The avalanche forecast for much of Utah this weekend has been off the charts; in almost all areas of the state the avalanche warning has been bumped up to Level 5 (Extreme), meaning their is a certainty of large, dangerous avalanches being triggered naturally. In most areas this is affecting all aspects and all elevations, something that again is a rarity.
Whilst most if not all of us, dear Reader, are not able to be out in Utah at the minute, it provides a wonderful case study for persistent weak layers (PWLs) and the risk they pose in relation to avalanches. Off the back of this extraordinary forecast from the Utah Avalanche Center (UTAC), I thought I'd delve into PWLs and how to grade their risk when skiing off piste.
Understanding Avalanches
Firstly, it is important to understand that avalanches, more often than not, don't just happen (although sometimes they do ...). Where and when an avalanche takes places can be forecasted by looking at several different factors. These include natural factors, such as the makeup of the snowpack (critical for us in this situation), the terrain the snow is in, and the weather forecast for the coming few days. There is also a fourth factor, the "human" factor, which is important because 95% of skiers caught in avalanches trigger the slides themselves or by other members of their party; the terrain, snow and weather creates avalanche conditions which are then set off by skiers travel through, over, or under the effected area.
It's worth mentioning that avalanches can and do happen naturally, without the human triggering it; this becomes more common as the risk from the other factors grows and grows. You can never rule out avalanche risk as a result, and should always stay on your toes when skiing in avalanche risk terrain.
Burying down into the snowpack factor, there are several ways the construction of the snowpack might elevate the risk of an avalanche. If it is a particularly warm and sunny day, the heat from the sun can begin melting snow deep in the pack, causing it to weaken and collapse, triggering a slide. Wind might blow old snow onto a slope, even if there hasn't been any snow for a while, suddenly overloading it to the point of collapse, or loading up a cornice that might break, collapse and trigger a slide.
Persistent Weak Layers
One of the biggest factors within snowpack is a Persistent Weak Layer (PWL). PWLs occur for several reasons, but primarily snow, usually early season snow, falls and is allowed to settled into a nice firm base.
Now, in an ideal world. snow falls consistently on top of this, with some but minor temperature variations which allow the steady build up of a nice deep snowpack. This is exactly what has happened in the French Alps, with reports (including first-hand accounts from yours' truly) of one of the most amazing snowpacks the Alps have seen in years.
However, if the snow stops falling and temperatures begin to vary considerably, with perhaps a sprinkling of rain along the way, the snowpack recedes into its shell, become one hard slab of boilerplate snow. This can happen throughout the season, with layers being created by freeze-thaw temperatures or sudden changes in the amount of moisture within the snow (Snow-Water Equivalency, or SWE).
We've all been there, skiing on a layer of a couple of inches of nice powder but scraping along a crust a little further down, or trying to make it down a steep piste that is rocked firm.
Now, later on, snow will fall on this layer. Because the layer has solidified, it can remain intact and refuse to bind to the snow above it. In extreme circumstances, the new snow doesn't then bind to itself either, thanks to temperature differences between the two layers. This results in individual flakes standing firm, sort of like a array of champagne flutes set out for a party. These are called "facets", and look and feel like a layer of sugar in or on the snow.
Finally, the icing on this cake of danger is new snow. Add this new snow, and the weight of a skier on top, and the champagne flutes will collapse; the bottom layer of hard snow, which nothing is bound to, acts as a slide, sending this combination of snow and human down the hill.
They can be buried, meaning a report of a PWL is not the end of the world. But until they are at least a metre or so down - the deepest a skier or snowboarder impacts the snow beneath them - they are a real danger. PWLs can also re-emerge later in the spring as the snow above melts and brings them closer to the surface.
Utah and PWLs: A Perfect Cycle
The avalanche cycle that Utah is currently experience (over the weekend 13 and 14 January 2024) is one of the best, most pure examples of this PWL system in practice.
So far this season, the snow in Utah has been sub-par. It almost always would be, with Utah experiencing its highest snowfall totals on record last season, but even looking beyond that snowfall totals have not been up to what they would be. The snow was later in coming, only arriving in November and December, a by Christmas a really poor snowpack had been formed.
Along the way, there had been some serious variations in temperature, along with a sprinkling of rain. The snowpack at Christmas and New Year was firm and hard packed, with a seriously dense crust in play.
Since New Year, temperatures have dropped and a couple of serious storm cycles have brought in some decent snow, adding about a foot or so to the snowpack. However, this has almost all remained as sugary faceted crystals, binding neither to itself or the snowpack below. Multiple field reports form the UTAC have been picking up on this faceted layer throughout this period, with some really interesting snow pit tests along the way.
Then, between Thursday 13 January and Monday 15 January, Utah will or has seen anywhere up to and over four feet of new snow. This new snow is falling on this mega-weak layer. As a result, it is slide city, with large natural avalanches popping off all over the place. UTAC has bumped the avalanche risk up to Level 5 (Extreme) and I could not agree more with this assessment; if this was me, I would be grabbing a good book, a hot chocolate, and setting up camp by the fire in the hotel bar. It is not a day to be outside let a lone skiing anything other than on piste on gentle runs, far from any danger.
Where Does Utah Go From Here?
There is a reason it is referred to as a cycle; the risk will eventually dissipate, as slopes that are prone to avalanches will slide, and those that aren't will collect more snow.
Two things will hopefully happen from here.
On slopes that have avalanched, this will hopefully wash the faceted champagne glasses away with them. New snow will fall and hopefully the conditions will be such that this snow will sinter and bind with the snow around, above and below it.
On slopes that haven't avalanched, this new snow will bury the PWL further and further down in the snowpack, eliminating them as a risk for the time being. Hopefully temperatures in the snowpack will stabilise and allow the PWL to blend with the snow around it, or eliminate the facets - under the right conditions, deep facets can be eliminated and made to bind with the snow around it.
A Word on UTAC
The Utah Avalanche Center is by far and away one of my favourite resources for growing my understanding of avalanche safety. They are continuously publishing videos explaining the risks they face as they see them, digging pits, running column tests, and getting out into the field to see the results of different conditions, alongside simply producing excellent, precise avalanche forecasts.
I haven't (yet) skied in Utah, but it sits very high on my bucket list! Until then I will just have to ski vicariously through the forecasts of UTAC.
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