Turnagain Pass

We received social media report of a skier triggered slab avalanche yesterday in the forecast area described as a “decent scare”. We hope to get a bit more information today, including the specific location.

West to northwest winds are forecast to be elevated in the afternoon today, 10-20 mph with gusts into the 30s. They aren’t expected to be quite as strong as last Friday but watch for blowing snow again. There is still snow available for transport including the fluff that fell Saturday. With this there will be the potential for newly formed small wind slabs. As Wendy mentioned for Friday, this wind flow direction is tricky for Turnagain Pass. It can funnel through the Pass from the south and load north aspects on the non-motorized side, while at the same time load the SE face of Seattle Ridge. It can also split around the Pass and not affect much of the terrain in the heart of Turnagain at all. You might find you are in an area that is seeing no wind, or not enough to move the surface snow into wind slabs. This wind pattern also increases through channeled terrain and can be more pronounced in Crow Pass and Portage. Pay attention to blowing snow and active wind-loading.  There may also be old lingering wind slabs from the past wind-loading. Look for smooth pillowed snow on convexities, cross-loading in gullies and feel for stiff snow over softer snow. As always, watch for signs of instability like shooting cracks from your skis or machine.

If you do find and trigger a new wind slab, remember it could step down to old buried weak layers deeper in the snowpack. In this case, a larger avalanche is possible (more on this in Problem 2).

Sluffs: It will be easy to trigger loose snow avalanches (sluffs) in steep terrain that has been sheltered from the winds. It is unlikely an avalanche like this would be big enough to bury a person, but they can have serious consequences if they carry you into terrain traps like cliffs, rocks, or trees.

Forecast elevated winds today looking at the NAM on Windy. Watch for blowing snow!

As mentioned above, it is still possible to trigger an avalanche on two different persistent weak layers in the upper 2’ of the snowpack. One is a layer of surface hoar that was buried on 1/28, and the other is a combination of near-surface facets and surface hoar that was buried on 2/9. Both of these layers are slowly becoming more stubborn to trigger, but they are still showing that they are capable of producing avalanches. There were human-triggered avalanches on these layers last Thursday and Friday and observers have been still been reporting whumpfing. We suspect that the skier triggered avalanche that occurred yesterday may have involved one of these layers as well.  The layers of weak snow have been found across the forecast area in all elevations bands. However, lower elevation (below 2000′) steep terrain, where there is a crust combined with the weak snow, seems to harbor the most reactive set-up.

In addition, these weak layers may be sitting under old wind slabs and hard to identify.  They are also potentially getting another layer of wind-loaded snow on top today. Triggering a fresh wind slab may step down to a buried weak layer. Overall the snowpack has to be viewed with some uncertainty and caution.  If choosing to push into steeper terrain, keep in mind that these weak layers exist, watch for signs of instability (with the caveat that they may not be present but the slope may be unstable), follow safe travel protocol and evaluate terrain consequences. Imagine an avalanche, 1-2′ deep, propagating across the terrain. If the slope does avalanche where will you end up?

Both buried weak layers visible in a snow pit wall on Sunburst, 2.17.21.