As predicted earlier, the maiden monsoon depression of the southwest monsoon season will form soon over the Bay of Bengal. This system will strengthen the monsoon surge in the Arabian Sea and drag the monsoon line to Mumbai and beyond. Heavy to very heavy rainfall is expected all along the West Coast reaching up to Mumbai between 11th June and 15th June 2021.
A cyclonic circulation is expected to form over the Gulf of Siam and adjoining Myanmar on 08th June and become organized in the subsequent 24 hours over the same area. Under the influence of this circulation, a low-pressure area is likely to form over the northeast Bay of Bengal on 10th June. This weather system will move along the coast and shift over the Head Bay of Bengal and also become a depression on 11th June. In all probabilities, the depression will cross the coast on 12th June and be consequential for the onset of monsoon over East India and some central parts covering West Bengal, Bihar, Jharkhand, Odisha, and Chhattisgarh. Simultaneously, along the West Coast, the monsoon will progress to cover Konkan, including Mumbai, and also cover parts of South Coastal Gujarat.
Meteorologically, monsoon systems forming over the Bay of Bengal, accentuate the monsoon surge along the West Coast in general and the Mumbai area in particular. The opening spell of onset of monsoon over Mumbai is invariably heavy and will be no different even this time. Rainfall will be heavy to very heavy over Mumbai and suburbs between 12th June and 15th June and 3 digit rainfall( >/= 100mm) in 24hr is expected over Mumbai, Alibag, Navi Mumbai, and Thane.
As per the revised schedule, the normal date of onset of monsoon over Mumbai is 11th June as against its earlier one of 10th June. Last year, the onset of monsoon over Mumbai was announced on 14th June. This year, the monsoon is expected to reach with military punctuality around its normal date with the least margin of +/- one day. The withdrawal of monsoon date has also been rescheduled for October 08th, as against its previous mark of 29th September.