SnowFlurry 4 2020/21 | Low probability - High consequence

Read the snow profile

The snow profile was recorded on December 11th at noon at 2790m on a 32° steep eastern slope. It is slightly windy from the south and the sky is about 50% covered by clouds.

At the profile location there is 235cm of snow, in which a whole series of different layers have been worked out, which have been created from all three types of transformation: Melt transformation, build-up transformation and degradation transformation.

In the comments field we find an assessment of the general hazard situation for the build-up transformed weak layers and an assessment of the further development as well as the assignment of the formation period of the two layers of 103 - 89 centimeters in height.

Two snow cover tests were carried out: An ECT in a modification that does not officially exist: the column was shoveled down to a height of 145m. This was done because they wanted to estimate the tendency for fracture propagation in the layer, but with an ECT, weak layers with such a thick "original layer" can hardly be made to fracture.

And indeed: In this modified form of the test, there was a fracture with fracture propagation over the entire block on the 8th blow from the shoulder - that is the 28th blow in total. The result is called ECTP28.

In addition, a Propagation Saw Test (PST) was carried out. A 30cm wide and 1m deep column running in the fall line was exposed and cut off at the back. A potential weak layer was then selected and a snow saw was run along the weak layer from bottom to top. An artificial fracture is created and observed to see whether it spreads backwards of its own accord. In this case, the saw was pushed into the same weak layer as the ECT. After a penetration depth of 15 centimetres, the fracture spread by itself to the end of the block. The result is thus: PST 15/100 (End).

SnowFlurry

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Snow profile Schöntalscharte (northern Stubai Alps), 11.12.2020

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Snow profile Schöntalscharte (northern Stubai Alps), 11.12.2020

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Interpreting a profile

For an inner-Alpine dry valley with an average of only 5.50m of fresh snow per year at 2000m above sea level and an average of only 1.5m thick, settled old snow cover in late winter, there is already more than a decent amount of snow here at the beginning of December.

In order to dig out such a profile with a snow depth of 235cm on your own, you will be busy for an hour, depending on the snow conditions. If you dig profiles frequently, you not only practise using a shovel and probe and the digging technique. You also learn to understand how small the chance of survival is for a victim buried over 2m deep - even if your companions locate them immediately and start digging them out. As a rule, an avalanche victim only has a maximum of 20 minutes to die of asphyxiation - a few minutes are already taken up with the avalanche transceiver search. Then there are perhaps 15 minutes left to shovel.

This reminds you once again that you should always prevent an avalanche from being triggered, no matter how small. As a result, the snow pusher sees some new approaches in avalanche science, which focus on terrain forms with rather low consequences, just because the slopes are small or there are no terrain traps, rather critical.

You can see three areas in the profile:

Red

In red from approx. 235cm to 115cm snow depth you can see the fresh snow since 04.12. This has already settled and solidified well, especially in the lower area. Like the rest of the snow cover, it is currently in the process of decomposing and is therefore becoming more and more solid. This can be recognized by the steep, i.e. weakly pronounced temperature gradient - this is the red line that connects the temperature measurements at different snow depths.

In addition, a subtype of degradative transformation is taking place in the entire snowpack, especially in the lower area: Mechanical transformation. The snow load of several hundred kilograms per square meter exerts massive pressure on the lower snow layers. This causes them to degrade even faster and more strongly.

Green

In green from about 115 cm to 70 cm, you can see a sequence of crusts and layers that have been transformed by building up. However, most of the structurally transformed layers are already in the process of transforming into angular-rounded crystals. They hardly ever become pure round grains - i.e. the actual end product of the degradative transformation. They are usually already too large for that. The end product is somewhat larger, angular-rounded crystals. This is because the decomposing transformation basically only creates the spherical shape and not necessarily the smallest possible, round-grained crystals.

The layers in the green area come from the changeable, snowy but sometimes warm October and November with foehn winds and heavy rainfall. The hardest crust with the widest deflection of the blue bar to the left is due to the influence of rain and, above all, the long-lasting, warm high-pressure phase in mid-November. The sun shines for several hours on this eastern slope - even in December.

Thick layer of snow after the extreme southerly dusting. — Thick layer of snow after the extreme southerly dusting
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Lukas Ruetz

Blue

In blue we see the heavy snowfall from the end of September, possibly also encrusted together with the snowfall from the beginning of October. A fusion crust has formed, which was able to build up and transform for a while. However, the crystals that have undergone a structural transformation have already changed back from angular to angular-rounded crystals.

Avalanche risk

The thick layer of snow has made it unlikely that the old snow layers will be triggered. To do this, you have to hit a spot with a less thick layer. However, as you cannot see these spots in the terrain, you can only continue to avoid the entire areas, i.e. the altitudes and exposures that the avalanche warning services assign to the old snow problem. Out of 100 areas with little snow that you enter or drive over, a maximum of 10% can be recognized as areas with little snow - the rest is difficult or impossible to recognize in the terrain. Accurate snow depth estimates for individual points in the terrain are simply hardly possible due to the equalizing effect of the subsoil forms through the snow cover. However, the break propagation is still good to very good in the weak layers of old snow. This means that if you find a place where you can trigger, huge avalanches can result.

In the regions south of the main Alpine ridge with the most fresh snow, trigger points suitable for skiers have become extremely rare and hardly conceivable. In the transitional regions with still plenty of fresh snow further north, up to about the Inn, there are already considerably more release points. In the regions with less than 1m of fresh snow, there are potential trigger points like in a patchwork quilt: regularly distributed across the affected exposures and altitudes with a relatively high density.