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WeatherBlog 17/2013 | Xaver and the air mass boundary

Low pressure Xaver taps into cold polar air and mixes up the Mediterranean region

by Lea Hartl 03/12/2013
Snowfall brings Frankfurt Airport to a standstill, there is a 100-car pile-up on a highway in Hesse, Spanish climbers are complaining about the incessant rain, Andorra is drowning in snow and, at least in the Eastern Alps, nothing is happening at all, although a few days ago masses of snow were predicted here too. What's going on?

Snowfall brings Frankfurt Airport to a standstill, there's a 100-car pile-up on a highway in Hesse, Spanish climbers are complaining about the incessant rain, Andorra is drowning in snow and nothing at all is happening in the Eastern Alps, even though a few days ago masses of snow were predicted here too. What's going on?

Changeable weather on Wednesday (13.3.) in Tyrol

Low pressure system Xaver was responsible for the snow and cold in Germany and parts of France. Xaver is moving along the pronounced air mass boundary that has been with us since the weekend and, blocked by Xaver, has only moved very slowly to the south so far. Considerable amounts of snow fell on Xaver's northern side with icy temperatures and lots of wind. In Normandy and on the islands in the English Channel, it has snowed more heavily in the last two days than it has for over 30 years. Due to its proximity to the Gulf Stream, the climate in the area is otherwise rather mild and public life came to a standstill in the face of meter-high snow drifts. After enjoying snow-covered palm trees along the English Channel for a while, Xaver moved southwards and has now reached the Mediterranean region, always with the cold polar air behind the air mass boundary in its luggage. An area of high pressure over the Atlantic is shoveling the cold air behind him. Xaver now visits the Gulf of Genoa. There he finds warm, moist air and the temperature contrasts (cold air at altitude, mild temperatures on the ground) intensify. As a result, the atmospheric stratification becomes more unstable and showers and thunderstorms develop. Xaver is now quite a strong fellow. It continues its journey and moves eastwards.

Lightning radar from Wednesday afternoon (13.3.)

At the beginning of the week, some models were still assuming that Xaver would take a north-easterly course and strongly graze the eastern Alps. On the back of the trough, such a situation would lead to northern congestion. In the meantime, however, it looks more likely that Xaver will remain a little further south, which should result in less precipitation. However, at least the Eastern Alps will not be left empty-handed and if the clouds clear at the weekend, there should be one or two sunny powder turns. Due to the large differences in pressure and the flow around the Alps, strong winds are to be expected, especially tomorrow, Thursday (14.3.).

Geopotential in 500hPa and ground pressure, forecast for Friday (15.3.)
Theta E at 850 hPa, forecast for Friday (15.3.)

The exact impact of Mediterranean lows on the Alpine region is complicated to predict, as small changes in the path of the cyclone can make the difference between a mega-dump and sunshine. Also, the lows usually form or intensify at relatively short notice and the exact location of the cyclogenesis is difficult to determine in advance. While the southern slopes of the Alps are quite reliably subject to congestion, the north is only affected if the low moves in such a way that it grazes certain areas in passing and causes congestion while it is actually already moving away. The north-westerly systems that are more relevant for the northern Alps are often characterized by low-pressure areas that last longer and repeatedly send fronts into the Alpine region. These then arrive there over a relatively large area. Shifts of a few hundred kilometers ensure that it snows 10 centimeters more in Salzburgerland, for example, than in eastern Switzerland, but do not completely change the weather, as is often the case with Italian lows. On Sunday, a strong south-westerly flow will set in, north föhn will turn into south föhn and our colleague Orakel will possibly comment on the southern slopes of the Alps. In the longer term, it looks like the weather will remain unsettled. Spring will probably remain somewhat wintry, at least in the mountains.

Photo gallery

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.

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