Representative image. Ashwin Kumar from Bangalore, India, CC BY-SA 2.0, Wikimedia Commons
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Why it matters
Heavy rain decides whether schools open, whether roads in Chennai flood and whether farmers in the delta can work their fields this week. The spell also matters for water: Tamil Nadu's groundwater is at its lowest in 16 years, and the northeast monsoon is the main chance to refill wells, lakes and reservoirs before summer.
Tamil Nadu rain is set to continue until 26 October 2026 after a low pressure area formed over the southwest Bay of Bengal off the state’s coast early on Wednesday, 21 October. Heavy overnight showers flooded roads in Chennai, and schools in the city and its neighbouring districts stayed shut for the day.
Where the heaviest Tamil Nadu rain fell overnight
MGR Nagar in Chennai recorded 15 cm of rain in the 24 hours ending 8.30 am on Wednesday, the highest in the region, according to figures reported by The Hindu. Vadapalani logged 14 cm, while Saidapet, Meenambakkam, Alandur and Kancheepuram each received 13 cm. Nungambakkam and Adyar got 11 cm each.
That is 150 mm of water in a single day and night. Roads in T Nagar and on Usman Road were waterlogged on Wednesday morning, going by visuals released by the news agency PTI.
District administrations declared a one-day school holiday in Chennai, Tiruvallur, Kancheepuram and Chengalpattu, PTI reported. The Hindu and Hindustan Times put the count at five districts, adding Ranipet to the list. Collectors announce these holidays one day at a time.
Tamil Nadu rain alerts: which districts are orange and yellow
The India Meteorological Department (IMD) placed Tiruvallur, Chengalpattu, Villupuram, Thanjavur and Cuddalore under an orange alert on Wednesday, Hindustan Times reported. An orange alert means very heavy rain is likely, along with thunderstorms, lightning and squalls. Most other districts, including Vellore, Namakkal, Erode, Kanniyakumari and Tirunelveli, were on a yellow alert, which signals moderate rain with thunderstorms.
The Regional Meteorological Centre in Chennai has also flagged the south coast. Thoothukudi, Tirunelveli and Kanniyakumari could see very heavy rain until Thursday morning, and the RMC said some places there may get up to 20 cm across Wednesday and Thursday. Karur, Dindigul, Pudukottai and Thanjavur are likely to get heavy spells as well.
The IMD forecast for Tamil Nadu, Puducherry and Karaikal shows isolated very heavy rainfall of 11.5 to 20 cm on Wednesday and Thursday, BusinessLine reported. After that, the Tamil Nadu rain should ease to isolated heavy showers for another four days from Friday, which takes the wet spell to Monday, 26 October.
Why the rain is moving south this week
The new low pressure area sits over the southwest Bay of Bengal and the adjoining Tamil Nadu coast, with a trough stretching from it up to north coastal Andhra Pradesh. That line is pulling rain clouds across the southern peninsula: on Wednesday morning, cloud cover ran from Vellore and Chennai inland to Salem and Madurai, and down to Kanniyakumari, according to BusinessLine.
The Tamil Nadu rain that battered the north coast earlier in the week is expected to shift towards southern, delta and interior districts, The Hindu reported on 20 October. Interior towns such as Madurai and Karur had been very hot until a few days earlier.
Second low pressure area and the cyclone question
Forecasters are already watching a second system that could stretch the Tamil Nadu rain into next week. A circulation over the Gulf of Thailand and Myanmar is expected to reach the north Andaman Sea by Thursday and form another low over the southeast and east-central Bay of Bengal around 23 October.
Accounts differ on how strong it could get. BusinessLine reported that IMD’s weather models point to a possible tropical cyclone as the system moves northwest and strengthens into early next week, though its track and strength are not yet clear. A day earlier, an RMC official told The Hindu that none of the systems lining up in the Bay was expected to turn into a cyclone. Strong El Niño conditions, which keep the sea warm, are helping systems form back to back, the official said.
What the northeast monsoon means for Tamil Nadu’s water
The northeast monsoon, which runs from October to December, brings Tamil Nadu most of its yearly rain. This year the state needs it more than usual. The New Indian Express, citing Water Resources Department data, reported that groundwater fell in all 38 districts in September 2026 compared with a year earlier, the worst position in 16 years.
Tiruvannamalai saw the sharpest fall: the water table dropped from 3.68 metres below ground in September 2025 to 7.56 metres in September 2026, so wells there must now go more than twice as deep to reach water. Officials blamed a weak southwest monsoon and El Niño.
The recent Tamil Nadu rain has helped on the surface. The state had a 12% rainfall surplus for October as of 20 October, The Hindu reported, after the active monsoon wiped out an earlier shortfall.
What Chennai is doing to prepare
The Greater Chennai Corporation is repairing roads across zones 1 to 15 at an estimated cost of ₹21.80 crore, according to an official release cited by PTI. The civic body also handed out 6,000 food packets to people hit by the rain in Virugambakkam on Wednesday morning. Chief Minister Vijay was due to chair a meeting on monsoon preparedness on Wednesday, The Hindu reported.
The Tamil Nadu rain outlook for the weekend now depends on how the two Bay of Bengal systems develop. IMD updates its district warnings every day, and school holiday notices usually follow from collectors the evening before. More stories from across the country are on our India news page.
Frequently asked questions
Till when will heavy rain continue in Tamil Nadu?
IMD and the Regional Meteorological Centre in Chennai expect rain to continue till 26 October 2026, with very heavy spells on 21 and 22 October.
Which districts declared a school holiday on 21 October?
Chennai, Tiruvallur, Kancheepuram and Chengalpattu, according to PTI. The Hindu and Hindustan Times also listed Ranipet.
Will a cyclone form in the Bay of Bengal?
A second low pressure area is expected around 23 October. IMD models show a possible cyclone, but forecasters say its strength and track are not yet clear.
What does an orange alert mean?
An orange alert means very heavy rain is likely, along with thunderstorms, lightning and strong winds. A yellow alert means moderate rain with thunderstorms.