2024-03-27 08:41:15
Favorable start to winter with little “semolina” or “sugar”
From late October to Christmas 2023, snow fell in abundance at high altitudes several times. “Start of winter with snowfall, storm and high avalanche danger”, “High avalanche danger in the west due to fresh snow and rain” or “Winter , for real!”, these were the headlines on the SLF AvaBlog. Above approximately 1500 m to 2000 m, snow depths significantly above average were recorded in the north around Christmas 2023. In the south, snow depths at altitude were within average values.
The situation was different in low-lying regions: because most of the precipitation there fell as rain, there was little or no snow in many places.
A lot of snow at the start of winter generally means a favorable snowpack structure with few marked weak layers – this was the case this winter. Indeed, when heavy snowfalls are over, the snow cover quickly stabilizes and, subsequently, avalanches become rare. Why that ? The main reason is linked to the temperature conditions in the snowpack. On the ground, the snow temperature is generally 0°C due to the warmth of the ground, while on the snow surface the values generally vary between 5 and -20°C depending on weather conditions; there is therefore a temperature gradient (variation per unit of thickness) inside the snow cover. If the temperature difference is, for example, 15 °C for a layer of snow only 50 cm thick, the temperature gradient is much greater (four times) than for a layer of snow 200 cm thick. The greater this temperature gradient, the more marked the metamorphosis of the snow. However, during the constructive metamorphosis of snow, large angular grains are formed. These have few links between them and therefore little cohesion. Pictorially, we then also speak of “semolina”, “sugar” or “flowing snow”. When such layers are covered with snow once more, they constitute typical weak layers in which avalanches can be triggered. The avalanche situation is then characterized by a problem of old snow.
With the thickness of the snowpack, we should not expect a significant problem of old snow at the base of the snowpack following this start of winter. The snowpack at the start of winter certainly presented some successions of crusts with intercalated fragile layers, but they did not play a critical role.
For winter sports enthusiasts, it was therefore a perfect start to winter – but unfortunately only at high altitude.
From Christmas to February: very soft, white at the top, green at the bottom; avalanche situation often favorable
At the change of year, avalanche conditions were very favorable. There were virtually no avalanches triggered by winter sports enthusiasts. The start of the year was characterized by repeated snowfalls and, over a period of seven days, accumulations of 50 to 100 cm of fresh snow were observed over a large part of the territory. In the absence of pronounced weak layers, however, the situation remained generally favorable. Sunday, January 14, 2024, however, was an active day for avalanches, with 25 human-triggered avalanches reported. The fresh blown snow had settled on surface frost in places, which resulted in avalanches being triggered very easily. In most cases, the avalanches were rather small and all those affected escaped unharmed.
By mid-January, snowfall was less frequent and there were also periods of stable, cool winter weather. During this time, the snow surface, and in the less snowy regions of Engadine and Ticino sometimes also large parts of the snowpack, were able to undergo a constructive metamorphosis. Covered by fresh snow which fell several times from January 17, this surface became a fragile layer above approximately 2000 m. This period was thus marked by a few avalanche accidents, including one fatality on January 24, 2024 in Orsières.
Subsequently, however, favorable avalanche conditions once once more dominated, interrupted by snowfall from February 9 to 11 in the south, but without major consequences.
At altitudes below 1500 m, the snow, already scant, has disappeared with generally very mild temperatures. Winter sports (except on technically snow-covered slopes) were therefore difficult at these altitudes during the holiday period.
From mid-February: return of winter, heavy snowfall in the south with the biggest avalanches of the winter up to that point; numerous avalanche accidents in all regions
On February 8, winter returned to the south following a break of almost two months. He seemed to want to catch up, and this is how following four situations marked by meteorological barrage on the south side (from February 8 to 11, from February 22 to 28, from February 29 to March 4 as well as from March 8 to 11) , we recorded accumulations of 3 to 5 m of snow. “Finally fresh snow in the south”, “New dams on the Gotthard! » or “Weather dam in the Gotthard, episode IV”, these were the titles of the AvaBlog this time. Large and even very large avalanches occurred on several occasions, most on March 10. No significant damage has been reported so far.
The avalanche situation was, however, not only critical in the south, but for people often in all regions. In the north there were also successive fresh snowfalls, but the main problem was caused by the fragile accumulations of wind slabs which had formed under the effect of the repeated southern foehn. There were numerous triggerings of avalanches by people, but these were generally small to medium sized. In the south of Haut-Valais as well as in the intra-Alpine regions and the south of Graubünden, a problem of old snow has also developed: there have been several occasions where avalanches have been triggered in the weak layers of the upper part of the snowpack, sometimes of large size in isolation. Most of the accidents occurred on March 2 (11 accidents, 13 people recorded, including four completely buried and three dead) and from March 8 to 10 (18 accidents, 25 people recorded, three completely buried and three dead).
A winter of sliding avalanches
The winter of 2023/24 was a “sliding avalanche winter”. The foundations were laid at the start of winter: snow fell in abundance on the still soft ground. The thick snow cover without noticeable weak layers began to slide at the start of winter. Sliding avalanches are less dangerous for winter sports enthusiasts than dry slab avalanches, because they cannot be triggered by people. For the security services of ski areas or roads, on the other hand, sliding avalanches represent a great challenge, because they can threaten communication routes or ski slopes for a long period of time.
This winter, more than 2,300 slide avalanches have been reported to the SLF. In comparison: during the entire winter of 2022/23 (from December 1, 2022 to June 1, 2023), 293 landslide avalanches were reported to the SLF. However, there have already been winters with even more numerous and especially larger gliding avalanches, such as the winter of 2011/12. Damage was recorded during 15 sliding avalanches, with search actions following the burial of a communication route also being counted as damage.
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