New methods for severe precipitation warning for Hungary

Under a project funded by the United States-Hungarian Science and Technology Joint Fund, research has been conducted to improve the forecasting of extreme rainfall events (often accompanied by flash flooding) in Hungary. Many of these events appear to result from mesoscale circulations, possibly small mesoscale convective systems (MCSs), thus presenting a very challenging forecast problem. Two keys to improving these forecasts are (1) a better physical understanding of the mesoscale circulations that produce severe rainfall, and (2) imptoved methods to identify and monitor the evolution of potentially dangerous systems as early in their development as possible. Three areas of research have been pursued. First, a variant of the Probable Maximum Precipitation (PMP), the Possible Maximum Precipitation (PoMP), has been derived and then employed retrospectively in the diagnosis of several incidents of extreme precipitation that occurred in Hungary during the summer of 1998. One of the events studied, a case of very heavy rainfall in Moson-magyarovar on 27 July, is described in detail. In this event as well as the others. PoMP results showed promise as a way to estimate potential severe rainfall. A second area of research involved the implementation and testing of an automated satellite rain estimation technique. Observations from the European Meteorological Satellite (METEOSAT) were used daily to provide precipitation fields over Hungary and neighboring countries. Detailed analyses were made of the extreme precipitation events. Precipitation patterns were generally very widespread and satellite estimates compared poorly with gauge data. Possible reasons for these discrepancies included errors arising from the quality and frequency of the satellite data, questionable applicability of rain-estimation parameters in the mountainous regions surrounding the Carpathian Basin, and the frequent occurrence of embedded convection in these cases. A third area of research involved attempts to qualitatively assess the frequency of occurrence of large MCSs. Although some statistical evidence (primarily the existence of nocturnal maxima of heavy precipitation rates in some regions of Hungary) suggested the existence of MCSs, a qualitative examination of satellite and radar observations during two summers (1997 and 1998) did not uncover the characteristic signatures of Mesoscale Convective Complexes (MCCs), the largest MCSs.