The state of Kerala, located on the west coast of India, experienced a record 100-year flood that resulted in major landslides from unprecedented prolonged and extremely heavy rainfall (50–480 mm·day−1) during August 1–19, 2018, causing extensive damage and about 500 causalities. Rainfall observations indicate that the heavy rainfall occurred over two spells (August 7–10 and 14–18) in association with an offshore trough, and a depression over the Bay of Bengal (BOB). High-resolution 38-year climatology data (5 km) and the ERA-Interim reanalysis dataset show a strong low-level jet over the Arabian Sea and a depression over the BOB with a southwestward tilt during the heavy rainfall. Very high-resolution (2-km) mesoscale model simulations suggest that this high convective instability due to the strong westerly jet along with the formation of offshore vortex, the transport of mid-tropospheric moisture under the presence of conducive vertical shear of horizontal wind, and transport of mid-tropospheric moisture from the BOB are the major factors (as shown in the schematic diagram) behind the extreme heavy rainfall over Kerala.