After the major onslaught of 4-5 feet of new snow with strong wind last Tuesday through Friday, another shot of precipitation just ended this morning. This last pulse dropped between 10 and 16 inches of new snow since Sunday afternoon; also with strong easterly ridgetop winds. The group on Tincan yesterday was able to easily trigger many fresh storm slab and wind slab avalanches. These were composed of just this last round of snow. Today, we are expecting those types of new snow instabilities to be a bit more stubborn, but still possible to trigger. What is more concerning however is what this past week’s worth of snow fell on.
Under the 4-6 feet of settled new snow since last Tuesday are various weak layers of buried surface hoar, near surface facets, and facets surrounding sun crusts. Around a foot under that is another layer of buried surface hoar (March 16th layer) and yet a third layer under that (March 2nd layer). We know for certain that these layers were causing extremely large avalanches on Friday when the storm was at its peak. Crowns propagated several thousand feet wide, many of which are now covered back up. With skies possibly clearing this afternoon and tomorrow, it is critical we understand these weak layers are in the snowpack.
If a person is able to trigger one of these buried weak layers, the size of the avalanche will be deadly. We are talking about a 4-6′ deep slab with the potential to take out an entire slope and propagate across terrain features. This exact thing occurred on Saturday at Pete’s North during a short break between storms. Even though the weak layer is quite deep and may be harder to trigger now, if we hit the right spot all bets are off. The folks on Pete’s North triggered a slab up to 4′ deep when they took their skis off and their boots sank deeper into the snow, impacting the weak layer. They triggered it remotely, on a flat spot on the side of the slope. With no good or safe access into the upper elevations yet, we have to be more cautious than normal due to the potential size of this kind of avalanche. This means sticking to the lower angle slopes with nothing steeper above you. With a lot of uncertainty and such high consequences, it’s simply best to avoid the bigger terrain and steep slopes.
In addition to this large avalanche issue are smaller avalanche issues. These would be those storm snow instabilities from yesterday’s snowfall: wind slabs in the 1-3′ deep range and storm slabs in the 10-16″ range. Cornices are also a big concern and could be teetering close to failure. With almost a week of strong winds with all the new snow, we can expect cornices to be dangerous.
For an idea of how much snow, and water weight, is on the most recent weak layer, which is the March 22nd layer that consists of buried surface hoar, near surface facets and sun crusts, see the numbers below. We call it the March 22nd layer because that’s the day the storm began and when it was buried.
Girdwood: 4-6 feet of settled snow (4.5 – 6″ SWE, Alyeska mid and top respectively)
Turnagain: 4-6 feet of settled snow (5.2″ SWE, Center Ridge)
Summit: 6-10 inches of settled snow (0.8″ SWE, Summit Lake)
New snow that existed on the south side of Turnagain Pass (Johnson Pass area) before yesterday’s 10-16″ of additional snow. This photo also shows the deepest of the three layers of buried surface hoar (BSH), the March 2nd layer.
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Skier triggered storm slab avalanche in the Tincan Trees yesterday – the slab depth is around 16-20″. Photo courtesy of Alaska Guide Collective 3.28.22.
