Case study of a severe windstorm over Slovakia and Hungary on 25 June 2008

A system of thunderstorms approached the Slovakia and Hungary in the late evening hours of 25 June 2008, causing extensive damage and peak wind gusts up to 40 m/s. This study examines the macro- and mesosynoptic conditions for the windstorm using soundings, analyses, and forecasts of numerical models (ALADIN, ECMWF). A derecho-like character of the event is discussed.Meteosat Second Generation imagery and convective indices inferred from satellite and model data are used to assess the humidity distribution and the conditional instability of the thunderstorm environment. An intrusion of the environmental dry air into the convective system and intensification of downdrafts is considered to be one of the reasons for the damaging winds observed at some areas. This is supported by the radar imagery showing a sudden drop of radar reflectivity and creation of line echo wave patterns and bow echoes.A numerical simulation provided by the non-hydrostatic MM5 model indicated the development of meso-γ scale vortices embedded in the convective system. The genesis and a possible role of such vortices in creating rear-inflow jets and intensifying the low level winds are investigated with the help of the vorticity equation and several other diagnostic parameters. In addition, the effect of various physical parameterisations on the forecast of the windstorm is evaluated.

Research Highlights
► The evaluated windstorm showed several characteristics of a derecho.
► Intrusion of the environmental dry air was one of the reasons for the damaging wind.
► Numerical simulations indicated the development of mesovortices.