Persistent weak layers can lead to large avalanches: A warning

Persistent Weak Layers and large Avalanches

Yesterday (July 18) I went into the backcountry from Porters and triggered two large avalanches. Known as remote triggering, I triggered each slope from a distance of 40-100m away. I did not get caught in either slide. The first one was on the Main Line off Allison Peak into Crystal Valley. I entered the top of the slope on the Northern aspect of the line (it wraps to face east). I took out my probe to gauge the snow depth and hiked down about 50m from the summit. The snow depth was no more than 40cm and I deemed it shallow enough that it would not harbour deeper weak snow. I’d skied Big Mama (similar aspect and elevation) on Saturday and saw the bomb craters left by the Porters Patrol. There had been no releases on Big Mama and I extrapolated that Main Line would have similar conditions. I was wrong. As I began to cautiously ski, something felt amiss. I kept pausing to flip my pole upside down and push the handle into the snow to gauge the snow depth. It was still shallow and I was hitting the ground at 40-50cm. On one of these pauses, I noticed cracking about 40m below me. I watched the slide propagate widely, all the way to the low saddle in Crystal Valley. At this point I headed down onto the bed surface as I’d be safer where the slide had already run. I took off my skis and booted back up the shallow snow to the Peak. The avalanche released 200m below it’s usual start zone. My group and I headed down the ridge to the shallower-angled saddle to study the crown wall. We had very easy test results there and everyone enjoyed the education!

On the second slope, our group was on a broad ridge. I went to traverse off the ridge to avoid some rocks, felt a collapse (whoomph) and a south facing slope 50m above and 100m to the side of me released and ran about 300m down and 300m wide.

For several years now – in fact, the past seven years, we’ve enjoyed a mostly stable Craigieburn snowpack. This year is not the case. I’ll describe the problem of persistent weak layers, how they form, what they look like and what it takes for them to go away. I’ll make some suggestions for avoiding them but at this point, it’s best not to go out into the backcountry (although the ski areas are fine). If you do go, stick to ridges and slopes of less than 30 degrees which are not attached to any steeper slopes. Only go if visibility is good and definitely avoid going if the weather is further contributing to instability (it’s snowing, blowing, raining or warming).

What is a persistent weak layer and how does it form?

A PWL is as it’s described – a weak layer in the snow pack that persists over time. It forms when snow falls early in the season. If that initial snow sits around for several cold, clear days, the snow crystal will begin to change (metamorphose) into a larger, sugary weak crystal known as a facet. This year we had snow early and it lingered on most aspects – but only above 1700m. The crystals deteriorated and became extremely weak.
On July 12-15 we had a large SE storm that deposited about 50cm of new snow along the range. The storm was followed by several warm and windy days with winds from both the SW and NW. The new snow plus wind-transported snow was deposited over the pre-existing snowpack. The initial storm snow did not quite provide enough weight on the weak snow. Ski patrols were able to trigger a few avalanches but really, it took the additional days of wind to create enough weight on the weak layer to create the widespread avalanche cycle that we began to see on July 17.

What does the PWL look like?

If you dig (and you have to dig or probe to get an idea of the location of the weak layer as it is variable), you will find a very dense slab about 40-70cm deep sitting over a distinct 5cm layer of weak sugary snow. If you expose a profile of the snow, this strong layer over the weak layer can be easily seen. This mess sits inconveniently on a hard, icy crust layer – which provides an effective sliding layer.

How long can a PWL last and what makes it go away?

It’s persistent so it can linger. Two things will eventually help the layer: One is rain to high levels to an extent where water percolates through the pack. When this all freezes up, the pack heals.
The second is to get enough snow fall (or wind distribution) that the weak layer becomes buried deep in the snow pack. The weight of a skier generally only affects the snow pack to a depth of about 100cm (or 120cm to play it safe). The problem with this scenario is that snow depth varies – it may be 100cm to the weak layer in one spot but only 50cm to the weak layer in another. This spatial variability is the bane of the backcountry traveller and avalanche practitioner and since you cannot be certain of the location of this weak layer, then safe travel protocols must prevail (skiing one at time, eyes on each traveller etc).
In addition, the problem goes from that of persistent weak layer to that of deep slab once more snow falls. This problem is especially thorny in that the likelihood of triggered diminishes but the consequences, if you did trigger a slide, become much higher. You will likely trigger a much bigger avalanche. This is known in the industry as a low probability-high consequence scenario.

At the ski areas, avalanche mitigation in the form of explosive control work will eliminate the problem. Skier compaction in the start zones also helps. We don’t have these luxuries in the back or side country.


For now, ski our excellent ski areas and study the terrain and any visible remnant avalanches. Read the advisory every day and make sure to check out the observation pages as well. This is a great year to take an avalanche class. You can learn a lot (check out the MSC website for accredited avalanche education providers. Chill also offers snow safety classes). Watch this next storm – it’s expected Thursday through Saturday and looks to be a big wind event, with snow.

If you have several solid years of experience you can still go but stick to ridges and low angle slopes (less than 30 degree) that are not attached to steeper slopes. Let someone know where you are going and carry emergency gear. Talk to Patrol before you go. Dig and do snowpack tests. Submit your observations to (your local area).

A PWL is not to be toyed with. You cannot outsmart it but must be systematic in your data gathering on it’s location (aspect and elevation). On every tour, you should look for this weak layer and record it’s location and test results on it. Ride slopes one at a time and realise that with PWL’s, it may not be the first or even the fifth rider who triggers the avalanche. Often all it takes is for someone to hit the deficit spot – the shallow spot which sets off the failure across the entire slope. Therefore, tracks on a steep slope do not necessarily indicate that a slope steeper than 30 degrees is safe.

Remember the bullseye clues: Avalanches running, deposition by wind or new snow fall (but especially wind), cracking and collapsing (whoomphing) – are all indicators that dangerous avalanche conditions exist.




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