<p>The progression of the moisture flux with the seasonal evolution of the Indian summer monsoon (ISM), as followed by the evolution of its onset and retreat isochrones, is examined using European Reanalysis v5 (ERA5) and the India Meteorological Department rainfall analysis. In this study, the onset and retreat dates of the ISM rainfall diagnosed at the granularity of calendar days of the year are aggregated to a few homogeneous regions of India for which a detailed moisture flux analysis is conducted. Our analysis reveals that the early onset of summer rains in northeast India is unique from the rest of India, with its southerly moisture transport, which is mainly from the Bay of Bengal. The onset of the ISM in other regions is characterized by strong cross-equatorial southwesterly moisture flux. The retreat of the ISM shows a southward retreat of the moisture flux across the Indian Ocean. Interannual variations in the onset and withdrawal dates are closely linked to anomalies in cross-equatorial moisture transport, indicating that the southwestern Arabian Sea and northeastern Bay of Bengal may serve as key sentinel regions for upper-air monitoring of ISM variability. These areas exhibit significant fluctuations in both the non-divergent and irrotational components of moisture flux during onset, providing potential early indicators of anomalous monsoon evolution and would nicely complement existing seasonal monsoon prediction practices.</p>