Mechanisms for convection development in a long-lasting heavy precipitation event over southeastern Italy

Observational data and outputs from the Weather Research and Forecasting (WRF) model are used to investigate a heavy precipitation event that occurred on 12 and 13 November 2004 in Apulia and Basilicata, southeastern Italy. The event lasted for more than 24 h and featured two large rainfall peaks, with values up to 250 mm in one day, recorded in two different phases over two distinct areas.The analysis indicates that a low-level jet stream (LLJS) induced by the large-scale pattern maintained a convectively unstable environment and advected moist air masses at very low levels throughout the event. During the first phase, the orography provided the lifting mechanism to develop convection. During the second phase, the convergence developed in the low levels was sufficient to maintain a quasi-stationary linear mesoscale convective system over the nearly flat terrain of Salento peninsula.High-resolution numerical outputs highlight that the shift of precipitation from the first to the second maximum was caused by the passage of a weak mesoscale cyclone and of an upper level short-wave trough. The delay in the modeled evolution of these subsynoptic features is considered responsible of the incorrect timing of the simulated precipitation.The study provides further evidence of the close relationship between the occurrence of a moist and unstable southerly LLJS and heavy precipitation over the Italian Ionian regions.

Research highlights
► A persistent moist LLJS leads to a long-lasting heavy precipitation event.
► Orography and convergence provide lifting to the potentially unstable LLJS.
► The wind profile contributes to the formation of a linear MCS.
► The passage of two subsynoptic features controls timing of precipitation.
► Southerly LLJS is confirmed to be a meteorological hazard for Italian Ionian areas.