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Weather situation
The general weather situation of mid-September to mid-October was characterized by a mainly south-westerly to southerly flow and the frequent passage of extratropical cyclones over Switzerland.
The general weather situation of mid-September to mid-October was characterized by a mainly south-westerly to southerly flow and the frequent passage of extratropical cyclones over Switzerland.
Southerly flows transport warm and moist air from the Mediterranean towards the Alps. As the air is forced to rise over the Alpine barrier, orographic precipitation is triggered on the windward side of the Alps. Cyclones, on the other hand, are accompanied by fronts, which produce precipitation due to the confrontation of different air masses.
The precipitation event of 22-25 September 1993 was characterized both by a southerly flow and by the passage of a quasi-stationary frontal zone over the Alpine region, which marked two phases in the precipitation event. First, on 22-23 September, the dominant south-south-westerly flow transported warm and moist air from the Mediterranean towards the Alps, triggering orographic precipitation over the southern side of the Alps. Then, on 24 September, a cold front passed over northern Italy, producing frontal precipitation over the Alpine crest and the southern side of the Alps.