Turnagain Pass

With sunshine and soft snow, folks that were out yesterday found the pot o’ gold and a shamrock of day. Overall signs are pointing to a snowpack that is trending to green (LOW danger) and that triggering a persistent slab avalanche is becoming unlikely. It has been a week since the last avalanche large enough to bury and kill a person was triggered on our weak layer/crust combination of concern. There have only been a few small wind slab avalanches reported since then. Over the past week there have been a number people out enjoying the snow with no incident. However, completely trusting a snowpack with obvious persistent weak layers is hard as there is always some uncertainty. There are a couple factors keeping the danger elevated today. One being increasing winds and snow available for transport. The other main data point is that for the second day in row skiers backed off of their objective due to a large collapse (whumpf) and then dug down to find a buried sun crust with facets. On Tuesday it was on Orca and yesterday it was on Goat Mountain. This is a sign of instability combined with poor snowpack structure. Observers on Maxs and Magnum yesterday found similar structure on southerly slopes but no collapses while traveling or propagation in snowpack tests.

Here are some things to keep in mind today:

1. The west/northwest winds are forecast to increase throughout the day. This may add a bit more load and slab over the buried weak layer (more on the winds in Problem 2). The most likely places to trigger an avalanche will be on steep slopes that have seen recent wind-loading, with stiffer snow on the surface.
2. Southerly slopes with the buried sun crust and facets seem to be more suspect than northerly slopes that just have buried facets and/or surface hoar and no crust.
3. There is more snow over the buried weak layer in Girdwood, Portage, Placer/Skookum and the northern end of the Pass then there is in the terrain towards the southern end of Pass. Where there is more snow there is the potential to have a deeper slab.
4. Slab vs. no slab really is the question. If there is just soft snow over the weak snow (with or without the sun crust below) then there is no issue, aside from sluffing. Sussing out if there is a slab is key. Hand pits and small slope tests are good tools for this. One thing that is working in our favor for stability and riding conditions is that the cold temperatures have continued to loosen the surface snow as it facets out.

What to do with this information? As always, pay attention to changing conditions, watch for signs of instability, use good travel protocol, and think about terrain consequences.

Sluffs: Steep slopes that have been sheltered from the wind have around a foot or more of loose snow sitting on top of firm surfaces. It will be easy to trigger dry loose avalanches today, and they can pick up enough volume and speed to carry a person. While it is unlikely they will be big enough to bury you, they can be dangerous if they drag you into terrain traps like cliffs, trees, rocks, or gullies.

Sun effect: Despite cold temperatures and winds, be on the lookout for sun effect on steep southerly slopes, especially under rocky areas at lower elevations.  There is sun crust that formed Tuesday on steep southerly slopes. Watch for the surface snow becoming moist and small roller balls.

Snow pit on Magnum at 3000′ on a south aspect yesterday, 3.17.21. Note the sun crust that is easy to see. We found a layer of small facets both above and below it.

The west/northwest winds are expected to increase today which may increase the likelihood of triggering a small wind slab avalanche. These will have the potential to step down to the weak layers discussed above, creating a larger avalanche. As we have mentioned before, 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.  This wind pattern also increases through channeled terrain and can be more pronounced in Crow Pass and Portage. Be aware of active wind-loading, and keep an eye open for signs that a slope has been recently loaded. This will feel like stiffer snow at the surface, and may have the appearance of a smooth, rounded pillow. If you experience any collapsing (‘whumpfing’) or cracks shooting out from you or your machine, the snowpack is giving you clear signs that it is capable of avalanching.

Wind texture on the north side of Magnum yesterday, 3.17.21. There was just enough wind in the afternoon to blow a little snow around. The north side was getting loaded from the south (northwest winds being funneled from the south through Turnagain Pass).