*5.1. Synoptic Analysis*

On 11 May 2019, the Po Valley was affected by strong convective activity that caused economic damage and seriously injured a person in the Lombardy region. The synoptic scenario at 500 hPa was characterized by a stretched trough extending from northern Europe to the Mediterranean basin. In the afternoon hours, the cold air mass at 500 hPa reached the Po Valley with values around −26 ◦C and the winds consequently shifted to the northwest [46]. On the other hand, the strong south-westerly flow at low levels moved a large amount of water vapor from the Ligurian Sea to the inland, thus increasing convective instability. These meteorological factors produced favorable conditions for the triggering of convective cells over northern Italy. In this context, a squall line hit MXP, producing intense precipitation and a large amount of hail on aprons and runways. The intense thunderstorm also caused several floods in the city of Milan, where some underpasses and metro stations were closed. Instead, the strong downburst winds caused the felling of trees and billboards that required the intervention of firefighters.

On 7 July 2019, the Po Valley was affected by precipitation with a prevalent character of thunderstorms, which was locally very intense and associated in various cases with hail and strong gusts of wind. From a synoptic point of view, in the north of Italy, an anticyclonic field of subtropical nature was retreating towards central-southern Italy under the pressure of more unstable North Atlantic currents at high altitude (Figure 8, left panel). The huge energy reservoir in the lower layers of the atmosphere (humid heat conditions) that had accumulated in the previous days favored the triggering of the first thunderstorms over the Alps as early as midday of 7 July 2019. The intense westerlies moved the first storms towards the plain around 13:00 UTC, but a second group of organized cells hit the central and eastern Po Valley between the late afternoon and evening, producing various forms of damage: numerous garages, basements, roads, and underpasses were flooded, and some buildings were evacuated.

**Figure 8.** European Centre for Medium-Range Weather Forecasts—High Resolution Forecast (ECMWF-HRES) analysis: 500 hPa temperature (◦C), wind (barbs), and geopotential height (dm, contours) on 7 July 2019 (**left panel**) and 6 August 2019 (**right panel**) at 12:00 UTC.

On 6 August 2019, an anticyclone circulation characterized the synoptic scenario over the Mediterranean basin and ensured atmospherically stable conditions and sunny weather over the southern regions of the Italian peninsula (Figure 8, right panel). On the other hand, a deep trough located over the British Isles affected central Europe by touching the Alpine chain during the evening and causing a worsening of the weather conditions over the pre-Alps. In this area, the presence of cold air at 500 hPa, with values around −13 ◦C, increased the instability of the air column, thus promoting the development of convective cells. In addition, the high values of specific humidity, namely the content of water vapor, at 850 and 950 hPa, in combination with wind shear conditions, easterly wind at 950 hPa, and southerly wind at 850 hPa, produced a very unstable environment. From the early evening, some convective cells affected the Bergamo and Milan provinces with moderate rainfall. Later, the passage of moisture advection over the Alps between 18:00 UTC and 00:00 UTC provided further energy for the convection. Consequently, a more intense convective storm hit the central sector of the Po Valley and produced heavy precipitation, strong wind gusts, and small hail.
