Snowpack stabilization due to wet snow and high temperatures
Current off-piste conditions in the Northern French Alps and surrounding areas, and forecast for new year’s week
Snow conditions in the N French Alps over the last few days are a classic example of how, after causing initial instability, wet snow, high temperatures and rain have contributed to stabilising the snowpack. At least something good as come of the rain, because following last Wednesday’s storms, it’s been an extremely tricky week for going off-piste and touring! (Images c/o Météo-France)
For more info on this stabilising effect after wet snow conditions, see my article here.
When we get fresh snowfall, accompanied by a rise in temperatures and likely rain, the extreme danger in the days during and the hours right after the snowstorm is very apparent and often over-emphasized. Despite this initial instability, the final effect, however, is snowpack stabilisation – which is often under-emphasized.
Does this mean we ignore the obvious, classic signs of danger on a slope that is subjected to a rapid rise in temperatures and incoming solar radiation especially just after a snowstorm storm? Of course not, but equally it is important to recognise the net – final outcome of warming, melting and especially rain (any situation where free water drips down through the snowpack), will be a stabilizing ‘glueing’ effect, especially when temperatures go down below freezing level after. This is often reflected by a drop in danger level in the forecast up to the altitude at which the snowpack has been fairly well saturated – at least in the top layers down to 40-50cm. This drop in danger level (stabilisation of the snowpack) happens sometimes in as little as 12 hours after the melting or rain has ceased.
As well as this ‘glueing’ effect causing stabilisation, many steeper slopes have purged themselves of snow – often virtually down to ground level taking with it most of the facets that made up the persistent weak layer on those slopes. But the persistent weak layer, that developed during the clear skies over the two weeks leading up to the storm, is still a serious factor to consider on North’ish facing slopes up above approx 2800 m on North’ish facing slopes.
The following avalanche danger levels in the N French Alps by Meteo-France show the diminishing danger level after our most recent snowstorm on Wednesday 29th Dec , with avalanche danger ratings of 4 on the Thursday, down to a 3 or 2 on two days later – then down to a 1 four days later.
Thurs (day after the storms) 
Sat (2 days after the storms)
Another interesting indication of the stabilising effect of the wet snow and rain at lower altitudes can be seen in this pictogramme below from Meteo France, where the the ‘warmer’ slopes below the rain/snow limit of 2400 m have now become extremely stable and solid due to the ‘glueing effect’ of snow melting and refreezing, within just a few days of the snowstorm.
How does the forecast look for the coming week?
Sat 1st Jan: A sunny day with relatively high temperatures (0° C at 3800 m by day). Light W/SW wind.
Sun 2nd: Sunny again, with some high cloud possibly coming in as the day goes on. 0° C at 3000 m in the day, down to 2300 m by the end of the night.
Mon 3rd: Sunny again with possible thin high clouds. Clouding over during the night with possible light snowfall above 2200 m. O°C at 2500 m.
Tues 4th: Lightly snowing in the day, getting heavier overnight. Temperatures down on previous days: 0°C at 1200 m.
Wed 5th: Snowfall will eventually die out as the day goes on. Getting colder: 0° at 900 m. Winds turning N.
Thurs 6th: A brighter day, but with more snowfall
Tip of the week
Over 90% of avalanche accidents involve slabs that are made up of cold, dry snow and the victims almost always trigger it themselves. However, this is not a reason to ignore the obvious signs of wet/humid snow avalanche danger!
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There’s a long list of evidence that shows how applying simple frameworks, checklists and memory aids reduce risk in ‘high consequence, low feedback’ risk contexts prevalent in: aviation, military, finance, health care, avalanche terrain etc.
See our HAT quick reference ‘Safety is Freedom Framework’ for accident reduction in avalanche terrain. The Framework is aimed at all levels of off-piste and touring: for beginners: a point of departure; for experts: a guide for further learning; for pros: it’s a great framework for client training and quick memory aid.
The Framework is best if accompanied by training such as HAT events and on-snow courses, but it’s also a useful companion for all training as it focuses on the basic key points that all avalanche training courses address – it helps you to keep focused on the essential accident reduction points.
For more information on upcoming HAT events (including the *ORTOVOX Off-piste awareness Tour) : webinars, online talks and other useful announcements, see our HAT Facebook page.
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