The present work is meant to investigate a new instability in moist air in the framework of the thermodynamic approach with the latent heat release of phase transformations in moist atmosphere. Severe weather events are well known to be strongly associated with heavy cloud systems. Hydrodynamic description of the instability responsible for these phenomena is obvious to take explicitly into account thermodynamics of moist air. The discovered instability can be responsible for the severe weather events, such as tornadoes and tropical cyclones.