Nowcasting and real-time monitoring of heavy rainfall events inducing flash-floods: an application to Phlegraean area (Central-Southern Italy)

Severe weather events cause considerable disruptions to social and economic activities. Thus, high-precision meteorological products and services should be provided in order to forecast high-impacting weather conditions a few hours in advance such as those causing flash-floods. Today, this goal is mainly fulfilled by nowcasting. However, to fully comprehend alluvial phenomena, it is necessary to analyze not only the ground hydrological aspects but also the atmospheric dynamics leading to heavy precipitating events whose territorial distribution is generally complex. Herein, we analyzed the evolution of two convective storms which occurred on September 7 and 11, 2017, in the Phlegraean area (close to Naples) and explain why the first one, but not the second one, led to a flash-flood in Bacoli and Pozzuoli. We found that the traditional forecasting techniques failed since the severity of the two storms, in particular that of the first one, was not correctly forecast by Limited Area Models (LAMs). Using rainfall data, we demonstrate that LAM failed because it did not take into account the meteorological phenomenon enhancement occurring in the Phlegraean area when storms come from south-west due to the presence of Mt. Epomeo (789 m) on the nearby island of Ischia. In fact, this mountain has a quasi-pyramidal shape and is located at just 15-20 km south-west of the Phlegraean area. Thus, short/very-short time forecasting and nowcasting methodologies are not fully satisfactory. On the contrary, real-time rain-gauge monitoring networks are necessary to achieve high-safety levels in areas subject to flash-flood hazards and to promptly alert the population.