Yesterday, a significant temperature gradient was observed along the Po Valley in N Italy. The strong pressure gradient between central Europe and the Mediterranean has caused a significant foehn effect across the Alpine region, leading to a sharp increase in temperatures in the afternoon over SE France, NW Italy and S Switzerland.
Looking at the temperatures (Figure 1) we can see that most of the areas downwind the NW’ly flow observed values above 20°C.
Some temperatures have broken the all time records, like in Turin (27°C, which is 20°C above the climatological average).
These extremely high temperatures were caused by unseasonably mild air coming from the mid-Atlantic/N Africa combined with the catabatic wind (i.e. foehn) flowing down from the Alps towards W Po Valley, Ticino and SE France. Thus, being the temperature at 850hPa (1500m a.s.l.) very mild (around 12/15°C), the warming effect of the air flowing down the mountains slope (1°C every 100m) has caused temperatures to reach 20/25°C in these areas, as well as clear conditions.
On the central/E part of the Po Valley the NW’ly wind wasn’t powerful enough to ‘clear’ the layer of fog/low clouds lying on the area for several days (Figure 2). Thus, in cities like Venice and Trieste, maximum temperatures barely reached 10°C (though being still 2/3°C above the climatological average).