A late monsoon, a downgraded forecast and an El Nino advisory: what June's rainfall numbers mean

The southwest monsoon reached Kerala on June 4, 2026, three days later than the June 1 date the India Meteorological Department treats as normal for the state's onset, and nine days later than the department's own forecast date of May 26. On the day, IMD said the monsoon had covered the whole of Kerala, Mahe and the Lakshadweep islands and had moved into parts of Karnataka and Tamil Nadu. A three-day slip is modest by the department's historical record, in which Kerala onsets have run well into mid-June. It was also the first item in a chain of news that included a downgrade to the national seasonal forecast.

IMD cut its forecast for the 2026 southwest monsoon season to 90% of the long-period average, attaching a 60% chance of a deficit or deficient rainfall. The downgrade came as the department expected El Nino conditions to develop over the Pacific during the season itself, a pattern that usually suppresses rainfall over India. The long-period average is the 1971-2020 mean rainfall for the season as a whole; a 90% forecast sits below normal without being a forecast of failure.

Why a late onset matters, and why it does not decide the season

The June 1 onset over Kerala is a scheduling device rather than a physical threshold. Rain can, and regularly does, reach the southern tip of the peninsula before the department formally declares the monsoon arrived, because the declaration tracks a threshold of rainfall across a defined set of stations. A three-day slip therefore says something about the timing of the first substantial spell, not about how much rain the season will deliver.

• Normal onset date over Kerala: June 1.
• 2026 onset: June 4, nine days later than IMD's own forecast date of May 26.
• Coverage on the day: the whole of Kerala, Mahe and the Lakshadweep islands.
• Revised seasonal forecast: 90% of the long-period average, with a 60% chance of a deficit.
• Principal reason cited: El Nino conditions expected to develop during the season.

El Nino is the mechanism that converts a Pacific ocean anomaly into an Indian rainfall outcome. During an El Nino, the Walker circulation shifts, trade winds weaken and the Pacific warms. Indian summer rainfall depends on the position and strength of the subtropical jet and on the Madden-Julian Oscillation, both of which tend to be suppressed under those conditions. It is a probabilistic relationship, which is why forecasters speak in ranges and why a single season's outcome cannot be read back as proof. Down To Earth reported that NOAA's forecasts had the 2026 El Nino becoming very strong and persisting into the Northern Hemisphere winter of 2026-27, and that some climate scientists expected an event rivalling the strongest El Nino on record, between 1876 and 1878. The Indian Ocean Dipole was neutral at the time, and the possible return of active western disturbances was the other variable in the department's outlook.

The practical consequence arrived quickly in the cities. Reporting in early June noted that Mumbai's Brihanmumbai Municipal Corporation had warned of roughly 45 days of stored water on the back of a below-average forecast, and that monsoon-dependent civic planning across Maharashtra was already being adjusted. Reservoirs in the state were reported well below their June-end targets, and farmers in several districts were advised against committing to kharif sowing until the rains settled.