SnowFlurry 1 2020/21 | Déjà vu on the early winter crust sandwich

Review from the perspective of the snow cover

In the last week of September, there was a significant snowfall right up to the higher valley levels. For many locals, it was the start of the new season with stone skiing on a good-natured surface. This was followed at the beginning of October by a sudden onset of foehn winds and light rain up to high alpine altitudes. In mid-October, there was a second significant fresh snow event followed by a moderate warming. This was followed at the end of October by a third heavy snowfall down to the valleys with half a meter of fresh snow in some places. On November 3, it rained lightly again up to almost high alpine regions and the following day the air remained very warm, foggy and damp. Since then, more or less warm high-pressure weather has prevailed with mostly clear skies and fairly dry air, rarely interrupted by any significant fresh snow.

The warm spells have caused a melting crust to form at the top of the fresh snow. Below this, the snow will remain powdery for the time being or, if already settled, compact. However, the high-pressure weather for just over two weeks is changing the snow cover massively - especially on the shady side where the snow has remained, but also on the sunny side on the high mountains, which are snow-covered in all exposures. On very steep south-facing slopes, melting takes place during the day due to the high temperatures and solar radiation - the snow melts on the surface. At night, the snow surface freezes again, cools down considerably and forms a strong temperature gradient to deeper snow layers. As a result, the build-up transformation takes over at night - even in sunny terrain. However, the melting transformation is so strong again during the day that hardly any or only slightly pronounced, constructively transformed weak layers can form.

Outlook

Weak layers of old snow are therefore already present due to the high-pressure weather since the beginning of November. It is impossible to predict whether and how this will affect the avalanche situation. That depends on the further course, especially on the frequency and intensity of new snowfall.

Theoretically, no relevant old snow problem could develop from this - but that is very unlikely. If it continues to stay clear for weeks with hardly any fresh snow, the scurrying ants will form huge cup crystals in the thin snow cover - which will trickle towards us just by looking at them. We could then have an old snow problem close to the ground throughout the winter as soon as the loose snow cover is overlaid by a suitable snow slab.

Both scenarios, heavy snowfall in the near future or continued good weather, have advantages:

If it snows regularly and in equal amounts, there will probably only be a problem for a few days to a few weeks at most.

If the skies remain clear and the weak layers of old snow only become an old snow problem sometime in December or January after new snow, you can admire the floating snow crystals, shivering snowpack test results and the multi-layered snowpack sandwich, which have been so rare in the last three winters, intensively for another season. Just like in 2014/15, 2015/16 and 2016/17, for example.