From the middle of the month cold air flowed into Switzerland and the snow line fell to 1,400 m, reported Meteo Swiss. The sudden drop resulted in an unusually cool average September temperature, it said.
At 10 degrees, the average monthly temperature was 0.4 degrees below the average between 1991 to 2020 norm. However, the difference was greater in some places. Higher altitude temperatures were significantly lower than normal. Monthly average temperatures at Jungfraujoch (3,463m) and Säntis (2,502m) were 1.3 and 1.5 degrees below the norm for September..
Rainfall was also heavier than normal. Towards the end of the month heavy rain fell in Western Switzerland. In some areas rainfall was 180% to over 260% of the norm. In the Chaumont region in the canton of Neuchâtel rainfall was 260% of the norm, making it the wettest September there since records began in 1864. On the southern side of the Alps some regions received rainfall 130% to 140% of the norm.