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SnowFlurry 13 2019/20 | Delineation of avalanche problems in the LLBs and useful functions of the Euregio Avalanche Report

Possibility of avoiding old snow problems over a large area

by Lukas Ruetz • 02/16/2020

Well-attended ski tour in Sellrain

Lukas Ruetz

The Euregio Avalanche Report has brought some innovations to the world of avalanche warnings since 2018/19. These include the function of highlighting or dimming areas with a certain avalanche problem - perfect for getting a quick overview of weak layers in old snow.

Handle old snow problems correctly

To manage old snow problems, you need to avoid them much more than most other avalanche problems. This means avoiding slopes at the described altitudes and exposures as much as possible, and if you do enter one of them, you should remain very, very defensive.

How explicit are the altitude demarcations and exposure specifications for the avalanche problems?

There are extreme differences in the severity and assignability of the five avalanche problems to the altitude and exposures. We discuss all five below.

New snow problem

The new snow problem always occurs in all exposures and becomes more critical with increasing altitude. It naturally snows the same amount in all exposures (without the influence of wind), the amount of precipitation increases with altitude and the fresh snow becomes colder, making it more brittle and easier to trigger as a slab avalanche. It is therefore not possible to differentiate the fresh snow problem according to exposure and not to use a sharp altitude demarcation, as it becomes continuously more problematic towards the top.

Drift snow problem

The same applies to the drift snow problem in terms of altitude demarcation. The higher up you go, the stronger the wind, the more fresh snow and the colder it gets. This means that the drift snow areas generally become more extensive towards the top and easier to trigger due to the lower temperatures. In the avalanche report, the drifting snow problem can be defined somewhat more clearly than the fresh snow problem in the altitude information - even if the demarcation should still be seen as a very rough, fluid transition. In terms of affected exposures, it works much better because most of the drift snow will always - but not only! - will always be on the leeward slopes. In other words, precisely in the exposures opposite the main wind direction.

On the other hand, the weak layers in the drift snow bond more quickly due to the heat and the drift snow can no longer be triggered as a slab avalanche. This means that after the formation of the last drift snow and rising temperatures with sunshine, the avalanche danger calms down more quickly on sunny slopes than on shady slopes.

Thus, the drift snow problem can be roughly narrowed down according to exposure at the time of formation, although fresh drift snow packs can also occur in other exposures due to local wind deflection. And as soon as the drift snow problem eases again in fair weather due to time, warmth and sunshine, the sunny areas (SW-S-SE) can be excluded from the drift snow problem much sooner than the shady slopes.

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Gliding snow problem

In the case of the sliding snow problem, it is also only possible to make a rough classification. There tends to be less sliding snow towards the top. On the one hand, because the ground becomes rougher. Smooth grass mats become less and blocky rocky terrain more. Secondly, because it gets colder towards the top, so less snow melts, there is less rain and therefore less moisture between the snow cover and the ground. As a result, the problem of sliding snow becomes less frequent with increasing altitude, but the transition here is also fluid and not sharp. The same applies to the exposure of sliding snow. Mainly sunny areas are affected (more melt, more water seeping in towards the ground), but there is just as much sliding snow on shady slopes. The exposure rosette in the situation reports is an indication of the "mainly affected areas" and not the exclusively affected areas - as in the drifting snow problem.

Wet snow problem

In the wet snow problem, the delimitation by height and exposure works much more precisely. When it rains, all exposures below the snow line are affected by the wet snow problem. If high temperatures are the main reason for the wet snow problem, the zero degree limit plays an important role. And if it is the radiation (usually in combination with the air temperature) that is the main reason for the wet snow problem, then it is altitudes and exposures that can be used to delineate the problem.

This is especially true with regard to the combination of exposures with the time of day in the classic spring situation: the radiation is very intense. In the morning, eastern slopes are affected first, then southern slopes, and only at midday or in the afternoon western slopes. Northern slopes are usually not affected at all or only much later in the spring, when it gets warmer and the sun is even higher. Basically, you can imagine the problem areas in the spring wet snow situation as a spiral staircase from low-lying eastern slopes to high-lying northern slopes. The lower and earlier sunny (eastern slopes) in the course of a day, the earlier problematic. The higher and later (or not at all, northern slopes) sunny, the later problematic.

Then the stability of the old snow cover plays a central role in the wet snow problem: Where are there weak layers of old snow that are very well connected again when dry, but could become a problem again with the penetration of moisture and the associated weakening?

This means that the situation report can usually allocate the affected areas of the wet snow problem quite precisely, the boundaries are much, much sharper according to altitude and exposure. Especially in combination with the time of day and the structure of the old snow cover.

These weak layers of old snow always originate from an earlier old snow problem that was no longer relevant before the snow cover became soaked. And this brings us to the last, but most easily definable avalanche problem in the avalanche situation report.

Old snow problem

Due to the generally relatively slow development of the old snow problem and the relatively long relevance of this type of weak layer (several days to many weeks), one has the time with dozens of snow profiles in a region at different altitudes and exposures to assign the distribution of the weak layers relatively precisely to certain altitudes or altitude bands and exposures. Then you combine the snow profiles with process thinking and the weather pattern and get the sharpest picture of all avalanche problems in the delimitation according to altitude and exposure!

It is not uncommon to be able to assign individual weak layers in the old snowpack to an accuracy of less than +/- 100 meters in the situation reports. In addition, these weak layers are almost always only formed in certain exposures - i.e. usually only on shady slopes or only on sunny slopes. It is often the case that the weak layer simply does not exist at other altitudes and slope orientations.

Slopes that are located at this altitude and face in this direction are then simply always affected by the old snow problem. The symbols for altitude and exposure for the old snow problem are not a rough guide value as for the drifting snow, fresh snow or sliding snow problem, but are almost always a highly explicit indication.

Drift snow problem above 2400m in slopes from west via north to east Old snow problem in an altitude band from 2400m to 3000m in slopes from west via north to east — Drift snow problem above 2400m on slopes from west to north to east Old snow problem in an altitude range from 2400m to 3000m on slopes from west to north to east

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Avalanche warning services are often able to pinpoint the old snow problem very precisely, but not always. There are situations where the best avalanche warning service in the world finds it difficult to indicate the distribution and, above all, the relevance of one or more weak layers of old snow in combination according to altitude and exposure.

In its last blog entry, the Tyrolean avalanche warning service speaks of a "diffuse old snow problem" in certain areas of North Tyrol, for example. The word "diffuse" refers to the distribution of the problematic areas. They are difficult to localize and not nearly as sharp as most other old snow problems.

Difference between localizability in the avalanche report and localizability in the individual slope

The avalanche problems thus show a progression in their "local localizability" via the avalanche report. The progression from "locally difficult to localize" to "locally usually very easy to localize" is: fresh snow problem - drifting snow problem - sliding snow problem - wet snow problem - old snow problem.

It is almost exactly the opposite of how the user recognizes and localizes avalanche problems on the individual slope on site. This is because fresh snow and usually also drifting snow problems can be easily recognized and assessed directly on site using your senses (sight, touch). You can also easily recognize gliding snow problem areas via the open mouths, but it is difficult to estimate when the area below the mouth will become a gliding snow avalanche. In addition, a sliding snow avalanche can also occur without a previously formed fish mouth, making it de facto unrecognizable and unpredictable. The wet snow problem can also be recognized very well, but it is usually difficult to estimate whether and when slab avalanches will actually occur or can be triggered. And in the vast majority of cases, you cannot perceive the old snow problem with your senses, as it lies hidden in the snowpack, is not visible on the surface and in most cases there are no warning signs.

The dimming/highlighting function with regard to old snow

The best thing to do is to choose an area for your tour that is not affected by relevant weak layers in the old snowpack in the first place. The highlighting/dimming function for various avalanche problems in the new Euregio Avalanche Report for Tyrol, South Tyrol and Trentino is perfect for this. You can use it to highlight or hide warning regions with certain avalanche problems. Particularly interesting with regard to the old snow problem and a first rough filter if you are thinking at home about which area to head for on your next ski tour.

Warning areas with an old snow problem in the Euregio on 16.02.2020

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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.

Show original (German)

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