Daily precipitation and daily mean temperature
During this event, most of the precipitation was caused by the development of a low pressure area over northern Italy, which then moved in direction of Eastern Europe (see Weather situation). While at first the precipitation was spread over a large portion of the country, the area of heaviest precipitation then shifted towards the eastern part of Switzerland, as the low pressure system progressed north-eastwards.
On 9 August, before the development of the low pressure area over northern Italy, Switzerland already received precipitation due to the inflow of moist air from the west. On the Alpine north slope, amounts between 5 and 39 mm were recorded, mostly in liquid form, as the snowline was located at an altitude of 2700-2900m on that day.
On 10 August, the precipitation in Switzerland intensified due to the confrontation of different air masses above the country. On the northern side of the Alps, up to 68 mm of precipitation was measured (maximum at station Schwägalp on the eastern Alpine north slope), while up to 112 mm was measured on the southern side of the Alps (maximum at station Ponte Tresa in Ticino). That day, there was also a slight west-east temperature gradient; with the inflow of colder maritime air from the north over the western part of Switzerland, the snowline descended to an altitude of 2200-2500m, while it was slightly higher in the central and eastern parts of Switzerland (2500-2700m) more under the influence of Mediterranean air coming from the south.
With the strengthening of the low pressure area over northern Italy the following days, warm and moist air from the Mediterranean was transported over Eastern Europe, flowing towards Switzerland from the north-east. This warm air not only caused temperatures to increase again (snowline at 2500-2900m on 12 and 13 August), but, as the moist air was lifted above the topography, intense precipitation also fell over the central and eastern parts of Switzerland, especially on 11 August. While 12-82 mm of precipitation was measured on the western Alpine north slope that day, 17-94 mm and 40-140 mm were recorded respectively on the central and eastern Alpine north slope.