Floods are a recurring natural hazard in India, and their frequency and severity are escalating due to the compounded effects of climate change and anthropogenic pressures. This study investigates the catastrophic flooding that occurred in the southern Tamil Nadu districts of Kanyakumari, Tenkasi, Tirunelveli, and Thoothukudi in December 2023, triggered by extreme rainfall associated with Cyclone Michaung. Analysis of long-term rainfall data from 1901 to 2023 reveals a significant increase in rainfall variability, seasonality, and the frequency of extreme events, particularly during the October-December northeast monsoon period. The rainfall recorded on December 17, 2023, exceeded the 100-year return period in Tirunelveli and Thoothukudi, and the 50-year return period in Kanyakumari and Tenkasi indicating a statistically rare and hydrologically severe event. Seasonality indices (PCI, PCD, SI) and onset-withdrawal trends further highlight a shift toward concentrated, high-intensity rainfall episodes and longer monsoon durations. HAND-based flood inundation modeling, validated using ground truth points, delineated over 150 km2 of affected area, particularly along the Thamirabarani basin. The research emphasizes the need for proactive flood management in the coastal districts of Thoothukudi, Tirunelveli, Tenkasi, and Kanyakumari. Key strategies include creating flood inundation maps through dynamic hydrological-hydraulic modeling, establishing early warning systems, implementing sustainable land-use practices, and developing green infrastructure. Long-term climate adaptation measures are also crucial, such as climate modelling for future rainfall predictions, investing in climate-resilient infrastructure, and educating communities about flood preparedness.