Daily precipitation and daily mean temperature
Between 30 April and 4 May 2015, temperatures on the northern side of the Alps gradually increased with the inflow of warm air from the south-west. Most of the precipitation was spread over three to four days depending on the region, and the largest daily sums were measured in most regions on 1 May.
During the afternoon and night of 30 April, the 0°C isotherm was located around 1800-2200m and the precipitation that started in the evening of 30 April mostly fell as rain in the affected area. The largest rainfall amounts were measured in the Jura mountains (e.g. La Dôle VD, 32.5 mm; Eptingen BL, 31.6 mm), at the foot of the Jura and on the Plateau (e.g. Riedholz-Wallierhof SO, 29.5 mm; St. Urban LU, 29.2 mm; Grenchen SO, 28 mm), as well as in the Prealps of western Switzerland (e.g. Col des Mosses VD, 29.3 mm; Fruence FR, 26.1 mm).
On 1 May, the temperatures increased only very slightly and the 0°C isotherm remained around 1900-2200m altitude. That day, the intensity of the precipitation also increased and very large rainfall amounts were measured in the regions just mentioned (e.g. La Dôle, 99.7 mm; Riedholz-Wallierhof, 109.8 mm; Fruence, 95.9 mm). Even larger daily precipitation sums were measured in the Valais (e.g. Morgins, 114.6 mm; Derborence, 114.4 mm; Emosson, 111.1 mm), a large part of which fell as rain. For all these stations, the precipitation recorded on 1 May already represents more than half of what is usually measured during a whole month of May at those stations (norm 1981-2010).
On 2 and 3 May, the precipitation continued but with lower intensity than on 1 May. Those days, and especially on 3 May, temperatures considerably increased with the passage of a warm front over Switzerland. The 0°C isotherm was shifted by almost 1000m in two days and was located above 2800-3000m on 3 May, causing the precipitation to fall as rain over most of the Alps. This region received the highest precipitation on 2 and 3 May, while slightly lower rainfall was measured in the Jura and on the Plateau. The table below lists the precipitation measured on 2 and 3 May at a few stations located on the northern side of the Alps.