SnowFlurry 2 2022/23 | Weak layer development

Winter is finally as it should be: cold and with fresh snow. Off-piste ski tours are now possible in many places in Tyrol. At lower altitudes, however, things are still looking bleak this year, with at most the ascent on skis possible, but not enough to ski down. In forest aisles and wind-protected areas above the tree line, there has been great powder skiing in the last few days, but you still have to watch out for sharks.

The development Part 1: Old snow avalanches in poor snow conditions

An early winter problem with old snow led to an avalanche-rich period around the Christmas holidays. Especially where there was a closed snow cover before the end of November, the snow was transformed and loosened (see SnowFlurries 1). A foundation of angular crystals and deep frost acted as the primary weak layer, but layers that had been built up and transformed around crusts could also be disturbed. Before the Christmas period, off-piste ski tours were not possible for the most part. There was also still no suitable board above the loose layers, which was one of the reasons why avalanche activity only started around 23rd December. On this day, temperatures rose with the onset of precipitation and the snowfall turned into sleet or rain.

This was followed on 25th of December by another jump in temperature - an additional indicator for the formation of a slab (bound snow) and favourable for avalanche activity. The avalanches triggered by winter sports enthusiasts in the Sellrain Valley were limited. However, if you look at it in relation to the number of tours possible there, the average is really high. (Avalanches were triggered on: Lampsenspitze, Pirchkogel, Rietzer Grieskogel, Praxmarer Grieskogel, Schöntal, Grieskogelscharte 2x, Zischgeles,....).

Unfortunately, many people have little understanding of the term "old snow problem" and don't know how to deal with it. In addition, people still pay more attention to the danger level than to the avalanche problem and danger patterns. Many people also still believe that a little snow is safer than a lot of snow. A frequently heard statement: "There is almost no snow, so no avalanche can happen". A misconception that is still firmly rooted in many people's minds. Winters with little snow are particularly prone to avalanches!

SnowFlurry

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Part 2: Christmas thaw and moisture penetration

From the end of December to the beginning of January, temperatures soared once again. On New Year's Eve, records were even set in some places. Due to the wet conditions and mild temperatures, the old snow problem solidified somewhat. But that doesn't mean it's no longer a problem and you can go anywhere on a 2-metre day! An old snow problem remains treacherous until the weak layer has solidified. Only the ratio of weak layer and board changes and the trigger points (hotspots) decrease slightly. However, if you catch such a hotspot, avalanches can also become larger, as fractures can be initiated in weak layers close to the ground.

Especially in places with little snow, weak layers close to the ground are easier to disturb, so there is also a GM7 (snow-rich next to snow-poor). In addition, many slopes are still untouched and weak layers are present over a large area, as they have not been destroyed by the many other skiers.

The Christmas dampness and the subsequent warming had a negative and positive effect on the snow cover:

Bad: Formation of the slab, which provided all the ingredients for avalanches
Good: Consolidation of the weak layers and the susceptibility of the old snow problem
Bad: Formation of new weak layers (Gm4)

Part 3: Crust sandwich

As we know, temperature fluctuations in the snow cover are not conducive to its solidification, as they are the starting signal for the build-up transformation. Before the onset of precipitation on 23rd of December, the snow surface was very cold and often consisted of loose crystals and surface frost. The precipitation deposited warmer and wetter layers on the cold surface, which led to a large temperature gradient at the boundary between the old snow surface and the new snow. In addition, the moisture input led to the formation of a melting crust. New weak layers of angular crystals were able to develop above and below this.

During snowpack investigations, an increasing number of partial fractures were produced, so the fracture propagation was not particularly good at this time.

Avalanche Praxmarer Grieskogel — Avalanche descent Praxmarer Grieskogel

Stefanie Höpperger

Part 4: Fresh snow and new weak layers

From 4.1. and 9.1.23 it snowed from various frontal systems, mostly accompanied by wind. The snow mainly fell above the tree line in very disruptive drifting snow packs that came to rest on loose layers.

Wild snow also fell from the sky in the frosty temperatures during windless periods. Wild snow is also known as "champagne powder" and is great fun to ski on. It really creates a dusting! However, if it is overlaid by bound fresh snow or drift snow, it represents a very weak layer that is prone to disruption.

In addition, the loose, unbound snow can be transported at wind speeds of just 15 km/h and deposited as very brittle and fragile drift snow. These avalanche problems and hazard patterns have led to further avalanches caused by winter sports enthusiasts.

Winter remains exciting!