Managing Avalanche Risk Part 11: Wind Slab Avalanche Problems

Managing Avalanche Risk Part 11: Wind Slab Avalanche Problems

In No. 10, I talked about a major dump of new snow that transitioned from great skiing to an increasingly touchy storm slab problem later in the day followed by an overnight cycle of natural avalanches.

By the morning after the stormy day, the natural cycle ended when the slab property (cohesion in the new snow), load (weight of the new snow), and bonds at a layer of plate crystals within the storm snow came to an uneasy truce—a situation forecasters refer to as “conditional stability.” Conditional stability means the snowpack is in balance: Natural avalanches aren’t happening, but it won’t take much in the way of change or a trigger to start avalanches or set off another cycle. We ran a more cautious program the second day but had still pretty good skiing in spite of the snowpack being near the tipping point. In this article we pick up the story as we came in from skiing on this day.

As is my routine, one of the first things I do at the end of my day is to look at the afternoon weather forecast. I saw little of little concern in terms of more snow, changes in temperatures, or potential solar radiation effects, but the forecast predicted wind would start in the evening and blow for 24 hours or so. Winds are notoriously tricky to predict in the mountains and our area is no exception, so I went to bed prepared to do a full update on our hazard analysis and risk-management plan the next morning.

The next morning, I noted the public forecast, issued the previous afternoon, predicted wind slabs. As soon as I stepped outside to have a look at conditions around the lodge, I knew things had changed overnight. In the moonlight, I could see dark green trees across the valley that had been snow covered the prior afternoon. Clouds were moving pretty fast across the sky and, even well below treeline at lodge elevation, it was breezy with drifts already visible in the yard. A review of nearby weather stations showed steady, moderately strong SW winds at upper elevations overnight and a bit of a mixed bag lower down with gusty SW, W, and NW winds.

“Under conditions like this, it’s easy to get caught by surprise if you’re not paying close attention.”

Winds can be tricky for even seasoned experts, especially in complex terrain that changes the wind direction.

Winds in general are a challenge in our area. The terrain is quite convoluted with features that cause twists and turns in the wind patterns, so it can be hard to tell where wind-blown snow has ended up. And the way the main valley lines up and funnels SW winds over our largely south-east and east facing terrain, a south-westerly produces a lot of cross-loading, which creates weird, disconnected slabs in the lees of even relatively minor gullies, rock outcrops, and clumps of trees. Under conditions like this, it’s easy to get caught by surprise if you’re not paying close attention.

In our morning meeting we all agreed we’d exceeded threshold wind speed—this is the minimum required to move snow and, while it varies depending on conditions, with recently fallen new snow it’s generally around 25 km/h. Given the upper flow was SW and steady both in terms of speed and direction, our hazard nowcast listed widespread wind slabs up to size 2.5 were almost certain on northeast-facing alpine terrain and smaller with less widespread, somewhat smaller wind slabs on adjacent east and north aspects. At treeline we figured we’d see mostly isolated, small wind slabs on a variety of aspects and noted cross-loading as something to keep an eye on. Below treeline, while the wind had blown there too, we figured wind slabs were unlikely, but the lingering storm slab might still be sensitive to skiers, especially in open glades. In addition, we expected cornices had grown and were likely not yet bonded.

This analysis, with several overlapping problems, was a major change from the previous afternoon when all we really had to worry about were storm slabs that were stabilizing. Just 12 hours of wind had created a much more complex risk-management challenge than what we’d been dealing with the last couple of days. And, given the winds were forecast to continue all day, conditions were quite dynamic—we predicted wind slab and cornice problems would become more widespread and more unstable with each passing hour.

This new situation required a pretty significant revision of our run list. Many of our bigger lee alpine features now were closed either due to on-slope wind slabs or overhead cornices. On the other hand, we were confident opening windward (south, southwest, west, and northwest) features, even large ones in the alpine where the recent storm snow would have been scoured off by wind. But we weren’t very excited about the prospect of skiing wind-hammered slopes, so even though they were green, we all knew these areas would be a less desirable fallback plan.

As we headed up the hill, we saw that cornices had, indeed, grown. Even though no new snow had fallen overnight, our tracks from the previous day were filled in on lee slopes. The snow surface had lots of wind ripples, drifts, and even sastrugi in some spots. In exposed areas, the storm snow from a couple of days ago had been stripped right down to the old snow surface, and most start zones lee to the wind had fat pillows of firm-looking snow in them. Overall, except in the most sheltered lower elevation terrain, it was clear the soft powder of the last couple of days was now anything from wind scoured to wind crust to wind slabs, depending on elevation, aspect, and terrain feature. There were no signs of natural overnight avalanches or cornice fall.

I started at treeline and was very careful on my first few runs. I skied short pitches from feature to feature, stopping my group above me each time the terrain shape or aspect changed. I had them keep an eye on me as I poked around, ski-testing small features with no terrain traps to assess conditions in places I suspected the wind had deposited snow. My ski-cuts produced minimal cracking but no significant results, even though I could clearly see and feel the wind slabs around me and under my skis.

This is my least favorite scenario. I knew the problems described in my morning theory existed—I could see and feel them. My analysis indicated there should be at least a few natural avalanches and skier-triggering would occur on more aggressive features. But I wasn’t seeing anything like this. So now, my questions were:

  1. Was my morning analysis too conservative and could I push out a bit?
  2. Had things not yet progressed to the tipping point, giving me a window of time in which I could ski a bit harder before it got worse?
  3. Had I just not yet found the locations where the problem was most acute?

I consulted with my team and the snow-safety crew, and everyone was seeing the same thing, so I decided it was likely scenario B: the problem was there and likely deteriorating, but it was taking a bit longer than we had expected to get to the tipping point. So, I decided to head for slightly more aggressive terrain to get a couple more exciting runs in before having to pull back when things got worse.

And…. I promptly got my ass kicked.

“I didn’t even have time to toss my poles or trigger my airbag much less get rid of my skis before I was pounded by a 50 cm thick slab that knocked me down and buried me to my shoulders.”

Having instructed my group to spread out a bit more than usual I rolled over a convex feature without bothering to do a ski-cut like I’d been doing all morning on similar features. The snow was definitely slabby—stiff enough that I was only penetrating 5 cm or so. Then, as I hit the transition to flatter terrain below the roll, the slab thinned and I broke through the stiffer surface into softer snow below. I flailed for a turn or two adjusting to the change, then stopped to warn the first skier coming down above me.

Either me breaking through or the added load of a second skier or both triggered a crack about 20 metres above me that quickly propagated across the entire 30 m width of the slope. I didn’t even have time to toss my poles or trigger my airbag much less get rid of my skis before I was pounded by a 50 cm thick slab that knocked me down and buried me to my shoulders. Even though it was years ago, I can still clearly see the surprised expression on the face of the skier who zipped past me, totally out of control, trying to ride the slab that was rapidly breaking up around him. He fortunately managed to stay on the surface as the slide came to a stop just below me.

Wind scours the windward side of the mountains and deposits it on the lee side. This is also how cornices form.

On inspection I found the slab on this pocket of terrain was a bit thicker and stiffer at the top of the slope than what I’d seen previously—could be the terrain and wind interacted a bit differently than in other places or another hour of wind had put more snow here since I’d last poked around on similar terrain. Either way, there was a bit more load and slab property here than in other places I’d skied earlier. And the failure layer was the very thin, almost invisible layer of plate crystals from two days prior that were still not well bonded to the snow above and below.

Embarrassing, yes, but nobody got hurt and we didn’t even lose any equipment, although the group did spend 15 minutes looking for one of my skis that got pulled off as I was getting pummeled.

As for the moral of the story, well, even with something as obvious as an expected and observable newly deposited surface wind slab, you can still be wrong. In this case, I was wrong in believing a more aggressive decision was appropriate. If I’d gone with scenario C above, I would have had a more conservative approach and probably nothing would have happened, but I would have missed out on a powerful learning opportunity. Who knows—maybe if I hadn’t got such a big surprise from this small event, I may have been more prone to ignoring things like it in future, risking a more significant incident later in my career.

Most importantly, we had a really good evening meeting where the most experienced guy in the group got grilled by the younger guides on the team. We did a deep dive into our day and analyzed my thinking and decision-making process, which hopefully added to everyone’s store of knowledge, allowing us all to be that much wiser next time. So yeah, other than a hurt ego from once again having to admit in front of others that I’d made a mistake, it was a win-win kind of day.

Karl Klassen took his first professional avalanche course in 1979 and spent the next 40 years working as a forecaster, guide, educator, and mentoring up and coming forecasters and guides. During that time he also sat on the Association of Canadian Mountain Guides board of directors including serving as vice president, president, then as executive director. He joined Avalanche Canada in 2004 as a public avalanche forecaster and became the warning service manager in 2009. Recently retired, he lives in Revelstoke, B.C., where he spends his time cycling in the summer and skiing in the winter.


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