It's never been so warm on the Jungfraujoch in September in the measurement history of Meteo Schweiz as on Tuesday, September 11th, 2018. The temperature was a whopping 10.8° celcius at 3580 meters altitude. That's above zero. The last record dates from September 9th, 2000. It then was 9.7° above zero. Anyway, Tuesday was another warm day in Switzerland with tropical temperatures with a maximum of 30.4° degrees in Sion, 28.9° degrees in Geneva and 27.7° degrees in Neuchâtel.
The temperature on the Jungraujoch has been extremely high this summer. Normally the lowest temperatures in July and August on the Jungfraujoch are average about 45 times below zero degrees celsius and remain on at least 12 days below zero the entire day. The lowest temperatures were only 33 times below zero this summer and it has been below zero the whole day only 2 times this summer.
The meteorological autumn started on the 1st of September and the image we see now in the higher parts of the Alps fits the trend of the last decades. Meteo Schweiz research shows that the meteorological autumn is getting warmer and especially in the higher parts there is a noticeable warming. Below you can see the change in the annual average temperature since the start of the measurements in 1864 (unit: ° C per decade, source Meteo Schweiz).
An analysis of the predicted temperature for the coming days at 850 HPa shows that the high temperatures aren't over yet. The graph below (source: meteoguru) shows the evolution of the temperature at 850 HPa (about 1500 meters altitude) as predicted by the European model ECMWF.
The American weather model GFS confirms these calculations. The graph below (source: wetterzentrale) shows the development of the temperature at 850 HPa (about 1500 meters altitude) as predicted by the weather model GFS. The red line is the average temperature at 850 HPa of the last three years. The white line is the expectation of the GFS for the coming period. If GFS will be right then it stays about 2 to 5 degrees on average warmer than normal in September until the end of the month.
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