Rapid warming, rain and wet snow avalanches

A common misconception about avalanche accidents is that they’re mainly related to rising temperatures. When I ask people about when and where they think most avalanche accidents occur, they nearly always reply ‘In Spring, on South facing slopes’. It’s an understandable theory because these wet snow avalanches can be very spectacular and destructive. However, as far as avalanche accidents involving humans is concerned, this is simply not true.

At HAT we emphasize that the vast majority of avalanche accidents involve cold, dry snow slab avalanches triggered by the victim on North’ish facing slopes in December, January and February. We emphasize this fact to counter the widespread misunderstanding above that avalanche danger is mainly related to rising temperatures.

Certainly a rapid rise in temperature, rain, etc., usually creates serious instability, especially when it’s the first big temperature rise above freezing and/or rain of the season on a previously cold, dry snowpack. And rain on fresh snow will almost always create instant avalanche activity. (That said, the net outcome of this warming and especially rain, when free water drips down through the snowpack, will be a stablilising ‘glueing’ effect, especially when temperatures go down below freezing level). But we do need to be aware of this rapid warming, rain effect on the snowpack and the resulting wet and humid snow avalanche.

However, this scenario does not contribute to many avalanche accidents where skiers are involved. I believe this is because common sense works well in these obvious danger scenarios – if one can be bothered to use it (e.g. not going onto closed runs, roads and paths when ‘nothing happens most of the time‘). Although these types of avalanches don’t claim as many lives as dry slab avalanches do, they still need to be treated with serious respect.

In sum, I think the way you have to look at it is that the danger from rapid warming, rain, etc. is the obvious, conspicuous ‘known’ enemy. The more deadly enemy is the covert, quiet trap of the dry slab avalanche, that you may unsuspectingly walk (ski or snowboard) into despite subtle but clear and ‘obvious’ clues indicating danger.

This video by Aaron Cassells shows a near miss when a wet snow avalanche came down across a CLOSED walking track, narrowly missing a person below (we’ve noticed that many accidents involving wet snow avalanches involve people in closed areas – closed due to avalanche danger).

Here in the N French Alps, it’s been another week of heavy snowfall. We already have substantial snow depths, recent snowfalls and more snowfall forecast (snow/rain limit of 1400-2800m in the N French Alps and surrounding areas over the next week or so). For the next few days, temperatures will be climbing considerably, with snow/rain continuing on and off. By the end of 3 Feb / Wednesday night, it is predicted that freezing level will have risen to 2800 m and crazy mild temperatures are expected for Thursday too (0° C at around 2900 m in the day, and barely falling at night!), even milder on Friday and Saturday before it starts cooling down again.

When we see a rapid rise in temperature, such as this, along with rain, snow, sun etc., this will create a risk of spontaneous, natural avalanches – especially just after a snowstorm. Some of these could be very large and spectacular. They will not be triggered by skiers but will happen naturally, especially below 2300m. With ski lifts currently closed, and the snow becoming very humidified, heavy and difficult to ski (so probably very few off-piste skiers around the next few days anyway), but there will be a real threat to roads and buildings.

Natural wet snow avalanches that release on steep slopes due to rain and a big increase in temperatures, during and just after snowstorms, can and do cause tragic accidents. We’ll need to keep an eye on this over the next few days (as well as the other issues of long term instability caused by cold temperatures, that we’ve been harping on about this season).

The net outcome of this warming and especially rain, when free water drips down through the snowpack, will be a stabilising ‘glueing’ effect, especially when temperatures go down below freezing level. You may notice that within 24 to 48 hours of a significant warming episode/rain, the avalanche danger rating often drops by at least one level.

Starting early next week, it’s going to get colder again, so this will help the stability of the snowpack. However, whenever there’s a persistent weak layer(s), like now, there’s still a serious potential for unexpected releases.

The post Rapid warming, rain and wet snow avalanches appeared first on HAT.