287 Mytilene, Lesvos, Greece HydroMediT 2024 | Book of Proceedings 1 Division of Hydraulics and Environmental Engineering, School of Civil Engineering, Aristotle University of Thessaloniki, Greece 2 Laboratory of Physical and Chemical Oceanography, Department of Marine Sciences, University of the Aegean, Greece 3 Department of Meteorology and Climatology, School of Geology, Aristotle University of Thessaloniki, Greece Abstract Thermaikos Gulf is an environmentally fragile coastal environment characterized by numerous anthropogenic and natural pressures. The environmentally protected areas along its western coasts and the large number of inhabitants that live over its northern and eastern coasts raise the need for systematic monitoring and prediction of the marine environment concerning the ocean circulation, weather conditions, sea level and wave characteristics, pollution events, coastal flooding, and freshwater input. Herein, we present recent advances of the Wave4Us operational platform that provides short-term forecasts of all the above conditions, freely accessed by local authorities, the research community, and the broader audience on a daily basis. Specified marine predictions (e.g., about pollution, flooding, and heatwaves) provide useful real-time information to first-level responders and managerial stakeholders during hazardous events that may threaten the quality of the coastal environment and the safety of the population residing on Thermaikos Gulf’s littorals. Keywords: Early warning system, numerical simulations, Aegean Sea, ocean circulation, Lagrangian *Corresponding author: Androulidakis Yannis (iandroul@civil.auth.gr) 1 Introduction Thermaikos Gulf (TG) is a semi-enclosed coastal area in the NW Aegean Sea frequently facing adverse conditions of seawater pollution, induced by numerous anthropogenic and natural factors. The high social and environmental impact of TG is mainly related to: a) the dense population together with the accompanying infrastructure that are located along the coastal zone, and b) the environmentally protected areas, mainly along its western coasts, where large river deltas and NATURA 2000 areas are located (Kaberi et al. 2023). Thus, the knowledge of the prevailing environmental conditions such as weather and marine characteristics on a daily basis, and moreover their short-term prediction, are of very high importance and demand. Such estimations and forecasts may provide useful information to all stakeholders involved with the marine environment (e.g., authorities and professionals), the scientific community, and of course to the citizens of the coastal zone. Their quality of life and safety level are both strongly determined by the state and quality of the sea, especially under the context of climate change conditions. Herein, we present the most recent advances of an integrated numerical forecasting platform that operates over the marine environment of the TG and offers daily forecasts of several met-ocean parameters to support the prevention and mitigation of different natural hazards (e.g., coastal flooding, marine heat waves) and anthropogenic stresses or unfortunate events (e.g., pollution, marine accidents, loss at sea). Field and satellite observations were used to evaluate the performance of the different components of the platform’s integrated modeling suite (Krestenitis et al. 2014; Pytharoulis et al. 2016; Androulidakis et al. 2021; 2023a; 2023b; 2023c; Makris et al. 2024). 2 Description of the operational system The main modules of the Wave4Us operational system and the respective predicted parameters are presented in Figure 1a. Results are mainly presented as 2-D horizontal maps, but timeseries and/or spatial distributions along cross-sections are also provided in areas of interest that can be viewed using the 'Results type selection' menu in the Wave4Us website: http://wave4us.web.auth.gr. The 3-day forecast results are updated daily at 08:30 UTC. The atmospheric simulation, based on the WRF-ARW model (WRF-ARW-AUTh; Pytharoulis et al. 2016) covers the Central Macedonia region (1.67 km; Fig. 1b) with a simulation cycle that starts at 18:00 UTC of the previous day. WRF-ARW-AUTh provides the atmospheric forcing to the ocean model (Delft3d-Thermaikos; Androulidakis et al. 2023a) that covers the broader TG on a curvilinear grid and 15 vertical sigma-layers with horizontal spatial steps varying from 300 m in the northern part to 700 m offshore. APPLICATIONS OF WAVE4US OPERATIONAL PLATFORM TO SUPPORT FIRST- LEVEL RESPONSE ACTIVITIES IN THERMAIKOS GULF Androulidakis Y 1,2 *, Makris C 1 , Kolovoyiannis V 2 , Krestenitis Y 1 , Baltikas V 1 , Pytharoulis I 3 , Mallios Z 1