SnowFlurry 7 2020/21 | Weak layers in the south from cold to warm

Profile 1, 07.01.2021, Tullkogel, 2515m, S, 27°

The snow profile was recorded by LWD Tirol at noon in the Deferegger mountains of East Tyrol on a 27° steep south-facing slope. There is 190cm of snow at the location, it is -9.5°C cold and cloudless.

For us, the structure of the snow cover down to a snow depth of 120cm is interesting. From 190cm to 125cm lies the snow slab for the potential triggering of an avalanche. With a thickness of 65cm, the weak layers underneath are no longer so easy to disturb. However, with this thick layer, even small avalanches can produce large amounts of snow that slide down into the valley as a slab avalanche.

At a height of 125cm to 120cm, there is a sequence of three thin, angular weak layers and two melt crusts in between at around five centimetres. The uppermost weak layer lies directly underneath the potential snowboard on a crust that was formed by rain and warmth before Christmas. Cold fresh snow fell on this crust in the last days of December. This caused the angular crystals to develop at the transition from the old snow surface to the fresh snow. As there have been huge amounts of fresh snow in East Tyrol since then, the thick slab now lies above the weak layer. The crystals of the weak layer are not particularly large, about 1 mm in diameter, but very soft (hardness 1, blue bar only goes out to the first line to the left).

Below the first melt crust, there is another thin, angular layer of very small crystals, which is relatively soft. Followed by the second, wafer-thin melt crust and a harder, not too pronounced, angular layer.

Below 120cm, we find a compact old snow cover with a less worrying, built-up transformed layer at about 55cm.

The test result is interesting: In the angular layer below the fresh snow and above the uppermost melt crust, a fracture could be produced over the entire block in two ECTs. This was on the 21st and 16th blow. From this we can conclude that the snowpack in the areas where this layer has developed - depending on the thickness of the overlying slab - can be triggered even with a low additional load, a fracture propagation can occur and subsequently a slab avalanche can develop. Or, with a lower steepness, a settlement noise including crack formation in the snow can be triggered.

Conclusion

The profiles impressively confirm the current situation south of the main Alpine ridge in the main precipitation areas over the last two weeks, in addition to the numerous spontaneous avalanches triggered by winter sports enthusiasts. The Euregio Avalanche Report currently assumes a main problem area of the angular weak layer due to the "cold to warm" hazard pattern on sunny slopes between 2300 and 2600m.

In shaded slopes in these areas, there is a weak layer that is no less prone to triggering, albeit formed from a different process: surface frost, which has been snowed in mainly in a narrow altitude band around 2200m and now poses a problem in the snowpack.

It is not only the ease with which the layers can be triggered and their spatial distribution that is relevant for tour planning, but also their potential to generate remote triggering: Both weak layers are part of the old snow problem and, in this constellation, readily lead to remote releases.