Severe storms and nowcasting in the Carpathian basin

Summer weather can cause severe situations in the Carpathian basin. Convective events such as thunderstorms (sometimes tornado-producing supercells) and squall lines occur frequently during the summer. In the first part of this paper, some typical convective storm events and their atmospheric conditions are presented. Most of the convective storms are associated with cold fronts but the most dangerous phenomena can be connected to waves on frontal systems or cutoff cyclones. To solve the problem of very short range forecasts of these phenomena, the Hungarian Meteorological Service (HMS) runs a project to develop a nowcasting system (the MEANDER system), which is described in the second part of the paper. This system supports a framework for several nowcasting procedures and produces complex objective analyses and forecasts as long as 3 h. The MEANDER system has two main segments: an analysis segment and a forecasting segment. Input and processed parameters are divided into basic parameters (such as pressure, temperature, geopotential, etc.) and derived parameters (such as visibility, present weather) which are calculated from the basic parameters. In the objective analysis, radar and satellite data are also involved. In the forecasting procedure, the objective analysis at the beginning time, and the (practically at 3 h forecast) fields of the background dynamical model at the final time are considered as nowcasted fields. Between these two times, basic parameters follow the tendency of the numerical model, but gradually approach the forecasted values of the background numerical model. To forecast radar parameters (precipitating systems), so-called replacement vectors are applied. These are calculated from the nowcasted basic parameters. The MEANDER system has both a special update segment to catch rapidly growing cloud systems and a warning segment to issue automated weather warnings as well.