Abstract <p>This work is devoted to studying the role of cloud merging in the formation of hazardous weather phenomena. Characteristics of a convective cloud with heavy rainfall over St. Petersburg and Leningrad oblast on July 1, 2023, are considered. The analysis is based on data of the weather radar DMRL-C, weighing precipitation gauges Pluvio² 200, and two lightning detection networks. The occurrence and development of the <i>Cb</i> under study was a result of merging two convective clouds. During their development the clouds slowly converged moving towards St. Petersburg from the southwest. Measurements have shown that the cloud merging led to explosive growth of the cloud top up to an altitude of 13.4 km and significantly increased the maximal reflectivity up to 69 dBZ, the volume of the cloud’s supercooled part with high reflectivity, the volume of the cloud with graupel and hail, precipitation intensity up to 140 mm/h, and the frequency of lightning. The occurrence of “a cloud bridge” was recorded, which consisted of graupel and wet snow particles. Our results indicate the importance of considering the cloud merging in nowcasting the hazardous weather phenomena.</p>