BC PWLs

I had a shitty day today getting two bodies out of slide in a permenant closure at kicking horse and it got me thinking about the snow pack out her in BC this winter... it reminded me of this post by Mark Klassen on the ACMG site a couple of weeks ago

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I thought I'd add to Eric's very apt recent post to the MCR regarding recent events and the snowpack on the coast. I've been watching the snowpack across the west since late fall and have recently been guiding and instructing trips and courses in a variety of regions. The persistent weak layers that Eric refers to in his post are widespread throughout the western mountains of Canada and, from what I can tell, they also exist in many areas south of the border. In recent days, there have been numerous avalanches initiated by natural and human triggers in various areas around the province. While the ones Eric reported on the coast are certainly dramatic, I've also heard about and seen slides triggered on the February and January layers in the interior and the rockies. In one case, near Rogers Pass, it's suspected that a large natural avalanche stepped down to one of the December layers or perhaps even the November layer. In the Rockies, a large avalanche released at or near ground in the November layer. This is all eerily reminiscent of last year when the layering in the snowpack was very similar to what we are seeing this season. Even the dates of this year's persistent weak layers are almost identical to last year. In 2008, between February 26 and the end of March over 1000 natural and human triggered avalanches size 2 or larger were reported, including numerous close calls and a couple of fatals. I suspect there were many that went unreported. Last year, PWLs were known or suspected as the failure layer in almost every avalanche fatality.

If anything, this year is weirder and more complex than last. Experienced practitioners with 30+ years of experience have never seen anything like the layering in this year's snowpack and many of us are waiting for the other (next?) shoe to drop. Usually, avalanches in the Coast ranges or the Rockies have little or no relevance to the interior ranges. This year, however, I think the recent incidents reported on the Coast and in the Rockies are directly relevant to what we might expect here in the interior. On the coast, the PWLs have received more load and warmer weather than the interior. Given that the layering on the coast is similar to what we have inland, I wonder if the slides Eric reported might be a premonition of things to come in the interior as load increases and spring arrives. In the Rockies, the snowpack is generally shallower and weaker than what we see in the interior. This year, however, I have seen many areas where the interior ranges snowpack is highly variable in distribution and depth. I wonder if shallow areas in the interior might react similarly to what happened in the Rockies last week. In my opinion, in a winter like this, it's wise to keep an eye on conditions farther afield than you might normally--my nearest neighbour in a PWL winter may not be geographically close by--it could be a place hundreds of kilometres away where the snowpack is similar. To me, the incidents in the coast and rockies lately are directly relevant to decisions I am now making in Rogers Pass, for example.

As Eric mentions in his report, the PWLs have been largely dormant in the recent dry spell and this has produced high confidence in people who may not realize that a) even when PWLs are dormant, the occasional large and highly destructive avalanche is common and, b) dormant weak layers often wake up, become more sensitive to triggering, and sometimes fully reactivate when stressed by weather factors. The most common weather factors that prod a dormant PWL are solar radiation, loading by wind or new snow, rain, and warm temperatures. Even if these factors do not trigger a deeply buried PWL directly, they often trigger cornice failures or smaller surface avalanches, which then step down to the deep PWLs.

While skiing in the Rogers Pass area last weekend, I saw direct evidence of what I think is an underestimation of the potential of this year's snowpack. The Pass had been more or less skied out during the PWL dormancy in the dry spell. Last week the region received a moderate snowfall, winds, warming temperatures, and some solar radiation during sunny breaks, which resulted in a pretty decent avalanche cycle. Natural activity died out pretty quickly (just not enough load or slab to sustain a major natural cycle) but skier triggering of smaller slides continued right through the weekend. Within 24 hours of the end of natural activity, people more or less jumped right back onto many of the very aggressive lines that had been skied during the drought. This might be reasonable if a path has slid or if you have a complete history of the snowpack and avalanche activity on a given slope but it's kinda out there if you are just repeating what you did earlier in a period of low hazard or if you are blindly following others who might know even less than you.

Even if you are not going to shred the gnar on the steepest lines, it's worth thinking carefully about what you are exposing yourself to. In my attempt to go into Asulkan Hut Friday, I found there was no debris in Asulkan Brook below the Mount Abbott slidepaths and saw no sign of any significant avalanche activity on the slopes above. With warm temps, sun beating on the slopes above, and big cornices on the ridge high above, I decided not to take the route everyone else was taking up the creek but rather broke a high trail through the trees above the trimline. At the Mousetrap, I decided to turn back given the variety of issues in the snowpack at different elevations and the sun hammering the start zones of the Ravens slidepath. In the meantime, numerous other parties walked up and skied down the valley using no safety measures and travelling the same route(s) that had been set (and were probably reasonable) during the drought. I saw a similar approach to terrain in other popular locations such as McGill and Connaught. Talking to folks on the trail, the most common reasoning was: "There's been at least 100 people through there in the last day or two and nothing has happened, so it's obviously okay."

We have been in a "low probability of triggering/high consequence if caught" situation for a couple of weeks--no one got killed so confidence is high. Recently, there's been a number of close calls and some serious injury accidents across the province--nothing dramatic enough to make the top of the evening news--so confidence has not waned the way it might of we were seeing bigger storms or more newsworthy accidents. We are now in a situation where many professionals feel the snowpack is in balance and just waiting for the next change in weather and/or a human trigger to set off something disastrous. Is it likely to happen? No. At least not until there's more weather factors in the mix. Is it survivable if you are caught? No.

I admit to getting old and I'm certainly more conservative now than I was even 10 years ago. I'm not trying to convince anyone to stay home. I do think, though, that people should make informed decisions about risk, especially if they are taking other, less experienced people with them into the mountains. What I saw in Rogers Pass last week disturbed me deeply, not because I'm against people taking risks but because I don't think everyone in the groups I saw was making fully informed decisions.

Here's some things I'm doing to hedge my bets this winter:
Get local information from credible sources about what the concerns are and where/when they are most likely to be sensitive to triggering.
Don't trust information from snowpack observations, the pack is too complex and many of the layers buried too deeply.
Do not assume that just because you (or others) have done something before with no problem that it's appropriate to do it again this year or this week. We are seeing numerous avalanches taking out 100 year old timber this season so there's a combination of factors at play that no living person has experienced.
Look up--all the time--and reduce exposure to large slopes that show little or no sign of significant avalanche activity, more so when the weather might be stressing the snowpack.
Travel on smaller terrain features with little or no convexity.
Travel on lower angled slopes that are well supported.
Give yourself a little more margin for error than you normally might: in avalanche terrain use routes, travel techniques, and group management practices that minimize exposure time, reduce the number of people exposed, and maximize the use of safer terrain.
Beware of warm day/blue sky syndrome: we tend to underestimate hazard and make more aggressive decisions when the weather is good.
Timing is key: a few days can make a huge difference. Give the snowpack more time to adjust after weather changes than you normally might.
If there is any doubt about the snowpack, there are three things that are of critical importance: terrain, terrain, terrain.
Be prepared to wait for a better winter before tackling more aggressive lines or trips.
There's lots of places where the hazards and risks are manageable. Assess your motivation, carefully think your decisions through, and consult with credible, experienced, practitioners if you think you may not have the training or knowledge to make a fully informed decision.
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