Severe convective storms phenomena occurrence during the warm half of the year in Bulgaria (1961–2006)

Disastrous events originating from severe convective storms (SCS) like heavy rainfall, hailfall and thunderstorms were analyzed for the warm half of the year (April–September) using long-term data records from 67 climatological and 26 precipitation stations of the meteorological network of the National Institute of Meteorology and Hydrology (NIMH) of Bulgaria. Daily rainfall over 30 mm/24 h measured in 4 or more provinces (from total 27 country provinces) is defined as a threshold for heavy rainfall. Similar approach was applied to hail and thunderstorm spreading. The analysis of the variation of those phenomena for the period 1961–2006 shows that the number of the days with heavy precipitation increases while the total rainfall as a whole decreases. During the last 16 years (1991–2006) the SCS phenomena and related thunderstorms and hail occurred more frequently in August and September in contrast to the period 1961–1990. Synoptic analysis shows prevailing cases of SCS associated with cold fronts, cyclonical pressure field and Mediterranean cyclones.