Turnagain Pass

A snowmachiner triggered a large wind slab avalanche just outside our advisory area near Whittier yesterday. He was able to ride off the slab and was not caught or carried. The avalanche was up to 3’ deep, 250’ wide, and ran 600’ vertical. More details here.

Photo of the snowmachine-triggered avalanche near Whittier yesterday. Photo: Warren Gage 04.01.2023

Our main concern is still the potential for triggering a really big avalanche on the weak layer of facets that was buried two weeks ago. Since last Saturday we’ve seen almost 20 huge avalanches triggered on this layer, and the problem remains today. These avalanches have been 3-6′ deep or deeper, and many of them were triggered remotely from low-angle terrain connected to steeper slopes. This is a remarkable amount of activity for a deep slab problem, and it is nothing to mess around with. As we get further out from our last major loading event this layer is becoming more stubborn and avalanches are slowly becoming less likely, but we are still on edge about it.

These deep persistent slab avalanches rarely give any warning signs prior to avalanching, and our normal stability tests don’t tell us anything about how reactive the snowpack is. All we have to rely on is the fact that we continue to see big avalanches failing on this layer, and we have seen a very poor setup in every snowpit we’ve dug over the past week. This includes snowpits on Tincan, Sunburst, Magnum, Cornbiscuit,  and Lipps. The avalanches we saw on the back side of Seattle Ridge and in the Skookum drainage are all the snowpack info we need to tell us that poor structure is consistent across our advisory area.  For now the only way to manage the problem is to avoid traveling on or below steep terrain, and wait for that weak layer to gain some strength.

This is the setup that is giving us so many problems. The combination of a 3′ thick hard slab on top of a reactive weak layer is not a good one. Photo taken Thursday on Cornbiscuit, but this is the same structure throughout the advisory area.

This avalanche occurred last Saturday, but it is the kind of thing we are still concerned about today.

With another day of northwest outflow winds on the way, we expect to see reactive wind slabs continue to form today. These will be most likely just below ridgelines, in steep gullies, or on convex rolls. Yesterday a snowmachiner triggered a wind slab avalanche big enough to bury a person just outside of our advisory area near Whittier, and we expect to find with similar conditions throughout our advisory area. While these avalanches are much smaller than the deep slabs we are concerned about (see Problem 1), they are still big enough to be paying attention to. Be on the lookout for snow blowing off ridgetops as you travel as an indicator that senisive wind slabs are forming. Fresh wind slabs may give you warning signs like shooting cracks, and the surface will feel stiffer than the snow that has stayed protected from winds. As mentioned above, the weak setup we’re dealing with already has us sticking to low-angle terrain, so this wind slab problem is really just one more thing to watch out for.

Loose Wet Avalanches: The sun will once again be a factor today, with mostly clear skies and mild temperatures on the way. We will likely see the snow surface heating up enough on southeast to southwest slopes to see some loose wet avalanches this afternoon. There is a chance that one of these loose snow avalanches failing near the surface could pull out a bigger slab as it moves downhill. Pay attention to changing conditions through the day, and be aware of increasing danger as you start to see rollerballs on solar aspects or feel those surface crusts breaking down in the afternoon.

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