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 areused to investigate a heavy precipitation event that occurred on 12 and 13 November 2004 inApulia and Basilicata, southeastern Italy. The event lasted for more than 24 h and featured twolarge rainfall peaks, with values up to 250 mmin one day, recorded in two different phases overtwo distinct areas.The analysis indicates that a low-level jet stream (LLJS) induced by the large-scale patternmaintained a convectively unstable environment and advected moist air masses at very lowlevels throughout the event. During the first phase, the orography provided the liftingmechanism to develop convection. During the second phase, the convergence developed in thelow levels was sufficient to maintain a quasi-stationary linear mesoscale convective systemover the nearly flat terrain of Salento peninsula.High-resolution numerical outputs highlight that the shift of precipitation from the first to thesecond maximum was caused by the passage of a weak mesoscale cyclone and of an upper levelshort-wave trough. The delay in the modeled evolution of these subsynoptic features isconsidered responsible of the incorrect timing of the simulated precipitation.The study provides further evidence of the close relationship between the occurrence of a moistand unstable southerly LLJS and heavy precipitation over the Italian Ionian regions.