In 2017 the HEC-HMS model for the Sava River Basin was embedded under the Flood Forecasting and Warning System in the Sava River Basin (Sava FFWS) and coupled with many hydraulic models. Since the model was initially calibrated as the event-based model, a lack of accuracy has been recognized during the continuous simulations within the Sava FFWS operational use. Therefore, the Sava FFWS users organizations: ten forecasting organizations from five Sava countries, agreed to upgrade and improve this hydrological model. The activities of the model improvement were performed in period January 2019 till June 2020. It was implemented by the national experts from the Sava FFWS users' organizations as a true joint action and coordinated by the Secretariat of the International Sava River Basin Commission. This paper presents the results of the Sava HEC-HMS model improvements and updated parameters, including a comparison of results of initial and improved models within the operational forecasting system. The paper also discusses the potentials of the remote sensing and radar-and satellite-based data that will be used for the future model improvements.