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SnowFlurry 5 2020/21 | Accident example of the low-snow trigger points in areas of old snow

Snow slab release Pirchkogel on 15.12.2020

by Lukas Ruetz 12/19/2020
On December 15, a medium-sized slab avalanche was triggered on the Pirchkogel in the Stubai Alps. No one involved came to any harm. The skiers hit an area with a thin layer of fresh snow and were thus able to initiate the break, which ultimately spread over a large area in the weak layer.

Let's take a look at the difference between a snow profile at the avalanche crack with an autumn old snow base and a snow profile without an autumn old snow base.

General situation

While the probability of triggering is only slowly improving in the areas affected by the old snow problem, very good conditions can now be found where there was no continuous snow cover before the beginning of December. The snow surface in shady and flat, sunny terrain remains powdery due to the superficial, accumulating transformation. This is directly related to the fine weather and clear skies. As a result, the snow surface can cool far below the air temperature and the superficial fresh snow turns into small, angular crystals. On steeper, sunny slopes, the high temperatures mean that a brittle crust can usually be found. In combination with the temperature, this also occurs with the actually only weak sun - we are almost at the lowest sun position of the year.

Snow profile 1, 17.12.2020 - Pirchkogel/Hinterer Grieskogel avalanche

The snow profile was recorded two days after the avalanche was triggered in the middle of the outcrop. We are here in a 40° steep area exposed to the northeast and close to the ridge. The avalanche was probably triggered very close to this location. The snow cover here is about 70 cm thick. The wind has probably removed most of the fresh snow here, as the heavy snowfall from December 4 - 7 brought about 1m of fresh snow here.

The snow from the snowfall since December 4 is between 70cm and approx. 35cm high. Underneath we find the old snow from October and November and perhaps the September snow has also remained here. Deep rime crystals had formed on the former old snow surface, which can now be found in the snow profile between a height of around 31cm and 28cm. Underneath is a melting crust, which has also been massively eroded by the accumulating transformation. This is why the spectacle symbol of the melting crust also contains a deep frost symbol.

In the extended column test, a fracture in the weak layer of floating snow was produced across the entire block on the 10th blow. This fits very well with the avalanche triggering.

The weak temperature gradient in the lower half of the snowpack shows that the degrading transformation is currently taking place there. In the upper half, on the other hand, the temperature gradient becomes stronger and the curve becomes increasingly flatter, especially in the uppermost area. This is where the constructive transformation is currently taking place. This can also be seen in the small, angular crystals that have already formed in the fresh snow. After a brief phase of degradative transformation after the end of the snowfall, the constructive transformation has now taken over due to the clear sky and therefore low surface temperature.

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Snow profile 2, 19.12.2020 - St. Sigmund im Sellrain

The second profile shows the stark difference to the areas where there was no snow before December 4th. We are here on an eastern slope at an altitude of 2360m.

There are no weak layers near the ground. Only a snow cover that has settled to about half the snow depth since the end of the heavy snowfall. There was about 135cm of fresh snow here immediately after the end of the snowfall - now there is still 70cm.

In the stability test, neither a partial fracture nor a fracture over the entire block could be produced after 30 impacts. The result is therefore ECT31.

The snow is already quite compact close to the ground, while angular crystals are forming further up these days - also due to the stronger gradient caused by the surface cooling. However, these are still tiny and not yet a cause for concern for the next snowfall.

In total, you will find top conditions at this location from a well-set base with 10 - 20cm of built-up converted powder snow on top.

Photo gallery

This article has been automatically translated by DeepL with subsequent editing. If you notice any spelling or grammatical errors or if the translation has lost its meaning, please write an e-mail to the editors.

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