Last year, the primary rain-bearing system reached Delhi and Mumbai together on June 25, first time since June 21, 1961, with Cyclone Biparjoy impacting its progress over southern India and the adjoining western and central parts of the country.

Normally, the monsoon reaches Kerala by June 1, Mumbai by June 11, and the national capital by June 27. On May 15, the weather office had announced the onset of monsoon over Kerala by May 31.

Weather scientists said cyclone Remal, which ripped through West Bengal and Bangladesh in May-end, pulled the monsoonal flow to the Bay of Bengal, resulting in an early onset over the northeast.

The simultaneous onset of monsoon over Kerala and the northeast is quite rare and has happened on four occasions earlier -- 2017, 1997, 1995 and 1991.

The normal monsoon onset date is June 1 for Kerala and June 5 for Arunachal Pradesh, Tripura, Nagaland, Meghalaya, Mizoram, Manipur and Assam.

Between 1971 and 2024, the earliest onset of monsoon over Kerala was in 1990 when the annual rains reached the coastal state on May 18. The onset of monsoon over Kerala happened on May 22 in 1999, and on May 23 in 1974 and 2009.

Retaining its April forecast, the IMD in a May-end presser said the country could see above-normal rainfall in the four-month monsoon season (June to September) with cumulative rainfall estimated at 106 per cent of the long-period average of 87 cm.

Below-normal monsoon rainfall is expected in northeast India, normal in northwest, and above-normal in central and south peninsular regions of the country.

India's core monsoon zone covering most of the rain-fed agriculture areas in the country is predicted to receive above-normal rainfall this season, the MeT office said.

The monsoon is critical for India's agricultural landscape, with 52 per cent of the net cultivated area relying on it. It is also crucial for replenishing reservoirs critical for drinking water, apart from power generation across the country.

June and July are considered the most important monsoon months for agriculture because most of the sowing for the Kharif crop takes place during this period.

El Nino conditions are prevailing at present, and La Nina may set in by August-September, scientists said. El Nino -- the periodic warming of surface waters in the central Pacific Ocean -- is associated with weaker monsoon winds and drier conditions in India.

La Nina -- the antithesis of El Nino -- leads to plentiful rainfall during the monsoon season.