Synoptic conditions of the derecho storm. Case study of the derecho event over Poland on August 11, 2017

This study presents a comprehensive synoptic analysis of one of the most violent storms recorded in recent years in the north-western part of Poland, which occurred on August 11, 2017. Its development took place ahead of a waving cold front in the tropical air mass, downstream of the upper-level trough. The thunderstorms formed over Lower Silesia in the afternoon and moved towards Gdańsk Pomerania to occur over the B altic Sea after midnight on August 12, 2017, where they gradually disappeared. As the thunderstorms moved through this area, they ranged from single convective cells and unorganized multicell storms through supercell thunderstorms to mesoscale convective systems in the form of bow echo squall lines and the mesoscale convective vortex (MCV). The convective system, evolving over time, fulfilled the derecho criteria. Its development was related to the presence of both the upper and mid-level jet stream, which supported the formation of a strong rear inflow jet (RIJ) in the rear part of the convective system, being one of the main factors generating the formation of a bow echo squall line with strong wind gusts. The maximum wind gusts recorded on August 11, 2017, are among the highest in the history of Polish measurements and amounted to 42 m/s in Elbląg, 36 m/s in Chrząstów, 35 m/s in Gniezno, and around 30 m/s at several other stations.