Interesting Bora effect across the Po Valley

On April 14th an interesting weather event occured across the Po Valley. Until this day a strong ridge extending across central Europe was causing warm and sunny conditions over the region. Then, a drastic change in the synoptic situation has seen a cold air mass moving S from Scandinavia and reaching the Alpine region/Balkans on this day.

The contrast between the pre-existent warm and dry air over N Italy and the cold air on the other side of the Alps triggered the development of showers and thunderstorms between Austria, Slovenia and the extreme NE Italy during late morning. Around midday the cold air (as Bora wind) reached Istria/Trieste flowing then across the N Adriatic Sea. The combination of moisture with the warm/cold air contrast enhanced the showers/thunderstorms across NE Italy, which are well visibile from the satellite (Figure1, see line of convection across Veneto).

Figure1. Bora entering NE Italy at 13UTC (source Sat24).

As the cold air flows over the Po Valley, it starts losing its moisture content becoming dry. Thus, by early afternoon most of the showers/thunderstorms ease and don’t move further W (Figure 2). Instead, the cold air still continue flowing at the lower level (below 850hPa, 1500m asl) reaching Lombardy by mid-afternoon whilst carrying all the dust picked up across the valley. Indeed, the previous weeks have not seen rain over N Italy, thus the strong winds have picked up all the dust accumulated previously and caused a sort of dust-advection across the W plans (Figure 3).

Figure2. Convection eases as cold air dries across the Po Valley (15UTC, source Sat24).
Figure3. Cold air flow (between red arrows) is completely dry and pushes all the dust across the dry Po Valley towards Piedmont and Lombardy (16UTC, source Sat24).

It is interesting noticing how the temperature observed in the main cities (Venice, Milan) has changed during the afternoon (see attached plots).

As the cold air reaches Venice during late morning, the wind speed increases significantly up to 30kt (60km/h), before dropping once the cold front passed. Temperature drops from 18/19°C to 11°C in one hour.

Further west, Milan sees very warm conditions until mid-afternoon. As the cold (though dry) air reaches the city, the wind suddenly veers becoming E-ESE with peak of 20-22kt (40/45km/h). The temperature drops by 10-12°C in one hour.

In summary, this has been a very interesting event (as this “cold dust-advection type” is very unusual over N Italy) especially after two weeks of settled weather over the region.

Published by

edof93

I am Edoardo and I am passionate about weather analysis and forecasing. I would like to share my knowledge and to learn more with others (both International and Italian community).

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