| Summary: | ABSTRACT In May 2024, the state of Rio Grande do Sul, in southern Brazil, experienced an unprecedented flood that severely impacted Porto Alegre and its metropolitan region. Water levels in the Guaíba River exceeded all historical records, reaching 5.37 meters on May 5, 2024, surpassing the previous peak of 4.76 meters recorded in 1941. In response to this extreme event, a team from the Hydraulic Research Institute at the Federal University of Rio Grande do Sul (IPH/UFRGS), including the authors of this study, mobilized efforts to perform daily operational forecasts of Guaíba water levels during the event. Despite the challenges, the forecasting system successfully identified critical moments of the flooding: flood levels exceeding 5 meters with a 3-day operational lead time; non-occurrence of levels above the 6 meters levees top; second peak above 5 meters with 8-day lead time; prolonged duration of the flooding around 30 days. The accuracy of the forecasts were crucial for mitigation measures such as preventive evacuations and emergency actions. This study highlights technical and operational aspects of the forecasts and discusses the importance of an integrated approach to managing extreme events.
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