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2020/2021 Avalanche Information

 Poster: A snowHead
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There's no avalanche bulletin in France at the moment but the forecasters in the Savoie have been out on the terrain and have warned that a persistent weak layer situation may be developing on high altitude >2500m N and E faces due to the lack of snow at the start of this winter. Something to think about later in the year.


Last edited by Poster: A snowHead on Fri 20-11-20 9:15; edited 1 time in total
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Good post davidof, Swiss SLF hinting in a similar direction.... and so it begins!
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For anyone who is interested, a summary of the avalanche incidents that results in injuries and some info on current conditions as I see them (I'm out most days at the mo.)

http://pistehors.com/_7DjbHYByuHDGsGAFnGs/current-avalanche-conditions
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Quite warm today, Zero iso around 2000m this PM in the mountains above Grenoble accompanied by light rain. It was 2800 meters yesterday. It doesn't make for great skiing at mid-altitudes but with a bit of a freeze / thaw will stabilize the snowpack but beware of wet snow slides on sun exposed slopes as this photo from Data Avalanche illustrates



http://www.data-avalanche.org/avalanche/1608016688191

We can hope that most of these purged with the sunshine and foehn on Wednesday. Higher up, above 2300 meters where it has remained cold the weak layer is still present in the snowpack on shaded north facing slopes.
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I hope that in the future, satellite technology will be used to predict avalanche convergence and assess snow conditions. The development of the space industry has already shown that space exploration is our future. Not only in terms of the possibility of colonizing other planets, but also in terms of caring for our home planet. The article "Activity Pack: Satellites Improve Life" https://www.skyrora.com/post/activity-pack-satellites-improve-life makes one think about it. The main thing in the future is to use these technologies for good, and then everything will definitely be fine.
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Nasty one, close to La Grave and the Lautaret

Hautes-Alpes : un skieur dans un état critique et un autre blessé dans une avalanche à Villard d'Arène

https://www.dici.fr/actu/2020/12/22/hautes-alpes-un-skieur-un-etat-critique-un-blesse-une-avalanche-villard-d-arene-photo-1475772



The group of 6 skiers without supervision, found themselves caught in this avalanche 500 m long and 150 m wide near the crest of Côte Plaine (north face of Chaillol). One of them, very buried, only had a tiny air pocket and luckily was only slightly injured.Most of these skiers were equipped with Arva, shovel and probe.

But the avalanche continued and mowed down another ski touring skier who is at this stage in critical condition, told us the Public Prosecutor of Gap.

The avalanche most likely occurred because of the exceptional warmth that we are experiencing.
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Sadly the critically injured skier has passed away....... Crying or Very sad
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More pictures and report here, it's an area we know very well, but I would never skin up that face, just too feckin steep!

And I have to say in the 20-30 times (I've lost count) that I've been up there, I've never seen tracks going up that way?

https://alpinemag.fr/avalanche-deux-blesses-dont-un-grave-dans-les-hautes-alpes

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I hope that the injured recover. That face looks just like the face of Mt Gauthier at Nax; the snow accumulates on the rocks and then lets go. It looks stable and safe because of the rocks, and I have known people assume that this creates enough adherence not to be concerned abut the accumulations. Although each location is unique in orientation etc, I have learned to be very wary of this kind of formation. It seems that they tracked up a debris field to the left, which should have been been a big red flag, I think. Something to learn from here and file away in the back of the mind - multiple signs. Weathercam, did you not ascend just because of gradient or because it gave you bad vibes.


Last edited by You'll get to see more forums and be part of the best ski club on the net. on Tue 22-12-20 21:39; edited 1 time in total
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@valais2, too feckin steep and too dangerous, and like I said I've never seen people skinning up there, that's more ski mountaineering terrain than ski touring.



They were probably going for the peak at 2,740m you can see the more mellow route to the Col at 2,639m
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weathercam wrote:
The avalanche most likely occurred because of the exceptional warmth that we are experiencing.


First of all this is a windslab type avalanche nothing to do with temperatures and those cold, north faces at altitude stay cold even when it is quite warm and pleasant out in the sun. The avalanche went on the persistent weak layer I've been talking about which is present in a lot of the Alps due to the long anticyclonic period in November with a thin snowpack. This has formed a weak layer of gobelet or depth hoar over much of the mountain. A layer that is cm deep in places and sensitive to remote triggering. There have been a number of similar avalanches over the previous days, fortunately none fatal but some with very serious consequences for the victims.

The victim in this case was buried under 2 meters of snow. Not impossible to be rescued alive but your chances are really slim if you are buried over a meter under snow. The delay, and the fact they brought a dog team from Grenoble by helicopter suggests he either didn't have a beacon or it was not working. It appears he decided to follow the tracks of the guided group in front who triggered the slide.


valais2 wrote:
the snow accumulates on the rocks and then lets go. It looks stable and safe because of the rocks, and I have known people assume that this creates enough adherence not to be concerned abut the accumulations. Although each location is unique in orientation etc, I have learned to be very wary of this kind of formation. It seems that they tracked up a debris field to the left, which should have been been a big red flag, I think. Something to learn from here and file away in the back of the mind - multiple signs. Weathercam, did you not ascend just because of gradient or because it gave you bad vibes.


This is a misunderstanding of how avalanches occur, or at least skier triggered slabs/windslab which constitute 90% of serious avalanche incidents. For a skier triggered avalanche you need: a weak layer + a layer of cohesive snow plus a slope of 30 degrees or more. It is the weak layer that causes the cohesive snow to slide as it collapses like dominoes. It is not a question of adherence for this type of avalanche. The debris field is just snow purging off the steep rocks, nothing to be too alarmed about and possibly a good sign if your idea was to ski the NE couloir under pt2739 on the ridge as it too should have purged. That couloir does get skied btw.

The bulletin gave the risk as 3 and mentioned the PWL I have talked about earlier in this thread. It said that snow could be easily triggered on the weak layer and could mobilize a considerable volume of snow but it also said that the number of slopes that were dangerous was limited - so it should be possible to limit the risk. However a strong southerly foehn type wind has blown over the last few days. You see the ridge is a nice convex shape, the wind would have scooped snow off the south face and as it came over that rollover it would have decelerated and loaded the top of the couloir. Now people often ski couloirs on higher risk days because they auto-purge their snow and are relatively safe but this kind of couloir is exactly the type not to ski because the entrance is a big funnel that accumulates snow. It is a configuration that comes up time and time again in avalanche incidents. You also tend to get gobelet forming by rocks and at the base of cliffs where you also get accumulations from the spindrift.

One might question the approach. Those big zig-zags under the cliffs are playing with fire. We have a very similar couloir opposite me but it doesn't have the funnel entrance at the top. However there is risk of windslab either side of the bottom and we approach it by skinning or climbing directly under the couloir where it is relatively safe, specifically on the purges.

A bit of pre (or for some of us, mid) season reading: http://pistehors.com/dbCNGG8ByuHDGsGAyHG6/old-snow-persistent-weak-layers-and-guided-groups
Read the last couple of paragraphs carefully. If you are ski touring or off piste skiing this season this may be the most important article that you read.


Last edited by snowHeads are a friendly bunch. on Mon 25-01-21 16:40; edited 2 times in total
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@davidof, thanks for a very clear explanation and the further reading. Sad news about the casualty.
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davidof wrote:
....
weathercam wrote:
The avalanche most likely occurred because of the exceptional warmth that we are experiencing.
......


Sorry that was not clear, that was not me saying that but the translation from dici.tv

Last evening my friendly guide rang me up, not about this but nagging about what we're going to do re travel restrictions etc

We then talked about this, and a whole load more, which I should really put in the ski-tour thread at some point.

But as @davidof, says
davidof wrote:
.......The avalanche went on the persistent weak layer I've been talking about which is present in a lot of the Alps due to the long anticyclonic period in November with a thin snowpack. This has formed a weak layer of gobelet or depth hoar over much of the mountain. A layer that is cm deep in places and sensitive to remote triggering......


He went on to describe how he and a group of guides went up in the lift at La Grave to assess the snow-pack, they dug pits at the entry to the Vallons, and from what they found, large layer of faceted snow / weak layer, goblets etc, they all then took the lift back down!!!

That layer will be there all season on N facing slopes at altitude though not so much of an issue on S facing slopes as the snow disappeared during the High Pressure.

As I said, not too much of an issue for me as I prefer to ski circa 25% slopes, and he said that he too would be joining the 25% Club where he could this season, and in fact, he went on to say that he reckons 80% of the season he skis 25-30%
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Weathercam wrote:
davidof wrote:
....
weathercam wrote:
The avalanche most likely occurred because of the exceptional warmth that we are experiencing.
......


Sorry that was not clear, that was not me saying that but the translation from dici.tv


Yes sorry I realized that and should have made it clear in the quotation. A lot of things wrt to avalanches are not exact science anyway and open to discussion. Thanks for posting what your guide friend told you.

I should add that my post was in no way a criticism of the guide who was involved in this slide or of anything anyone else has posted above. It is easy to be wise after the event and both he and his group (who probably spaced out for the skin up) were safe bar an injury. It is always a PITA when people start following your tracks when you are maybe not totally sure of the conditions.
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indeed thanks Davidof - very interesting analysis and shows the importance of pits. I recall in 2012 a very weak base layer from November cycles and all the pits in the Faverges had this dominant weak layer a few cm from the soil surface. There were a lot of whale-mouth cracks/movement on South facing slopes that year. Re purging, I was told to be very suspicious of surface purging since it indicated slopes which were highly loaded or prone to windslab formation, and wet snow or rain onto these areas could lead to very unstable, highly loaded areas. Was that incorrect?
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@valais2, The weak layer you identified and whale mouths (glide crack) are not related. You tend to see, although not exclusively, the glide cracks on warmer slopes where melt water has got down to the base of the snowpack and reduced friction between the ground and snow so that it can slide under its own weight. This is often on grassy or smooth rock surfaces. This may occur even on relatively mild slopes and the cracks can open over a period of days to weeks to months. When and if they do release it is not due to skier activity but a natural event but the resultant avalanche of heavy snow can do a lot of infrastructure damage, they are rarely a threat to skiers. There has been some research into triggering them with water injection. A guide from the PGHM told me that they are more likely to release when the weather turns cold but that is just his observations.

I did a quick search and Henry Schniewind wrote this article

https://henrysavalanchetalk.com/glide-cracks-mean-snowpack-stability/

Regarding your second point. Purging from a steep cliff onto the slope below builds up snow on that slope and if you look at the photo above this has broken as a slab. You can also see that the cone entrance to the couloir as well as the left bank have also slide in that photo. With steeper sided couloirs where you don't get snow buildup the purging can stabilize the slope immediately below the couloir - the snowpack is deeper burying weak layers, the snowpack is deeper so less temperature gradient allowing weak layers to form and the movement of the snow purging either breaks up surface weak layers or consolidates the snow - but you have to be really under the line of the couloir for this to apply, and it is an inexact science.

The couloir du Pas du Pin in the Belledonne near me is like that. From very early in the season it has a good snow depth and you'll see in this picture the skiers (not me in this case) have climbed directly under the couloir on the cone where the purged snow falls. But this slope will avalanche right and left of the couloir.




Wet snow - well this will either cause the slope to slide after a sufficient amount falls due to the load, or it won't. If it doesn't it may help stabilize the slope with a bit of time. Rain, again this can help stabilize the snowpack. We have a clear break between where it rained last week and above (>2300m) where it snowed. The rain has penetrated the snowpack and refrozen forming a crust that has bridged weak layers lower down or it has purged unstable slopes.

Of course the crust can be a catalyst for future weak layers to form as you often get a high temperature gradient above them.

The important point for most skier triggered avalanches is a weak layer, less than 1 meter from the surface of the snow, more or less cohesive snow on top and a slope of around 30 to 45 degrees. The skier breaks the weak layer which is like dominoes, this collapse propagates through the snowpack giving the slab above the impetus to release and gravity does the rest.
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That’s an excellent piece by Henry S - he is able to dig deep into the literature and avalanche discussion and is such a valuable source. And thanks indeed for the time on the response above; these are really interesting perspectives on purging - particularly the issue of purging potentially breaking up the snowpack and allowing consolidation of the pack onto which it falls. But I note your ‘inexact science’ point. Although I have read and read about avalanche safety and learnt a lot from guides, I find small ‘inputs’ like your above really stick in the mind, and an added photo helps it stick in the memory. When worrying about a situation on the hill a photo makes me think ‘is this situation like that photo?’ .. which aids quick thinking and decision-making a lot.
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Some big slabs have gone here - looks like 2 or 3 layers down to the ground. Stories if multiple avalanches in the Siviez area


https://www.lenouvelliste.ch/articles/suisse/attention-danger-d-avalanche-dix-personnes-ont-ete-ensevelies-en-un-jour-en-valais-1023918
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From Joe Vallone after kiting up at the Col on the 27th before the dump.....

Side note: 3 days of wind and snow coming. There was a decent amount of wind today before the storm arrived. The snowpack is still an upside down poo-poo sandwich in the worst spots and just a regular poo-poo sandwich with sides of relish, pickles and mustard almost every where else .
There will be storm problems, those are easy to recognize but don't forget about the other problems. Don't get fooled, it's still December.
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Yes, another 5-10cm here last night, so more up top, but really windy, not exactly adding to snowpack stability..........
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Still got the PWL problem at altitude across much of the French Alps

http://pistehors.com/R86qz3YBbNihPQ79Xq-0/danger-persistent-weak-layers-at-altitude
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@davidof, that PWL, is that there now till freeze thaw? How much snow on top to bridge it or will it always be a problem till spring?
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The weak layer forms extremely quickly at the start of the season, but can take several weeks or longer to revert, so the safe thing is to assume it's there for the season, that said you'll almost always get a PWL early on, it all depends on the difference in ground to snow temps how quickly, if at all, it will diminish over the following weeks
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@KenX, I know a bit about snow science but I'd like to know even more.
other than a freeze thaw which is highly unlikely in Jan and feb as such altitudes, what would we need to make that PWL go away?
Has that PWL formed in the snow pack due to differing temps in the snow pack or was it snow fall on surface hoar?
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@Mother hucker, The crystals - once they have transformed into depth hoar (large cup shaped crystals) are in their final state. Taking away the thing that formed them (a temperature gradient) won't transform them back. So they can melt, which is what happens with the freeze/thaw cycles or they can be destroyed by mechanical means: piste bashers, skiers skiing the slope regularly. You can end up with the whole snowpack transforming into hoar, say after a long anti-cyclone in winter, in which case you have something akin to powder again. The Americans call it "loud powder" because the crystals make a swooshing noise as you ski through. This is relatively safe as there is no cohesive snow slab to slide... until it snows again.

If they are covered with a meter of snow that will probably be enough to insulate them from any skier triggering. They could also be buried under a rain crust. This is the case below 2300m currently where there was rain before Christmas: rain crust or rain getting into the layer and then freezing.

The thing is you never get a nice even 1 meter layer of new snow in the mountains. Even if the snow falls without wind over the following days there is bound to be some wind which moves snow around the place and it is where the snow is thin that you can trigger a slab. For example a convex slope will see the wind move snow from the top of the rollover and deposit it lower down, leaving the weak layer nearer the surface and a nice big slab just below.

The layer is at high altitude, so localized. Not that many people are touring above 2500 meters at this time due to the cold and weather. So it is an issue but avoidable.

The PWL formed because there was a thin snowpack in the autumn and an anticyclone during November creating a very strong temperature gradient from ground (zero) to surface (-25C at 2800 meters has been recorded).

This was in the Maurienne yesterday, 2500 meters, West facing slope. Alain who is talking says they already remote triggered some slides (something the Meteo France observer in Bourg St Maurice noticed on the ground at the end of November). Remote triggering means there is a weak layer from where the trigger is (you) right up the slope to where the slab has enough angle to slide. So the layer is widespread and very sensitive. In the video you see the big sugary grains at the base of the snow-pack.


http://youtube.com/v/ObCVX4VSYdo&t=4sn

and similar at 2550 meters on a south facing slope


http://youtube.com/v/ZOdv2oPQotA

and here at the end of last season (May 17) and you see the old depth hoar layer formed in the winter is still active


http://youtube.com/v/KJ_ACbA9rf0

Now I would say in all those videos the depth of snow on top would make it unlikely a single skier would trigger that layer at the point the tests were made but maybe a group of skiers on the same slope?
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@davidof, Thanks for that, I appreciate you going so indepth and taking the time to find the videos to help. Theres so many factors to compute. I really would like to understand in greater depth snow science and not just read the bulletin with basic knowledge.
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I will second the thanks from @Mother hucker, that is a really useful and well written piece regarding PWL.
It is to be hoped that all SH's who venture off-piste are reading and absorbing the content.
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Thanks.

It is important to remember that it is far from an exact science - at least with respect to forecasting. The problems we are seeing at the moment are localized (high altitude) and relative infrequent but worth thinking about if you are planning a big day out at altitude.

There are still some avalanches lower down - a ski tourer triggered one on a local "trade route" on Monday (pas de la coche in the Belledonne) - probably wind blown snow on a temporary weak layer at 1900 meters but it was a small slide that he skied out of but he was on his own.

The French are starting to mention the typical risks in the bulletin now

Quote:
NSTABILITÉS PLUTOT EN ALTITUDE
Situations avalancheuses typiques : neige ventée, sous-couche fragile persistante en altitude.


Wind blown snow and a persistent weak layer at altitude, exactly the problem we've been discussing here.
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davidof wrote:
Thanks.

It is important to remember that it is far from an exact science - at least with respect to forecasting. The problems we are seeing at the moment are localized (high altitude) and relative infrequent but worth thinking about if you are planning a big day out at altitude.

There are still some avalanches lower down - a ski tourer triggered one on a local "trade route" on Monday (pas de la coche in the Belledonne) - probably wind blown snow on a temporary weak layer at 1900 meters but it was a small slide that he skied out of but he was on his own.

The French are starting to mention the typical risks in the bulletin now

Quote:
NSTABILITÉS PLUTOT EN ALTITUDE
Situations avalancheuses typiques : neige ventée, sous-couche fragile persistante en altitude.


Wind blown snow and a persistent weak layer at altitude, exactly the problem we've been discussing here.


Just about, avalanche prediction is very imprecise. A couple of years ago we rested in the mountains, we also did not promise avalanches, but I don’t know what a miracle it was - we literally returned to the hotel 5 minutes before the avalanche. And if we decided to ride for another hour, then I'm afraid to imagine what would happen.
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davidof wrote:
Now I would say in all those videos the depth of snow on top would make it unlikely a single skier would trigger that layer at the point the tests were made but maybe a group of skiers on the same slope?


That's interesting. I was thinking that the second video showed a relatively thin snow pack.
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@davidof, great commentary. Thanks
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The second video actually shows 2 weak layers, with only a thin snow pack on the upper one, but a bigger pack on the lower. It suggest to me that a single skier could easily trigger the top layer. No idea how large a group might trigger the lower one.

Whether the upper layer moving could trigger the lower layer would be the question?
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brianatab wrote:


Whether the upper layer moving could trigger the lower layer would be the question?


Yes it could.
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AndAnotherThing.. wrote:


That's interesting. I was thinking that the second video showed a relatively thin snow pack.


Yes that's more accurate than what I said.
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@davidof, Ta.

This early season, what sort of depth of snow stuck around to get the temp gradient and grow the faceted crystals ? 15cm or so as per the video's ?
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@AndAnotherThing..,
very rapidly, and these are big generalizations for the French Northern Alps

Skiable snow fell at the end of September, around 40cm in places which let some lucky folks wink get out on their skis. This snow hung around > 2500 on very cold NW slopes. Snow returned, but only 5 - 10 cm in mid November and at altitude. Things got going with 50cm of snow at the start of December, another 50 cm a week later then around 80cm in the second half of December.

November saw stable anticyclonic conditions but it was only the first half of December that was really cold but remember a clear night sky can radiate energy away from the surface of the snow making it much colder than the air temperature in a weather station.

This is what the meteo france forecaster found on the 2nd of December in les Arcs

Quote:

+ On the west and north slopes no base until around 2350 / 2400m, only 5 to 10cm very light fresh snow that fell yesterday ... On the descent, we skied along patches of manmade snow that were sometimes frozen, sometimes crusty and sometimes very pebbly ...
+ Above 2350m, 10cm of powder on a hard base with rocks present. Then, quickly 10 / 15cm of very light powder on a brittle ice crust (and a layer of hoar underneath) for a total of 40 to 60 cm of snow, good appearance, correct skiability, but the crust is breakable in places
+ A sloping survey of around 30 dg on the west side of Roche de Mio 2650m.
- On the surface 8cm of very light fresh snow (080 kg / m3) and without cohesion. Fall of 01/12.
- 1cm of facets and depth hoar (from the mini snowfall of Friday 18th Nov metamorphosed)
- 3cm of hard but brittle crust (initially corresponding to the light rain on Thursday 19/11).
- 8cm of very large "superb" depth hoar of 2.5mm to 5mm !!! (fall of 17/11 transformed)
- 20cm of very hard round grains (concrete base)

This is generally well representative of the sector between 2400m and 2700m with more or less variable layer thicknesses, the crust being more often at 1 or 2cm than 3cm.
Same observations at Arc 2000 these days with a much thicker crust in the west (inskiable towards Grand Col 3 cm hard but brittle) than in the north (1cm brittle on the north side Aiguille Grive) and large depth hoar without cohesion below on 8 to 15 cm, but no refreezing crusts above 3100m (Aiguille Rouge) just a few sintering crusts on loose snow.
To watch the surface layer which will tend to metamorphose then form slabs or accumulations with the wind on Friday before another small fall ...


Hope this gives you a snapshot for that location - already a lot of depth hoar or gobelet. The observers triggered a number of small slides a few days previously on the opposite side of the valley.
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@davidof, Thank you.
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@davidof, I was looking for some information on response times for Avy rescue and came across your article here:

http://pistehors.com/backcountry/wiki/Avalanches/Avalanche-Survival-Curve

It's a few years old now but I assume it still roughly holds ?
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Managed to get myself slightly avalanched while skinning up the other day. Posting here as a cautionary tale.

Trade route that I've skinned-up 100+ times. There is a very small rock band on the way up which can be slightly awkward. The climb is south-facing and exposed to wind, so this spot can often have very hard-packed snow. A couple of kick-turns can be required to thread through the rocks so it's often a newbie splitboarder's nightmare.

Negotiating this spot the other day, as always the snow was very hard (although it was mostly pretty soft and fresh elsewhere). I was kicking hard with my skis to form a track in order to make life easier for my client. As I did so, a small slab released. It only moved a couple of metres, but it took both of us with it and, with a terrain trap below, we were temporarily not unconcerned for our well-being.

Looking at the snow afterwards, the hard-pack that was causing us trouble was hard wind-slab formed by the wind swirling over a rock at the top of the slope. This slab had formed in the last 24 hrs or so (I'd been there 2 days before). It had formed on top of a layer of surface hoar from the recent cold, clear nights which resulted in a weak layer. The surface hoar layer failed and the windslab moved on top of it. The crown wall went from almost zero at one end of the slab to around 50cm at the highest, right in the lee of the rock. Slab was only around 10m wide.

Scientifically interesting to have such a localised phenomenon from a small rock feature. We didn't really find any windslab elsewhere on the mountain all day although another small slab had released not far away and I should have paid more attention to it. Probably would have if I wasn't so familiar with the route. I will definitely be approaching this spot with more caution in the future. It's always a tricky spot anyway, particularly with splitboarders, and I'll be strongly considering taking the longer route around in future.

TL:DR - Small features can create very-localised avalanche danger. Over-familiarity is an important heuristic trap. Shocked
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 Ski the Net with snowHeads
Ski the Net with snowHeads
@stevomcd, thx for that......probably rings true for many out there this week about complacency with familiar routes, and the widespread surface hoar from last Friday

Am on a local PdS WhatsApp group that posted this update today, first level 4 of the season, maybe 5 by thurs/fri

https://float.kifftech.eu/new-snow-on-surface-hoar/?fbclid=IwAR3Y_-YAckRd3ZsvyIZ2hhwMQHt51qXzBGYTx68mIqK6BDBI3nmNiKZQVrk
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