Managing Avalanche Risk for Recreationists Part 13: Deep Persistent Slabs

Managing Avalanche Risk for Recreationists Part 13: Deep Persistent Slabs

For those of you who find yourselves champing at the bit, ready to start skiing in October, I’m going to put this out front—I’m not a big fan of early snowfalls. To me, having enough snow to ski before mid-November is no cause for celebration. Just the opposite, in fact.

In many cases, early snow is followed by a dry spell before winter really takes hold. Or we get rain-on-snow events followed by colder weather. Both of these early season scenarios lead to the potential for a persistent weak layer (PWL) that lies dormant, deep in the snowpack all winter. Then, in spring, just when we stop thinking about it, that PWL produces isolated, sporadic, but massive avalanches that catch the unwary by surprise. These are deep persistent slabs, typically associated with what’s commonly referred to as low-probability/high-consequence (LPHC) scenarios.

Deeply buried PWLs that remain long-term issues are far less likely to occur in winters when snow starts accumulating later, storms are colder, and there are no long dry spells or rain-on-snow events.

My first direct experience with an LPHC situation (although I didn’t really get the big picture at the time) was in the late ’80s at Marmot Basin near Jasper in the Canadian Rockies. This area has a cold, dry, continental snow climate. Thick early season facet and depth hoar layers on the ground are common, and practitioners there are used to them. Within the ski area boundary, we spend a lot of time in the early season ski- and boot-packing to compact these layers. In this season however, we had a warm, dry spell after early season preparation work that created a crust on the surface of that artificially compacted basal facet layer. A light snowfall followed by a cold snap then created faceted grains on top of the crust.

The facets were small and the layer was only a few millimeters thick, so it was hard to see or find after it got buried. On open runs within the ski area, this crust/facet layer got destroyed by grooming and regular, heavy ski traffic. But in out-of-bounds areas, the skiing sucked early in the winter, so no one was really going after it there. By the time the out-of-bounds runs were being skied, the crust/facet layer was deeply buried under firm snow and skiers were not affecting it.

In March, the ski area was preparing to host a downhill race and, to meet the required elevation drop, they needed a start just above our highest lift in an out-of-bounds area. We dug a snow profile at the proposed location and identified the early season crust (nearly two meters below the surface by now), but the thin facets on top were almost impossible to see so we weren’t overly concerned. Stability tests produced no results, indicating triggering at the crust was highly unlikely.

Being a wind loaded area we’d seen a fair amount of avalanche activity earlier in the winter but, before committing to sending the snowcat in, we hedged our bets by bombing the hell out of that slope. First by applying standard 2 kg hand charges then burying large charges near the crust—all to no avail. Finally, we set up an air blast: 10 or so 4 kg high explosive charges taped to bamboo poles a meter above the surface, spread across the entire slope. We connected them all up with detonating cord and set them all off together. Other than seriously ringing the ears of everyone nearby and scaring the upper lodge staff over a kilometer away when their big picture windows rattled so hard they worried they might break, we got no result—not even a crack.

So, we sent the snowcat operator up (to our credit, we insisted he wear a transceiver).

As we watched from the flats below, the cat started digging into the hill when suddenly a fracture line shot across the entire slope above.

 A split second later, 10 x 10 meter blocks of 2-meter-thick slab came roaring down at us, with a 5,000 kg snowcat doing 360s in the middle of it all as we scrambled out of the way.

Adding insult to injury, a professional avalanche class was riding the chair and witnessed the whole event. They swarmed up the slope to do fracture line profiles and reported that the bed surface was a crust and the failure layer was a very thin, hard-to-see facet layer on the crust. Thank you. Thank you very much….

An example of a deep weak layer fracture propagation. These slides all occurred simultaneously and extended trim lines on two adjacent but disconnected runouts. Photo credit: Karl Klassen

While I’m sure there were other deep persistent slab seasons after this eyebrow-raising event in the Rockies, my next very clear recollection of an LPHC event is the 1996-1997 season. I was now working as a guide in the Columbia Mountains of British Columbia, where the snowpack is deeper and warmer, and you don’t see the basal facet/depth hoar as much as in the typical Rockies snowpack I’d grown up with. But interior BC is no stranger to LPHC scenarios associated with deeply buried early season PWLs.

Early in the 1996-97 season, the snowpack got warmed up and rained on. I don’t recall if this was an atmospheric river event, but ARs are commonly the cause of what I’m about to describe. As the warm, rainy storm tapered off, temperatures fell and the wet snow surface got covered by colder, dryer snow. Faceting went nuts at the warm-wet/cold-dry snow interface, and eventually a crust formed resulting in a crust/facet combo.

Skip ahead to March 1997. I’ve just lifted from a backcountry lodge with a helicopter full of skiers. We’ve seen almost no avalanche activity for quite a while. It’s a warmish, sunny spring morning after a getting a bit of nice dry powder overnight. Spirits are high, and everyone’s excited to hit the new snow even though it’s not a big dump. It’s looking like a typical great spring skiing day in the Selkirk Mountains: 20 cms of fluff on a firm base, under blue skies with no wind.

A few minutes into the flight, I turn to the pilot and say, “Do you smell something?”

My immediate concern is some kind of helicopter problem, but it quickly becomes apparent it’s the smell of freshly cut timber. There are no logging operations in the area and we’re a couple hundred metres above the ground, so this is weird.

As we turn the corner into the side valley where we’re planning to ski our first run, the smell becomes intense, and we’re faced with a scene of destruction unlike anything I’ve ever seen. There’s a huge fracture line across the entire alpine bowl above us. Massive swathes of mature timber are down—trees a meter or more at the butt are splintered like matchsticks. As we fly further, I can see the fracture line dips down into the treeline at the ridge that forms the far end of the bowl. It runs across the ridge, connecting clumps of trees, boulders, and shallow spots. The fracture line continues on the other side of the ridge and climbs up into the next bowl, which has also been completely cleaned out with hectares of trees destroyed, creating a significantly wider and longer avalanche path than what existed there just hours earlier.

What triggered the thing is still a mystery to me. But probably it had to do with a bit of new load from the minor snowfall overnight combined with sun warming the upper slopes, and maybe a small cornice fall? Anyway, we quickly revisited our hazard analysis and added a lot more red to our run list.

This was the only avalanche of note we saw that day or the following week. Other operations reported one or two similar events but none near us. But it turned out to be the onset of an ongoing series of intermittent, isolated, and often unexpected avalanches—a classic LPHC situation where the chance of triggering was low but the consequences of hitting the wrong spot at the wrong time were unacceptable—for me as a guide anyway. The LPHC conditions persisted right through to the end of the season that year. Lots of people all over the province were far more aggressive than the professional community that spring, and a lot of them got away with it, although I suspect more by chance than by sound risk management and informed decision making.

That said, the 1996-1997 season saw nine fatal accidents resulting in 13 fatalities, with the November crust/facet layer being a major player throughout all the ranges of western Canada. Yes, you coastal skiers in maritime climates, take note: While it’s true that PWLs and LPHC cycles associated with deep persistent slabs are less common for you, they do happen in even the warmest and deepest of snowpacks when conditions are right.

Deep persistent slab avalanches are routinely size 3 or larger. They often run full-path, from ridgetop to valley bottom. It’s not uncommon for them to extend historical trimlines, running wider and farther into bigger trees than you might imagine. These avalanches are essentially un-survivable if you are caught in one.

It’s extremely difficult to assess when and where a deep persistent slab avalanche might occur and what it will take to trigger one. Even if you are willing to dig deep (remember these PWLs are generally a minimum of 2 meters and often more below the surface), snowpack observations and stability tests aren’t reliable indicators of sensitivity to triggering or propagation potential. LPHC patterns are, by their very nature, defined not by a series of related events that we’d typically call an “avalanche cycle.” They are characterized by isolated, intermittent, large to very large avalanches that are often widely dispersed over space and time.

In winters when conditions are conducive to deep slab problems and LPHC scenarios, I spend a lot of time looking for isolated, out of the ordinary, anomalous events. I must do this across ranges, on a daily basis, for weeks on end. If I do see deep persistent slab occurrences, I need to have a really sound understanding of how the snowpack has evolved throughout the entire season over vast areas to determine if the slope I want to ski presents undue risk or not.

Even the most highly trained and experienced professionals struggle with assessing and managing risk in LPHC scenarios. While I think it’s beyond the scope of most recreational skiers to analyze and forecast low-probability/high-risk, I do think they can learn and use professional strategies for managing it.

I suggest you consult your public avalanche forecast or other professional information sources to determine if the conditions for deep persistent slabs exist. Read deeply and broadly for talk of low-probability and high-risk and to get a feel for how experienced professionals are thinking.

When I’m talking to my peers in LPHC situations, we don’t speak much about snow science, results from avalanche control, or avalanche observations. We tend to discuss things like risk tolerance, the psychology of heuristic decision making (human factors), our motivations, and managing our own and our guests’ expectations of what a good day of skiing means.

I dial my risk meter way back. I think carefully about why I’m doing what I’m doing (especially if I feel like stepping out into steeper, larger, more complex terrain). I work really hard at figuring out how to find great skiing on moderate, low-threat slopes. I work hard to recognize and manage the feelings of being pressured to ski more aggressively.

I take a highly structured approach to route planning and trip preparation (using the daily process I’ve been harping on throughout this series) even when there’s seemingly little or no change in conditions.

In my experience, thinking you can outsmart deep persistent slabs in LPHC scenarios is unwise; the only low-risk strategy is to wait them out.

So my expectations are that I’ll have to avoid avalanche terrain and overhead hazard for extended periods of time, weeks for sure and probably months, perhaps the entire season. This requires extreme discipline and vast reserves of patience.

Choosing low-angle simple terrain is the answer when dormant persistent weak layers might wake up. The author, pictured here, enjoys a low-threat slope with his son, age 12. Photo Credit: Colleen Gentemann

If your risk tolerance is higher and you do decide venture into bigger terrain, here’s some ways to reduce risk:

  • Stay away from slopes with thin or variable depth snowpacks—avoid shallow areas like the plague.
  • Be extremely mindful of what’s above you at all times. Sluffs, small avalanches that would otherwise be of little concern, small cornice falls—even sudden warming from morning sun are often the triggers of deep persistent slabs during LPHC periods.
  • Stay much farther back from the flanks and runout zones of avalanche paths than you normally would.
  • Make sure everyone in your group is aware of and agrees to the same level of risk as you.
  • Apply enhanced safety procedures: Spread out farther, go one at a time more often than normal, and do not group up in runouts even when the start zone is far away.
  • Pull back early at the onset of changes in weather, even if the change seems insignificant.
  • Stay conservative during changing weather, especially if rapid changes are occurring, like a quick temperature rise or a short burst of intense precipitation.
  • Wait far longer than normal after weather changes have ended and conditions seem to be stabilizing.
  • Be extremely cautious in the onset, during, and for quite a while after the first big warmup of the spring. This is often the first big stressor of those deeply buried PWLs that have been dormant for months.
  • By hyper-aware of your decision-making on warm, sunny, bluebird spring days. Not only do these days stress the snowpack, this is also when the threat of avalanches seems less acute and people tend to make very aggressive decisions.
  • Always have a fallback/exit strategy and never hesitate to use your plan B if you are feeling unsure or nervous.


Perhaps the most important thing I’ve figured out over the years that helps keep me safe in LPHC seasons is to not feel pressured to ski aggressively—this includes internal pressure I put on myself to provide my group with an exceptional experience and external pressures from peers, guests, or company owners who are suggesting we do things I’m not comfortable with. I’ve learned to ski joyfully  on simple, low-angle terrain. And I’ve found if I can express my joy at just being in the mountains to those I’m with, all our lives are much simpler and happier. Coming home alive knowing you can ski harder in the future when conditions are better is the best reward.

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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