The precipitation event of 19-23 August 2005 consisted of two successive phases of precipitation. During the first phase (19-20 August), the precipitation, caused by the passage of a cold front over Switzerland, was not very intense (maximum of 34.1 mm and 56.3 mm measured respectively on 19 and 20 August). Taken alone, these precipitation amounts would not have had high impacts in Switzerland.
However, this first phase was followed by a second phase of intense precipitation. Because of the transport of warm and moist air from the Mediterranean and Adriatic Seas towards Switzerland from the north-east and the interaction of this air mass with the Alpine range, heavy precipitation was triggered on the northern side of the Alps and large amounts of water fell over Switzerland, especially on 21 and 22 August.
At first, the precipitation affected mostly the Plateau and the Prealps (21 August), where high rainfall amounts were measured. On 22 August, the zone of intense precipitation shifted eastwards, causing intense precipitation to fall on the northern flank of the Alps and the north-east of the Grisons. That day, the highest daily precipitation sum during the whole event was also measured (241.8 mm at station Weesen in the canton of St. Gallen).
On 23 August, rainfall amounts over Switzerland decreased considerably and only some regions in eastern Switzerland recorded precipitation.