Off-piste Snow Report – N French Alps (week of 22 Feb): Rapid Warming, Wet-Snow Avalanches & Still Hidden Weak Layer(s)

Rapid warming, wet-snow avalanches… and a hidden weak layer on the shady side of the mountain (north’ish facing slopes above 2200 m)

We’ve had record-breaking accumulations of 2 to 3 metres of fresh snow over the last 10 days in the N French Alps and surrounding areas. This has led to lots of avalanches of all sorts : accidental (set off by skiers), natural and artificial (by explosives). There’s some great footage circulating on social media!

Rapid warming this week will quickly turn recent powder into heavy, humid snow , especially on sun-exposed slopes, triggering plenty of noisy and impressive wet-snow avalanches – and the danger is relatively predictable. At the same time, a far more deceptive danger remains on the cold, shady side of the mountain, where a persistent weak layer is still in place and there is still a possibility of triggering unexpectedly large avalanches, especially above 2200 m on the colder N’ish facing slopes, and approximately 3000 m on the more S facing ones.

Don’t let all the visible wet-snow activity distract from this hidden, longer-lived “persistent” instability.

Rapid warming : humid & wet-snow avalanches (main visible problem)

The big change this week is temperature. We’re going from around 0°C to +8°C at 1800 m in just a couple of days. That, plus strong incoming solar radiation (sunshine), will quickly turn the recent cold snow into heavy, humid / wet snow, even up to 3000 m on S facing slopes.

This will trigger natural wet-snow avalanches and sluffs, some of them large, especially just as that rise in temps and sun hits the fresh snow and during the warmest periods. It’s a classic example of rapid change: the snowpack, like people, doesn’t have time to adjust – it gets stressed and falls apart.

The good news (as in my wet-snow article below) is that this danger is conspicuous and relatively predictable. You feel the increase in temperature, you see the sun on the slope, and then there is avalanche activity.

This cause and effect relationship helps to make wet / humid snow avalanche danger relatively predictable. It’s this predictability that helps us to be aware of avoiding steep slopes that have been subject to an increase in temperature – especially when there is fresh snow. Given the relatively few accidents (those involving skiers, boarders and climbers)  in wet snow avalanches, this general rule seems to work. And remember this applies to closed runs and roads; many victims of these natural wet snow avalanches were in areas that were closed by local authorities. 

Persistent weak layer on cold slopes 

At the same time, the persistent weak layer from the dry, cold December period is still very much an issue, particularly on N’ish facing slopes above 2000 m (and on S’ish facing slopes above around 3000 m).

For more on this persistent weak layer problem, e.g. the serious accidents we’ve seen over the last couple of weeks, see last week’s blog Nothing happens most of the time and the Safety is Freedom framework.

Safety is Freedom!

So that’s a summary of this week’s snow report and current conditions. For more on the subject of wet and humid snow avalanches see the article below.

___________________________________

Wet Snow Avalanches Put Into Perspective!

Very few avalanche deaths or accidents occur in wet snow conditions.

“So we don’t need to worry about wet snow avalanches at all?” you may well ask. The answer is “Yes we do”!! The tricky thing seems to be that even though there are more avalanches in wet snow conditions (which leads to an enormous misconception about where and when avalanche dangers exist), there are actually less accidents. The following puts this into perspective from an accident prevention point of view.

Even though wet snow avalanches are numerous and dangerous, the danger is relatively predictable and less ‘deadly’. Cold, dry snow slab avalanches are much less predictable and are the biggest killer. That is why, in our accident prevention training, this is the hazard that we mainly focus mainly upon.

Predictability of danger

It may not be easier to predict the release of a wet snow avalanche than it is the release of a dry snow slab avalanche in a specific place. However, it is much easier to predict the danger of wet snow avalanche activity.

Once temperatures rise above freezing and/or rain starts falling, the danger of wet snow avalanche activity increases. When temperatures get above freezing on a slope just after a snowfall or for the first time in days or weeks, the avalanche activity is most obvious, usually immediate and often spectacular. Even the casual visitor to the mountains can connect the relationship of these conditions with a wet snow avalanche thundering down a slope that’s been subjected to melting or rain. An unstable cold, dry snowpack rarely gives any feedback – so the current danger is not so intuitive.

So, even though you may instinctively think that most avalanche accidents occur in spring, on south facing slopes, this is incorrect. Almost all accidents involve cold, dry snow slab avalanches triggered by the victim(s). The vast majority occur in December, January and February on north’ish facing slopes. (That is north’ish for the northern hemisphere, and south’ish for the southern hemisphere). See graph below.

Most accidents happen due to victims triggering dry snow slab avalanches during cold months of the year and/or on the cold side of the mountain. Hence, in our HAT talks, courses and accident reduction framework, we concentrate on the elusive danger from the cold, dry snowpack and slab avalanches, rather than on the more predictable (and thus manageable/ more avoidable) danger from rising temperatures and wet snow avalanches.

However, wet snow avalanches are dangerous if you are exposed to them – and they contribute to a significant minority of total avalanche accidents each year. So it is important to respect the danger and stay out of harm’s way.

As we’ve said, danger from wet snow avalanche scenarios is much more noticeable, predictable, and thus avoidable than the less obvious danger from cold, dry snow slab avalanches. Dale Atkins, former president of the American Avalanche Association, likens wet avalanches to ‘bees’ because they’re conspicuous and noisy, and dry avalanches to ‘snakes’ because they’re secretive and stealthy. (See his case study towards the end of this blog).

The most obvious place where common sense can be applied, but very often isn’t, is on closed runs and tracks under steep slopes… closed due to wet snow avalanche danger. In these places, there’s no need to apply avalanche accident reduction techniques because common sense clearly works best (if we can be bothered to use it). i.e. don’t go onto closed runs, roads and paths even when ‘nothing happens most of the time‘.

The following video from ski instructor, Aaron Cassells, shows a near miss when a wet snow avalanche came down across a closed walking track, narrowly missing a person.

While cold, dry slab avalanches are involved with the the vast majority of accidents (and therefore our biggest preoccupation in terms of accident prevention), natural wet snow avalanches which release on steep slopes due to rain and/or a rapid increase in temperature, especially during and just after snowstorms, can and do cause tragic accidents.

‘Glueing effect’ of warming & rain

A commonly forgotten, but important, outcome of warming, melting and especially rain (i.e. any situation where free water drips down into the snowpack), is a net stabilizing of the snowpack via a ‘glueing’ effect, especially when temperatures go below freezing afterwards. This stabilizing/glueing effect is often overlooked by those biased towards the flawed rule that ‘increased temperatures and springtime is the primary avalanche danger’.

It is often reflected by a drop in the avalanche danger level up to the altitude at which the snowpack has been fairly well melted or ‘saturated’… and often higher up the mountain too as the warming effect of snow itself (even below freezing ) increases the settling/bonding rate. You will notice that very often within 12 to 48 hours of a warming episode/rain, the avalanche danger level will drop by at least one level on the scale. For example, in January 2018 the Haute Tarentaise region of Savoie, France was hit by a series of wild storms. In the middle of the month, over 1.5 m of snow fell within 7 days. The danger level rose to a ‘very high’ level 5 on the 22nd (the first day after the biggest storm). It then plummeted down to a level of 2 below 1800 m within just 24 hours.

This rapid stabilizing of the snowpack was mostly due to the above freezing temperatures and rain up to 2200 m or so. This saturated most of the snowpack up to around 1800 m and the top layers of the snowpack well above that altitude. Then there was a drop in temperature (O° C at 1800 m), which significantly solidified the snowpack below 1800 m and stabilised the snowpack higher up the mountain. The pictogram of the mountain (above left) clearly shows the forecast for rapid decrease of danger levels due to this stabilization.

Melt-Freeze

Once melting and freezing cycles become regular, usually in spring time, it’s a huge relief to people who spend a lot of time exploring avalanche terrain. Oddly though, I get a lot of feedback from people – even some experienced people – who think that this is the most dangerous time of year. It is certainly the time of year with the least amount of accidents – at least in the areas where there are wet / melt-freeze snow conditions. Here’s why it can be the safest time of year, even though there’s more avalanche activity.

Melt-freeze cycles make the wet snow avalanche danger even more predictable and intuitive despite the fact that there are more avalanches that release during these springtime conditions. Avalanche expert, Roger Atkins, explains this scenario clearly and concisely in his outline Mindset for Mitigators. It’s an outline aimed at pros, but is one of the simplest descriptions I’ve ever seen for putting springtime, melt-freeze conditions and wet snow avalanche danger into perspective.

‘Spring Diurnal‘

Typical conditions: The hazard assessment suggests that the only substantial hazard is from wet loose avalanches during the afternoon thaw phase of the diurnal freeze-thaw cycle

Typical operating strategy: Watch closely for adequate overnight freeze and consider closing avalanche terrain during the thaw phase of the cycle. Mitigation success is strongly ‘timing dependent.’

From Mindset for Mitigators: ‘Spring Diurnal’ Mind Set by Roger Atkins

…. Of course, I should probably just stop right here with the melt-freeze description, but that wouldn’t be as much fun! So, for skiers, here’s a link to an article I wrote on how to how to get the most out of spring snow skiing. During spring, timing is everything. If you get it right during the spring snow ‘melt/freeze’ cycles, you can experience some of the smoothest and safest snow conditions of the whole season. Nothing quite beats skiing fresh powder, but the older I get, the more I love the feeling of the ‘Spring Diurnal’ Mind Set !

Stories are usually the best way to put things into perspective. So, finally, I’ve edited a story in an article by Dale Atkins, former President of the American Avalanche Association. Here, Dale uses my observation of a wet snow avalanche accident which occurred in Val d’Isère a number of years ago, to advise rescuers about how to anticipate wet snow avalanche danger. Dale is another top expert, and all of us can learn from what he says too.

In short, a skier was swept over a cliff by a wet snow avalanche(s). He fortunately survived, thanks to the rescue services, but permanently lost the use of his legs due to the impact of the fall. Varied depths of approximately 15-25 cm of wind blown fresh snow had accumulated over the previous few days. There was a negligible freeze the night before (minimum temp of +4° C at the base of the slope) and the avalanche danger rating that day was a ‘considerable’ 3.

Case Study edited from: ‘Springtime Wet Snow Avalanches And Rescue‘ by Dale Atkins

Spring means a whole lot of changes for the snow and avalanche conditions, and spring can be a most dangerous time for rescuers. Last Friday there was an avalanche accident off-piste at Val d’Isère, France, which resulted in a serious and risky rescue. Fortunately the knowledge, skill, and professionalism (and probably some luck, too) of the ski patrollers saved the skier’s life. This article focuses on the differences of wet and dry avalanches and what this means to rescuers.

Accident: Face du Charvet E Face, 11:45 Val d’Isère April 1

The pisteurs (ski patrollers) at Val d’Isère faced a challenging and dangerous lunchtime rescue after an avalanche swept a skier down the east-facing Face du Charvet. During the rescue several additional avalanches threatened the patrollers, but they were able to reach the victim and short-haul him away in a helicopter.

American avalanche professional and ex-pat Henry Schniewind, who runs Henry’s Avalanche Talk, happened to be in the area and watched the rescue unfold. Take a look at his website to get more details including pictures and video. (If you’re going to France, especially the Val d’Isère, Tignes, or Meribel areas, be sure to go to his local avalanche talks. Even if you’re not headed to France, but you do want to learn about avalanches and if you’re willing to spend a little money, his website has some excellent online avalanche education through his Safety is Freedom program.)

According to Henry some new snow had fallen the night before (and three days earlier) on the 600m-tall east face of the Face du Charvet. Just before noon a small point release [I am sure it was not the first] turned into a large and wet loose snow avalanche as it cascaded down the rocky faces and chutes. The slide swept one skier from a group of five over a cliff. Ski patrollers responded and “at least” 3 or 4 additional “big” avalanches spilled down the face. During one of these avalanches the rescuers had to race to the side to escape. Henry said the roar of the avalanches was impressive. Fortunately, the patrollers dodged the avalanches and were able to reach and evacuate the injured skier but at great risk.

A Short Editorial Comment

The group’s decision to be where they were was wrong. It was the wrong time to ski the face. It should have been an easy decision not to go. Fresh snow, rocky and east-facing terrain, light freeze overnight and intense morning sun should have triggered alarm bells. By the way, the avalanche danger was rated “marqué,” 3 – which is the same as “considerable” in the North American ratings.

Wet Snow – A Different Type of Avalanche Dragon

A wet, springtime avalanche is a different beast compared to its mid-winter, dry-snow cousin. Avalanche savant Bruce Tremper summarizes these differences in his excellent book, Staying Alive In Avalanche Terrain. He writes, “Wet avalanches are triggered differently, they move differently, they’re formed by different conditions, you forecast for them differently, their deposits are different, and the scars they leave on the vegetation are different.”

Bruce goes on to point out that when it comes to how avalanches start the main difference between dry and wet snow avalanches is that dry snow avalanches are caused by overloading the strength of buried weak layers, and wet avalanches are caused by decreasing the strength of buried weak layers. In general, the primary culprit for dry avalanches is the additional stress, or load, from the weight of more new snow, wind-drifted snow, or even the weight of a skier or snowmobiler. In wet snow often it’s thaw conditions that melts bonds between snow grains, reducing the snow’s strength that’s the culprit.

Snakes and Avalanches

… In dry snow, avalanches tend to act more like a snake. Both are secretive, stealthy, and many people might say both are unpredictable. Snakes and dry avalanches are solitary and are often silent. You can travel all day and not see or hear either, but to get bit, generally you have to provoke the snake or avalanche.

Bees and Avalanches

Wet avalanches, on the other hand, are more like a swarm of angry African bees out looking for trouble. Both bees and wet avalanches are overt, conspicuous, noisy, and often come in swarms. When you see one, you’re certain to see more. It’s this fact about wet avalanches that poses a serious threat to rescuers (like in the above accident and rescue story).

Is It Safe?

In dry snow you have to find the trouble; most avalanche accidents are caused because the victim (or a friend) triggered the slide by venturing into the avalanche starting zone. As long as conditions (winds, temperature, precipitation, etc.) stay the same and other triggers (people) can be kept away, rescuers can work on avalanche debris with little or no threat from additional avalanches. In many cases, rescues can be performed with remarkably little risk.

In wet snow more accidents are caused by naturally occurring avalanches where the victim was in the wrong place at the wrong time. (Certainly, education, experience, timing and safety margins can help keep one out of harm’s way.) More often the avalanche crashes down on the traveler who might not even be in the steep slopes of the starting zone. Unlike dry snow, when conditions (sunshine, temperatures, etc) stay the same, with wet snow you can count on additional avalanches. When a natural wet avalanche has resulted in an accident, rescuers – either companions or professionals – will face the immediate threat of additional avalanches. Rescues in wet avalanches can be very dangerous affairs.

Video Summaries

In these 2 videos below, I summarised the accident as events unfolded for me as I came on to the scene during the rescue.

Summary: the clear danger is not the most deadly, … but that is not a reason to ignore it!

When wet snow avalanches occur, they are often very large, noisy and impressive. Their conspicuous nature makes us notice them, and associate the danger they present with certain slopes (warm often sunny ones) and with rain, rising temperatures and incoming radiation from the sun : all contribute to unstable and dangerous wet snow avalanche conditions – which are almost always a ‘natural avalanche event’. We therefore instinctively – naturally – tend to avoid the sort of areas where we have seen that these events take place.

The danger from cold, dry slab avalanches, on the other hand, is not so immediately obvious to us. Over 90% of the time these are triggered as an ‘accidental avalanche event’ by the victims themselves. Dry slab avalanches are far more deadly to humans than their wet snow counterparts. This explains our focus: we concentrate mainly on helping people manage the risk posed by cold, dry slab avalanches – so that they can have the most successful and enjoyable time out on the mountain…, but that is not a reason to ignore the obvious danger of wet snow avalanches!

Conclusion – solutions for accident prevention

The solution for avoiding wet snow avalanche accidents “caused by naturally occurring avalanches where the victim was in the wrong place at the wrong time“, as avalanche expert Dale Atkins put it above, is to avoid steep slopes that:

• have been subject to above freezing temperatures and/or rain ESPECIALLY during and just after a snowfall
• and/or where the overnight (re)freeze was not adequate, as Roger Atkins puts it in his Mindset for Mitigators: ‘Spring Diurnal’ Mind Set.

Tip:

Following a sufficient overnight freeze after a few days’ adequate melting phases and no fresh snowfall, this is when we need to start looking out for wet snow avalanche danger. The first place this will start to occur is on slopes directly exposed to the sun which have seen several hours of above-freezing temperatures.

Cold, dry snow snow conditions will almost always exist at the same time as wet snow conditions e.g. at higher altitudes and on north’ish facing slopes (in the northern hemisphere). The accident prevention techniques associated with these snow conditions are outlined in our ‘Safety is Freedom Framework’ and are the emphasis of our HAT Talks and Training since these are the conditions that need the most care and attention.