(IMD) said. The weather department said moderate to heavy rainfall lashed the Nicobar Islands over the past two days. The strength and depth of westerly winds over the south Bay of Bengal, Nicobar Islands and Andaman Sea increased in this period, with wind speeds exceeding 20 knots at 1.5 km above sea level and extending up to 4.5 km in some areas.

The Outgoing Longwave Radiation (OLR), an indicator of cloudiness, also decreased over the region. These conditions met the criteria for the monsoon's onset over the region, it said.

The weather office said conditions are favourable for the monsoon to advance further into more parts of the south Arabian Sea, Maldives and Comorin area; more areas of the south Bay of Bengal; the entire Andaman and Nicobar Islands; remaining parts of the Andaman Sea; and parts of the central Bay of Bengal over the next three to four days.