safe travel protocols by only having 1 person in avalanche terrain at a time, stopping in safe areas, and always spotting your partners.