Rainy Days For Chennai Ahead: Onset of Northeast Monsoon Soon

Key Takeaways:

• Chennai is likely to experience a week-long spell of Northeast Monsoon rainfall between October 14 and 21, with the onset expected around October 15.
• Nungambakkam has recorded no rainfall so far this October, while Minambakkam has received only 5 mm against Chennai's October monthly average of 300 mm.
• A cyclonic circulation over the Southwest Bay of Bengal and Gulf of Mannar, supported by a north-south trough, is expected to intensify rainfall across Tamil Nadu.
• Heavy rain, lightning, thunderstorms and gusty winds are likely during the week, with the most intense activity expected at night and during early morning hours.
• Forecast Validity: October 14–21, 2026.

October and November are the rainiest months for Chennai, with average monthly rainfall of 300 mm and 374 mm, respectively. The regional observatory at Nungambakkam has not received any rainfall this month so far. The airport observatory at Minambakkam recorded light showers on 5th October, registering 5 mm of rainfall. The capital city is likely to enter a week-long spell of monsoon rains, commencing soon.

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The Northeast Monsoon is expected to set in around 15th October. The onset, as such, is heralded by typical Northeast Monsoon showers beginning over Coastal Tamil Nadu, with Chennai as a prominent location. The rains travel quickly deep inland, right up to the shared borders with Kerala and Andhra Pradesh. The Southwest Monsoon rains were inadequate for the state and left a large rainfall shortfall of 20%. The Northeast Monsoon is expected to fill the deficit during the three-month-long season between October and December.

The main trigger for the onset of the Northeast Monsoon is the cyclonic circulation lying over the Southwest Bay of Bengal and Gulf of Mannar, in proximity to Coastal Tamil Nadu. A well-marked north-south trough is extending from the circulation, keeping a safe distance from the state coastline and stretching up to the West-Central Bay of Bengal. The trough will move eastward to pass through the coast tomorrow. A day later, it will cross over to the interiors of the state and run along the border between Tamil Nadu and Kerala.

As a precursor to the onset of the Northeast Monsoon, light rainfall is expected to start today during the late hours. The clouds will become thicker and denser tomorrow, beginning the inaugural spell of showers before the arrival of the mainstream monsoon. The monsoon showers will become more frequent and intense during the subsequent 48 hours. The interiors of the state will also start experiencing lightning, thunderstorms and strong winds. Watch out for safety and security throughout the week, between 14th and 21st October 2026. Heavy weather activity will be confined to the night and early morning hours. Showers will be relatively light and fleeting during the forenoon and afternoon hours. Exercise caution against lightning strikes and strong gusty winds during thunderstorm activity.

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