Some have improperly called it a flash flood. Justifiable considering the wounds of the disaster of May 2023 that are far from being marginalized. An intense downpour sent a good part of the city under water, hitting, as already happened on May 25th, also the historic center of Mercurial. This time, however, Jupiter Pluvius spared the gray clouds from pouring hail, which fell in the stormy downpour on Tuesday. The flooding was caused by a storm system of the Qlcs type, Quasi Linear Convective System, or a linear multicellular system, which poured 68.6 millimeters of rain into the heart of Forlì, a few steps away from Aurelio Saffi (Arpae data), 59 of which in the time slot between 3 and 4 pm.
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Extreme rainfall values
The storm system, details Pierluigi Randicertified meteorological technician and Ampro meteorologist (Professional Meteorological Association), “triggered on the Forlì-Cesena Apennine belt, and a rather intense cell belonging to the system very slowly extended to the urban area of Forlì, discharging a noticeable quantity of rain Despite accumulations varying according to the various areas of the city district, amounts of between 60 and 70 millimeters of rain were detected in just over an hour, which is equivalent to the rain that normally falls in the entire month of June and in the Furthermore, an amateur station in the Ospedaletto area recorded 81 millimeters of rain between 3.10 pm and 4.50 pm (1 hour and 40 minutes), a truly sensational value contributions correspond to 600-800 tons of water per hectare (the surface of approximately one and a half football pitches) spilled in less than two hours, and can be considered extreme values for our precipitation climatology”.
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Extreme phenomena are increasingly frequent
Just 24 hours ago, a violent storm had hit the city, while on Monday it was the hinterland that had to deal with an intense phase of bad weather. “These extreme precipitations in a very short time (which have already occurred several times this year in May between Ravenna and Forlì) are becoming more and more frequent due to the acceleration of the water cycle determined by temperatures in the lower layers and on the surface of the sea (from where the flows that feed the storm systems often arrive) constantly higher than normal with high quantities of water vapor that can be converted into precipitation – Randi details -. Beyond the intensity of the storm cell, the biggest problem arose from the fact that it appeared almost stationary, moving extremely slowly and even regenerating for some time on site. This was due to the fact that the currents at high altitude are very weak, and that the so-called “windshear”, i.e. the sudden variation of the wind with the altitude in direction and speed, is very low. When the currents at high altitude are weak and the shear in the wind field is low, the biggest danger brought by the storms is represented by the huge accumulations of rain, even if localized, due to their low mobility”.
“In general, the propagation of these systems from the reliefs to the neighboring plain occurs through the “lakes” of cold air that precipitation spills towards the ground and which then spread, opening up like a fan, towards the plain – continues Randi -. Once they have arrived these mini fronts of cold air (outflow boundaries) raise other warm air that may be present, generating new cells in the immediate vicinity, and this happens precisely when the currents at altitude are mild. At a more extensive level, thunderstorm activity is widespread and frequent throughout the area Apennine belt, was caused by the presence of a depression vortex at high altitude, full of cold air, which is slowly moving away but which is still active, triggering predominantly orographic storm activity, i.e. forced by the presence of the reliefs in the atmosphere which remains very unstable given the high temperature difference between the lower layers and the higher altitudes (strong vertical thermal gradient with cold air at altitude and warm air in the lower layers)”.
What will happen in the next few hours
But what will happen in the next few hours? Randi explains: “The cold vortex at high altitude, albeit slowly, is moving away towards the East and the weather is destined to improve from tomorrow (although there may still be some local thunderstorms in the afternoon on the mountains) and above all from Friday, when a sub-tropical anticyclonic promontory rising from North Africa associated with stable weather and rapidly increasing temperatures. However, the stable phase may not be long-lasting, as there are new unstable signals in the first half of next week, which will obviously need to be confirmed in the coming weeks. days”.
