Sequential Moisture Dynamics Driving Multi‐Day Flood‐Producing Rainfall Across India

ABSTRACT Extreme floods during the Indian Summer Monsoon often follow multi‐day heavy rainfall, yet the physical sequencing from large‐scale moisture supply to local flood‐triggering precipitation remains insufficiently resolved for prediction. We analyse severe flood events spanning coastal and inland regimes by integrating rainfall observations, reanalysis data, radiosonde profiles, moisture back‐trajectories and vertically integrated moisture budget. Across events, a consistent precursor cascade emerges where low‐level vertically integrated water‐vapour transport (IVT) strengthens 1–3 days before peak rainfall, column moisture convergence (VIMFC) peaks in close correspondence with the heaviest precipitation, and low‐level transport contributes roughly 41%–51% of total IVT, confirming dominant lower‐tropospheric control. The moisture budget shows vertical advection as the dominant contributor to precipitation. Distinct regimes are identified from moisture back trajectory: (i) coastal floods governed by oceanic low‐level jets and orographic ascent; (ii) inland floods governed by continental moisture recycling and coupling between monsoon lows and synoptic scale disturbances. These results identify a physically consistent precursor cascade linking synoptic moisture pathways to flood‐producing convection across contrasting flood regimes in India. The framework provides a compact diagnostic basis for improving short‐range flood forecasting and offers process‐level insight relevant to future flood‐risk assessment under intensifying monsoon extremes.