Monsoonal depressions over the Bay of Bengal are fundamental components of the Asian monsoon system, linking oceanic and atmospheric processes that regulate moisture transport, deep convection, and extreme rainfall over South Asia. Despite advances in numerical weather prediction, the mechanisms controlling the persistence and predictability of monsoon depression-induced rainfall remain incompletely understood, particularly over the complex coastal and orographic environment of southeastern Bangladesh. In July 2026, a monsoonal depression that formed over the Bay of Bengal generated a prolonged heavy rainfall event that persisted for nearly two weeks, producing widespread flooding, more than 51 fatalities, and extensive socio-economic losses. This study investigates the dynamical evolution and predictability of the event by integrating synoptic analyses, satellite observations, weather radar, ERA5 reanalysis, and operational numerical weather prediction products with convection-permitting Weather Research and Forecasting (WRF) simulations. Particular emphasis is placed on the role of Bay of Bengal moisture supply, low-level monsoon flow, air–sea interactions, upper-level divergence, and terrain-induced lifting in sustaining quasi-stationary convective systems. Model performance is evaluated using rain-gauge observations and satellite-derived precipitation through statistical verification metrics, including bias, root mean square error (RMSE), correlation coefficient, equitable threat score (ETS), and critical success index (CSI). The analyses reveal that persistent moisture transport from the Bay of Bengal, enhanced by favorable large-scale circulation and orographic forcing, maintained deep convection and produced cumulative rainfall exceeding 300–400 mm across several locations in southeastern Bangladesh. The convection-permitting WRF simulations realistically reproduced the spatial evolution and temporal persistence of the rainfall and demonstrated improved forecast skill relative to operational guidance, although localized intensity biases remained over complex terrain. These findings highlight the importance of resolving monsoon–ocean interactions and mesoscale atmospheric processes for improving prediction of long-duration precipitation extremes and contribute to a better understanding of Bay of Bengal monsoon variability and its implications for operational forecasting and climate resilience.

Keywords: air–sea interaction; moisture transport; convection-permitting modeling; forecast verification; orographic forcing