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The precipitation event of 19-23 August 2005, which caused particularly important floods in Switzerland, was related to the development of two successive cyclonic systems at ground level over Europe.
The precipitation event of 19-23 August 2005, which caused particularly important floods in Switzerland, was related to the development of two successive cyclonic systems at ground level over Europe.
The first cyclonic system was located over the Netherlands at the beginning of the event and caused the passage of a cold front over Switzerland on 19 August. The cold front was accompanied by severe thunderstorms that caused first precipitation over north-western Switzerland and then, as the front moved towards the south-east, over eastern and southern Switzerland.
As this cyclonic system weakened, a second cyclonic system developed on 20 August over the Gulf of Genoa at ground level. As the air is advected in a counterclockwise direction around such systems, warm and moist air from the Mediterranean and Adriatic Seas was transported over the Balkans towards Switzerland, arriving towards the Alpine range from the north-east. As the air was forced to rise over the relief, heavy precipitation was triggered. At first the precipitation affected the western and central part of Switzerland (21 August). Then, as the low pressure system moved from Italy towards the Adriatic and the Balkans, the precipitation shifted eastwards, affecting the central eastern Alps and north-eastern Grisons (22 August). As the low pressure system continued to move east towards Hungary (23 August), the zone affected by heavy precipitation shifted towards Austria and the south of Germany and only a few regions in eastern Switzerland received precipitation.