Bulgaria is a country with a high frequency of hail and thunderstorms from May to September. For the May–September 2010–2015 period, statistical regression analysis was applied to identify predictors/classification functions that contribute skills to thunderstorm forecasting in the Sofia plain. The functions are based on (1) instability indices computed from radiosonde data from Sofia station F1, and (2) combination of instability indices and Integrated Water Vapor (IWV), derived from the Global Navigation Satellite System (GNSS) station Sofia-Plana, F2. Analysis of the probability of detection and the false alarm ratio scores showed the superiority of the F2 classification function, with the best performance in May, followed by June and September. F1 and F2 scores were computed for independent data samples in the period 2017–2018 and confirmed the findings for the 2010–2015 period. Analysis of IWV and lightning flash rates for a multicell and supercell thunderstorm in June and July 2014 showed that the monthly IWV thresholds are reached 14.5 and 3.5 hours before the thunderstorm, respectively. The supercell IWV peak registered 40 min before the thunderstorm, followed by a local IWV minimum corresponding to a peak in the flash rate. In both cases, an increase of IWV during severe hail was registered, which is likely related to the hydrometeor contribution to GNSS path delay. The results of this study will be integrated into the Bulgarian Integrated NowCAsting tool for thunderstorm forecasting in the warm/convective season.

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Abstract⎯ Bulgaria is situated in a geographical area with high frequency and intensity of thunderstorms and hail events. Like in many other countries, an upward trend of weatherinduced damage has been observed during the last 15 years in Bulgaria. Much of it was caused by extreme weather events such as torrential precipitation associated with severe thunderstorms or/and wind storms. The series of flash floods and extreme hail storms, which hit Bulgaria in the warm half of 2014, were in line with that trend.On July 8, 2014, three supercell storms developed over West Bulgaria and heavily impacted urban areas in the afternoon. The extremely strong hail storms over Sofia and Montana were accompanied by strong wind gusts (about 85 km h -1 ) and torrential rain. The giant hail stones in Sofia had diameter of up to 10 cm and irregular shape. The severe hail and rain, and very strong wind caused substantial damage to infrastructure, buildings, vehicles, and agriculture. More than 40 people were injured by hail stones or collaterally by broken windows. One man was killed by a fallen tree. In Sofia alone, the reported damage was worth more than 100 million euro according to data from insurance companies.The paper presents the analysis of the evolution of the convective clouds based on radar data. The Doppler radar data revealed the existence of a mesocyclone, mesoanticyclone, microburst, and three-body scatter signature. The cloud top reached up to 17 km, and the maximum radar reflectivity factor was 69-71 dBZ. Analysis of the available radiosounding data, simulation with a numerical model, and the synoptic patterns are also presented.

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