Turnagain Pass

Storm totals from our most recent storm cycle were around 3-4′. During the tail end of this storm, Friday night winds were blowing 40-50 mph with gusts in the 80’s, creating a heavy load and sensitive slabs. Here is what we know happened immediately following the storm:

• Multiple medium sized avalanches released in paths along the Seward highway yesterday, some of which ran to the valley.
• We received multiple reports of soft slab avalanches roughly 2′ deep that failed at lower elevations on Seattle Ridge sometime during the most recent storm (Friday or Saturday).
• Riders were able to trigger small avalanches on wind-loaded test slopes in the Seattle Ridge flats yesterday.

Over the past 24 hours, average ridge top winds between 15-20 mph, with sustained periods between 40—50 mph, have drifted snow into sensitive slabs 3-5’ deep or deeper. Today it will be easy to trigger an avalanche on this layer. A fresh wind slab will feel denser and more consolidated than storm snow. It may also feel ‘punchy’, or hollow. You might notice cracks shooting out from your snow machine, skis, or snowboard as you approach one. Expect to find fresh wind slabs on the leeward (downwind) side of ridges or rollovers, and they are often deposited immediately below cornices. Pay attention to all of these signs, and give any suspect slopes a wide berth. The good news is that it doesn’t take very long for a wind slab to gain strength. For today, however, they will be easy to trigger, and they could be large. It is possible that a wind slab triggered by a person could step down to a deep weak layer near the bottom of the snowpack.

Cornices: We are starting to see some bigger cornices developing as strong winds have plenty of snow to move around after this week’s storms. These new cornices will be prone to breaking, and they have a tendency to pull back much farther up the ridge than you would expect. If you are unsure of where the ground ends and the cornice begins, look for visual cues like rocks or vegetation poking out of the snow.

Tincan. Plenty of snow to get loaded above treeline at Tincan. Photo: Tully Ward-Hamer. 11.28.2020

The weak layers at the bottom of our snowpack are now buried by 3-5’ of snow throughout our advisory area, and they may be as deep as 6-8’ in areas that have been wind loaded. As this layer gets buried deeper and deeper, it is becoming more difficult to trigger an avalanche near the ground. The scary thing is that the potential for destruction is increasing at the same time as the likelihood of triggering is decreasing. These avalanches have the potential to propagate very wide, and may be triggered remotely from below or adjacent to a slope. Further complicating the issue is that our typical stability tests (Extended column test, Compression test) are not equipped to give us any information on whether a person could trigger an avalanche on a weak layer more than about a meter deep. It is also unlikely that you would experience typical warning signs such as cracking or collapsing with a deep slab problem.

Given the difficulty of predicting just how sensitive a deep persistent weak layer might be on a specific slope, and considering the major loading event we have had (including fresh wind loading in the past 24 hours), the only way to manage this problem is to avoid playing on or below slopes steeper than 30 degrees. The most likely places to trigger a deep slab avalanche are areas where the snowpack is thinner. This may be near rocks or trees, or in areas that were previously wind scoured. Generally, as you move further south down the pass and towards the Summit Lake area, we are seeing a thinner snowpack with more sensitive weak layers at the ground. For now, we need to be patient and avoid playing on or below steep terrain until the deep weak layers have more time to heal.

Seattle Ridge. This weak snow at the ground has now been buried by an additional 4′ of snow. This is a scary setup. 11.26.2020