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.

Global Navigation Satellite System (GNSS) is an established atmospheric monitoring technique delivering water vapour data in near-real time with a latency of 90 min for operational Numerical Weather Prediction in Europe within the GNSS water vapour service (E-GVAP). The advancement of GNSS processing made the quality of real-time GNSS tropospheric products comparable to near-real-time solutions. In addition, they can be provided with a temporal resolution of 5 min and latency of 10 min, suitable for severe weather nowcasting. This paper exploits the added value of sub-hourly real-time GNSS tropospheric products for the nowcasting of convective storms in Bulgaria. A convective Storm Demonstrator (Storm Demo) is build using real-time GNSS tropospheric products and Instability Indices to derive site-specific threshold values in support of public weather and hail suppression services. The Storm Demo targets the development of service featuring GNSS products for two regions with hail suppression operations in Bulgaria, where thunderstorms and hail events occur between May and September, with a peak in July. The Storm Demo real-time Precise Point Positioning processing is conducted with the G-Nut software with a temporal resolution of 15 min for 12 ground-based GNSS stations in Bulgaria. Real-time data evaluation is done using reprocessed products and the achieved precision is below 9 mm, which is within the nowcasting requirements of the World Meteorologic Organisation. For the period May–September 2021, the seasonal classification function for thunderstorm nowcasting is computed and evaluated. The probability of thunderstorm detection is 83%, with a false alarm ration of 38%. The added value of the high temporal resolution of the GNSS tropospheric gradients is investigated for a storm case on 24–30 August 2021. Real-time tropospheric products and classification functions are integrated and updated in real-time on a publicly accessible geoportal.

A long-lived supercell developed in Northwest Bulgaria on 15 May 2018 and inflicted widespread damage along its track. The first part of this article presents a detailed overview of the observed storm evolution. Doppler radar observations reveal that the storm acquired typical supercellular signatures and maintained reflectivity values in excess of 63 dBZ for more than 4 h. The thunderstorm was also analyzed through lightning observations that highlighted important characteristics of the overall supercell dynamics. In its second part, the study investigates the predictability of the severe weather outbreak. In the medium forecast ranges, the global European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble indicated the presence of favorable conditions for the development of deep moist convection 4 days prior to the event. A set of three convection-allowing ensemble simulations also demonstrated that the practical predictability of the supercell was approximately 12 h, which is considerably higher than some previously reported estimates. Nevertheless, the skill of the convective forecasts appears to be limited by the presence of typical model errors, such as the timing of convection initiation and the development of spurious convective activity. The relevance of these errors to the optimal ensemble size and to the design of future convection-allowing numerical weather prediction (NWP) systems is further discussed.

In 2024, several supercell thunderstorms were registered in Bulgaria between April and June. A long-lived isolated supercell developed on 13 June 2024. This supercell persisted for over 7 hours, traveling more than 350 km and producing hail larger than 2 cm in diameter, which caused significant damage.A thermodynamic analysis was conducted for estimation of the severity of the thunderstorm development. The obtained high values of CAPE, strong 0–6 km wind shear, and a typical curved hodograph shape indicated the potential of development of supercells. To investigate the supercell’s evolution, radar data (updated every 4 minutes) and high-resolution visible (HRV) satellite imagery (at 5-minute intervals) were analyzed. Key radar parameters included maximum reflectivity (Zmax), the Vertically Integrated Liquid above the freezing level (dVIL), and VIL Density (VILD), defined as the ratio of VIL to the height of 15 dBZ contour. Four cycles of intensification were distinguished during the mature severe stage of the supercell development. Most of the typical radar signatures of powerful convective cells, such as V-shaped radio echo, comma shape, Bounded Weak Echo Regions (BWER), Three-Body Scatter Spikes (TBSS), and Side lobe, were registered.During the evolution of the supercell:- V-shaped radar echo was registered several times, followed by a significant deviation in the storm motion of up to 60–65 degrees from the mean wind flow.- There is a pronounced relationship between the storm's cyclic behavior and variations in dVIL and VILD.- The intensification of the cycles was associated with the new formation of feeder cells and their subsequent merging with the supercell.- Maximum radar reflectivity exceeded 65 dBZ for more than 2 hours, with peaks reaching above 70 dBZ during the severe stage of the supercell.

Severe weather events, such as intense precipitation, hail and thunderstorms, are common summer phenomena in Bulgaria and are associated with large economic losses. An active hail suppression is taking place in North West and South Central Bulgaria over the dense agricultural regions from May to September. A joint venture between Sofia University "St. Kliment Ohridski'' and the Bulgarian Hail Suppression Agency contributed to the project “Balkan-Mediterranean real-time severe weather service” (BeRTISS, 2017-2020). As part of the BeRTISS project, a pilot operational service was established by exploiting GNSS tropospheric products in support of safety, quality of life and environmental protection in the region. To facilitate the service, in 2018 a GNSS network with 12 reference stations was installed and since February 2020, the Sofia University GNSS Analysis Center provides operational near-real time products. First results of combining GNSS derived Integrated Water Vapour (IWV) and Instability Indices (InI) are reported for Sofia plain for the period May-September (2010 - 2015). Based on statistical regression analysis, classification functions are obtained that contribute to the thunderstorm forecasting skill. The majority of the classification functions combining IWV and InI are reported with the best performance in May, followed by June and September. In this work, IWV and InI classification functions are presented for South Central Bulgaria. The first results for the period May-September 2020-2021 indicate the probability of detection 0.71 for IWV, 0.85 for InI, and 0.89 for IWV and InI combined. While false alarms decreased from 0.42 for IWV to 0.39 for InI and 0.28 for IWV and InI combined. These results will be implemented in the GNSS Strom Demonstrator web portal.  

The impact of climate change suggests that the environments favorable for severe thunderstorms development, in particular supercells will become more frequent across Europe.Bulgaria is a country with a high frequency of thunderstorms during the warm season. Every year, the development of supercells is observed over its territory, and storm splitting process is not rare. This phenomenon can pose difficulties for the forecasters. For this reason, correct radar analysis and estimation of wind shear are of great importance to the forecasting and the nowcasting.A case of a splitting supercell that developed on the 29-th of May 2022 is analysed. Radar data from S-band Doppler radars were used. All radar products shown throughout the study were generated by the Interactive Radar Information System (IRIS) Analysis of Vaisala.On the 29-th of May 2022 over the territory of Bulgaria there was a 500-hpa trough with the center of the upper level low over the Scandinavian Peninsula. On the surface pressure chart there was a sequence of cold fronts. The flow on the upper and near-ground level converged (from SW, following the circulation of the trough and from SE, following the movement of the Mediterranean cyclone respectively). This leads to strengthening of the vertical wind shear. This synoptic setup was favorable for severe convection development. Data for instability indices and wind hodograph were additionally used.In the afternoon an isolated convective cell first appeared at 15:45 EET in the layer between 5th (-9.4°C) and 10th km (-42.4°C) with maximum radar reflectivity Zmax = 29 dBZ. The cell grew rapidly and after 20 minutes Zmax was 60 dBZ and the storm had characteristics of a supercell. A mesocyclonic vortex was visible on the Doppler radar. The cell acquired a V-shaped structure, and later on it split into two cells, both of which retained high radar reflectivity. Based on the radar data the presence of the typical supercellular radar signatures, such as weak echo region (WER), bounded weak echo region (BWER), hook echo, and TBSS are presented. After splitting, the left-moving (LM) and the right-moving (RM) cells kept high values of radar characteristics, but RM had shorter life time and its intensity was lower. Both cells produced multiple convective hazards along their paths. The hazards associated with the RM were severe hail (hailstone sizes greater than 2 cm in diameter), strong wind gusts, heavy rain and intense lighting activity. The storm severity and the presence of large hail stones were confirmed by maximum radar reflectivity, VIL (Vertical integrated liquid) and VILD (VIL density).