SnowFlurry 5 2017/18 | Why fresh snow increases the risk of avalanches

Wind

In high winter, precipitation usually comes from fronts and accumulation, the convective part of the precipitation is low. Fronts and accumulating precipitation bring wind and increase the risk of avalanches simply due to the formation of a drifting snow problem during the new snowfall.

Decisive: The additional load & temperature fluctuations

Extreme amounts of new snow over several days or even weeks also only occur due to a combination of fronts and accumulation effects. A cold front is followed by a warm front, then the next disturbance, and so on. Especially for the areas north of the main ridge, the Atlantic, humid westerly weather conditions are responsible for this, as was the case a week ago. For fresh snow masses south of the main ridge, a low pressure complex in the western Mediterranean is needed, as we are seeing this week.

The fresh snow pack has different layers, mainly due to the constant ups and downs of the temperature when changing from one front to the next and their influence on the snow crystals. Just as there are different layers in an old snowpack.

An example: The cold front initially deposits cold, loose fresh snow crystals. The wind speed then increases and harder snow packs form on these new snow crystals. This is followed by the warm front, which drops large, moist flakes from the sky. A bound, moist layer of snow is deposited on fresh drift snow, both together form the snow slab - this lies on cold, loose fresh snow, a so-called weak layer. A tasty recipe for a slab avalanche! And all of this without a build-up transformation and therefore without the problem of old snow!

Large amounts of fresh snow: a problem even without wind

The fluctuation in temperature causes problem areas to form within the fresh snow even without wind. The critical amount of fresh snow is reached at 50 cm at the latest, even under favorable conditions. There is a simple reason for this: There are always layers of loose crystals somewhere in the fresh snow pack of a prolonged heavy snowfall. Above this there is often a somewhat denser layer of fresh snow that has fallen at a higher temperature, is therefore bound in a very short time and can form a snow slab. This can usually be found in the aforementioned sequence of a cold front followed by a warm front.

However, a homogeneous new snow pack can also become a problem: Due to irradiation or rising air temperatures, the fresh snow begins to bind to the surface and compact faster than the fresh snow deeper in the fresh snow pack. It is therefore very common for fresh snow to become more dangerous in the short term after the end of the snowfall than when it is completely fresh. The upper areas of the new snow begin to form the snow slab due to the consolidation, while deeper areas remain loose and unbound for a little longer.

Main problem: Loading of weak layers in old snow

The main problem of extreme precipitation events, however, is the abrupt loading of already existing weak layers in old snow. With 1m of fresh snow, for example, approx. 100kg/m² more load is placed on the existing snow cover, and this in a relatively short time. The weak layers in the old snow may not be able to withstand this additional load. However, if it snows 1m of fresh snow in three weeks, the weak layers have time to "adapt" to this snowfall by snowfall.