Turnagain Pass

The last known avalanches were two large slabs that occurred last Saturday on Cornbiscuit Ridge. This was a unique occurrence as a very large slab was triggered by a skier on the SW face of the ridge, which then triggered another large avalanche on the north side. One skier was caught, carried, and partially buried. See the Near Miss Report HERE.

A fast moving weather system rolled through yesterday with light snowfall and strong ridgetop winds. The peak snowfall was overnight with 3-5″ of new snow at Turnagain pass, a couple inches to the south at Summit Lake, and the winner was Portage and Girdwood Valley where the mid and upper elevations saw 5-8+”. The system is moving out now and quiet weather is forecast today. That means we can expect some lingering wind slabs in the Alpine, which will be today’s avalanche ‘problem 2’. More on that below.

Avalanche ‘problem 1′ is the deadly, but less likely to find and trigger, large slab that breaks in buried weak layers, potentially taking a whole slope out and then some. This type of avalanche is what has been occurring over the past week, and most recently on Cornbiscuit. Slab depths are in the 3-6′ range with thinner areas around 2’ deep. Last night’s snow and wind wasn’t too much of a new load onto the snowpack as a whole, meaning it wasn’t likely enough to cause avalanches in buried weak layers. Yet, with so much uncertainty in the pack right now, our hackles should still be up that a bit of change could push slopes closer to the breaking point; so maybe it’s a hair easier for a person to trigger a large slab today. Things to think about if headed out to Turnagain Pass or surrounding areas:

• Triggering a large dangerous slab is still possible.
• No signs of instability are likely to be seen to warn us – this was a common theme voice by groups on Cornbiscuit last Saturday – and is common with this kind of avalanche problem
• Slopes can release after many people have already been on them. Tracks do not mean the snowpack is stable.
• Most likely places to trigger a large slab is near rocks and in thin areas of the snowpack; these areas can be hidden and hard to avoid.
• Terrain with a shallower snowpack in general is more suspect – such as the south end of Turnagain Pass, Lynx Ck, Silvertip.
• Travel Advice: sticking to the lower angle slopes and out from under large slopes is the best way to manage this issue.

Andrew investigating the large slab on the SW face of Cornbiscuit last Sunday (avalanche occurred Saturday 1/7). Note the hidden rock in the crown face. Photo: Matti Silta 01.08.2023.

As time goes on and no rapid loading by snow or wind occurs, it will be harder to trigger these large slabs. We are entering into the Deep Persistent Slab category. Meaning harder to trigger, triggering is generally from thin areas in the slab, no red flags to warn us, and stability tests don’t tell us much. In fact, tests can even give false positive results, luring us even more into thinking all is ‘OK’. Unfortunately, we’ll be talking about this layer for a while.

This is the same avalanche on the SW face of Cornbiscuit, showing the five tracks from just prior to the avalanche. A portion of the up-track was also covered in debris from this avalanche. Photo taken the day of the avalanche by the group involved, 1.7.23.

Unlike the hidden dragon above, wind slabs should be much easier to see and feel. Look for areas of wind loading and stiffer snow over softer snow. These should be found in the higher terrain just off ridgelines and in cross-loaded gullies. Wind slabs could be up to a foot thick and still touchy as they are likely sitting on surface hoar and faceted snow. Before the wind and snow yesterday, widespread surface facets and surface hoar existed. Something we’ll be watching when the next snowfall rolls in. It is worth noting, a small wind slab does have the potential to trigger a large slab, turning a small problem into a much larger one. One more reason to keep playing it safe today and sticking to mellow terrain.