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Informist, Friday, May 1, 2026

MUMBAI – The onset of the upcoming southwest monsoon is likely to maintain its normal date over the Andaman and Nicobar Islands due to the timely development and strengthening of southwesterly winds in the second half of May, which signals that monsoon onset over India's mainland is also likely to keep its normal date, the India Meteorological Department said in its outlook for rainfall and temperature for May. The second half of the monsoon is likely to be impacted by the development of an El Nino event in July, the weather department said.

In a first, the weather department furnished its early predictions for the onset of the southwest monsoon. It is based on an extended range forecast for 850 hectopascals and 500 hectopascals winds, calculated through the department's southwest monsoon monitoring mechanisms. These two wind fields are essential meteorological tools for analysing atmospheric circulation. The 850 hectopascals level represents the lower atmosphere or boundary layer, highlighting low-level jets and temperature advection, while the 500 hectopascals level represents the middle troposphere, mapping large-scale troughs, ridges, and steering winds.

"During the third week of May, the southwesterly winds, as of today, are expected to be there. We are saying that there could be progress of southwest monsoon over Andaman Islands, Nicobar Islands, southeast Bay of Bengal, and Andaman Sea, around the third week of May," said Mrutyunjay Mohapatra, director general, India Meteorological Department.

The normal date for the onset of the southwest monsoon over the Andaman and Nicobar Islands is around May 22, based on the new rainfall criteria. Before 2020, the normal date of monsoon onset over the region was around May 20, according to the weather department. The normal date of monsoon onset over mainland India is around Jun. 1, starting in Kerala. 

The southwest monsoon develops when land heats up faster than the sea during summer. This creates low pressure over northern and central India. At the same time, the Indian Ocean stays cooler, leading to higher pressure over the sea. Moist winds flow from the ocean to the land, bringing rain. These winds are called southwesterlies because they blow from the southwest. These winds split into two branches, with one travelling along the Arabian Sea and bringing rain to the western coast and central India, and the other moving across the Bay of Bengal and reaching the eastern and northeastern parts of the country. As these winds hit mountain ranges such as the Western Ghats and the Himalayas, they rise, cool, and release rain. 

Currently, neutral El Nino-Southern Oscillation conditions are prevailing over the equatorial Pacific Ocean, the weather department said. Many international climate models, along with the Indian bureau's Monsoon Mission Climate Forecast System, indicate the development of El Nino conditions between June to September. "According to our forecast, El Nino conditions could develop in July and continue till the early months of 2027," Mohapatra said. It is difficult to pinpoint whether the event will develop in mid-July or at the end of July, he added.

The El Nino-Southern Oscillation refers to the fluctuation between El Nino and La Nina conditions in the Pacific Ocean. El Nino is linked to drier and hotter weather in India, while La Nina results in more favourable rainfall.

The Indian Ocean Dipole is neutral, the weather department said. Some international climate models indicate the development of positive Indian Ocean Dipole conditions during the southwest monsoon, the department said. According to the Indian bureau's latest climate model forecast, a positive Indian Ocean Dipole is expected to develop during the monsoon, it said.

The Indian Ocean Dipole is the difference in surface temperatures between the eastern and western tropical Indian Ocean. A negative Indian Ocean Dipole leads to less rainfall in India, while a positive index leads to more rainfall, and a neutral phase signals no significant change in precipitation. However, Mohapatra pointed out that the impact of an El Nino event is typically more powerful than that of a positive India Ocean Dipole.

In its first estimate for the upcoming southwest monsoon on Apr. 12, the weather department forecast below normal southwest monsoon rainfall due to the development of El Nino conditions. Overall rainfall in Jun-Sept is likely to be 92% of the long-period average with a modelling error of plus or minus 5%. According to the weather bureau, there are chances of normal rains in June and July, while rainfall is likely to be below normal in August and September.

The weather department will announce its forecast for the onset of the southwest monsoon over Kerala around May 15. The department will give out its final forecast for the 2026 Indian southwest monsoon in the last week of May, Mohapatra said.  End

Reported by Shreya Shetty

Edited by Akul Nishant Akhoury

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