The India Meteorological Department confirmed that northeast monsoon rain began over Tamil Nadu, Puducherry and Karaikal on 15 October 2026, the same day the southwest monsoon withdrew from the whole country. For Chennai, this is the season that matters most: IMD notes that Tamil Nadu receives about 48% of its annual rainfall between October and December.
IMD’s seasonal outlook says rainfall over south peninsular India for October to December is most likely to be normal, meaning 88% to 112% of the long period average. The fine print, though, is more mixed for the north coast, and the first week of the season has already shown how quickly a normal forecast can turn into a flooded street.
What the official outlook says
IMD released its October to December 2026 outlook on 30 September. The key figures:
- The long period average (LPA) for south peninsular India in this season, based on 1971 to 2020 data, is about 334 mm.
- The most likely category is normal, between 88% and 112% of that average.
- Strong El Niño conditions are present in the Pacific and models expect them to strengthen through the end of the year. The Indian Ocean Dipole is neutral and is likely to stay that way during the monsoon.
- Maximum and minimum temperatures are likely to be above normal over most of the region.
The more detailed outlook for the south peninsula issued on 1 October adds a regional split that Chennai residents should read carefully. It says southern parts of Tamil Nadu are likely to get normal to above-normal rain, while most remaining areas are likely to be below normal. For October alone, it expects normal or above-normal rain in many areas of south Tamil Nadu and some areas of north Tamil Nadu.
In plain terms, the probability maps lean wetter for the south of the state than for the Chennai coast. That is a statement about season totals, not about individual storms. A below-normal season can still deliver one or two extreme days.
The first week: a reminder of how rain arrives here
IMD’s 15 October press release forecast fairly widespread to widespread rain over Tamil Nadu from 15 to 19 October, with isolated heavy rain until 21 October and gusty winds of 40 to 50 kmph during thunderstorms.
North Chennai got the heavy end of that. On Saturday evening, 17 October, DT Next reported that parts of north Chennai received about 100 mm in roughly an hour. By midnight, Oneindia listed Kathivakkam at 310.2 mm and Parrys at 300.9 mm, with Sathangadu, Thiruvottiyur and Ennore close behind. Nungambakkam, by comparison, recorded about 108 mm. Waterlogging was reported in Perambur, Thiruvottiyur, Royapuram and parts of Navalur.
That pattern is typical of this monsoon. According to the Regional Meteorological Centre’s forecast quoted by DT Next, showers usually fall in the evening and around midnight, and the director, VR Durai, said widespread heavy rain was likely across Tamil Nadu until 26 October.
What it means for water supply
The season starts with the city’s biggest reservoir running low. DT Next reported on 15 October that Chembarambakkam held 1,254 million cubic feet against a capacity of 3,645 mcft, compared with 2,056 mcft on the same day last year. Officials said the lower level does not mean an immediate drinking water shortage, and the state has sought additional Krishna water from Andhra Pradesh.
A normal monsoon would refill the lakes. A weak one over north Tamil Nadu, which is what the probability maps hint at, would leave Chennai more dependent on desalination and Krishna water through the summer of 2027.
What the government has set in motion
Chief Minister C. Joseph Vijay chaired a monsoon review at the Secretariat on 21 September, PTI reported. Directions included keeping relief shelters stocked with water, food, power and medical care, holding boats and dewatering pumps ready in every district, and sending advisories by SMS and public alert systems. Revenue and Disaster Management Minister K.A. Sengottaiyan submitted a preliminary report from an expert committee on El Niño and disaster management at the same meeting.
In the city, the Greater Chennai Corporation says it has prepared relief centres, community kitchens and pumps, which we cover in our separate guides on flood-prone areas and storm water drains.
How to follow the season
- Check the Regional Meteorological Centre Chennai page for the city forecast, nowcasts and radar.
- IMD updates its extended range forecast, covering four weeks ahead, every Thursday.
- Watch district colour codes rather than state headlines. An orange alert for Chennai district is what should change your plans.
- Report waterlogging to the Corporation on 1913.
Quick answers
When did the northeast monsoon begin in 2026?
IMD declared onset over Tamil Nadu, Puducherry and Karaikal on 15 October 2026.
Will Chennai get normal rain this season?
IMD expects normal rain for south peninsular India as a whole, but its maps show below-normal rain as more likely over most of Tamil Nadu outside the south.
What is the long period average?
For south peninsular India, the October to December average is about 334 mm, based on 1971 to 2020 data.
Does El Niño matter?
IMD reports strong El Niño conditions and expects them to persist, which is one of the factors behind its regional outlook.
Information checked against official sources on 18 October 2026. Fees, dates and rules can change; confirm on the linked official pages before acting.
Sources
- IMD OND 2026 seasonal outlook press release, 30 Sep 2026 (India Meteorological Department)
- Outlook for the Northeast Monsoon over South Peninsular India, 1 Oct 2026 (IMD)
- IMD press release, 15 Oct 2026 (India Meteorological Department)
- IMD forecasts normal post-monsoon rainfall over south peninsular India (News On AIR)
- Cloudburst-like showers lash north Chennai (DT Next)
- Chennai rains: Kathivakkam tops list with 310.2 mm (Oneindia)
- TN CM Vijay chairs high-level meeting on monsoon preparedness (PTI via ThePrint)
- Chembarambakkam levels dwindle (DT Next)
