A conceptual model for the development of tornadoes in the Po Valley

Why is the Po Valley a hot spot in Europe for tornadoes? In this study the authors address this issue, studying a tornado outbreak that affected the Po Valley on 19 September 2021. During that event seven tornadoes (four of them ranked as F2 according to the Fujita scale) developed in the area in a few hours. The case study was analysed using observations and numerical simulations obtained with the convection permitting MOLOCH model. Observations showed that during the event there were two low-level boundaries in the Po Valley: a cold front coming from the Alps and a dry line generated by the downslope winds from the Apennines. A strong correlation between the area of tornadoes development and the low-level boundaries was observed. Numerical simulations with 500 m grid spacing proved that the cold front was important to the supercell development due to the baroclinic production of streamwise vorticity. Furthermore, the dry line played a key role creating locally large amounts of instability and windshear near the surface: kinematic and windshear parameters were comparable to those observed in US-tornado events only along a narrow path near the dry line. Finally, it was found that a warm and moist air tongue from the Adriatic Sea was fundamental in generating the supercells. In conclusion, comparing these results with previous papers, a conceptual model for the development of tornadoes in the Po Valley is proposed, which explains why tornadoes are relatively common in Northern Italy.