Frequency shifts in thunderstorm patterns as key precursors to flash flood events

Abstract Abstract: This study presents the research of thunderstorms in Slovakia using records of thunderstorm days from weather observation stations from 1965 to 2023, as well as radar data, and data from lightning detection systems from 2010 to 2023. An algorithm was developed to identify thunderstorms using radar and lightning data. The temporal distribution of thunderstorms is examined across annual, seasonal, daily, and hourly timescales. Additionally, we focus on the spatial distribution of thunderstorms, where lightning data according to a proposed algorithm were used. The occurrence of thunderstorms is also analyzed in the light of macrosynoptic types, according to a 28-class classification of the Slovak Hydrometeorological Institute. The results show that close thunderstorms (within 3 km of a given location) in Slovakia occur on average on 15 days a year, most frequently in the summer months of June and July, and primarily in the late afternoon between 3 and 5 p.m. Central European Summer Time (CEST). The spatial distribution of thunderstorms in Slovakia is non-homogeneous, with an increased occurrence in Central Slovakia, particularly in the area of Muránska Planina, Stolické, and Volovské Vrchy Mts. Conversely, the fewest thunderstorms occur in the westernmost part of the territory. Severe thunderstorms are most numerous in the Gemer region. The occurrence of thunderstorms is significantly influenced by synoptic types, with the most favorable conditions associated with types B (trough over Central Europe), Bp (traveling trough), C (cyclone over Central Europe), and Cv (upper cyclone). In recent decades, significant decrease in the number of thunderstorm days per year has been observed. This may be due to the observed decreasing trend of cyclonic synoptic situations in Europe, where a substantial number of thunderstorms occur.