CHENNAI: Tamil Nadu is set for an unseasonal spell of January rainfall as a well-marked low-pressure area over the southeast Bay of Bengal is likely to intensify into a depression, triggering heavy to very heavy rainfall across several coastal and delta districts from 9 to 11 January. The development is expected to revive the ‘dormant’ northeast monsoon (NEM) and extend it into January, prompting Orange and Yellow rainfall warnings for multiple districts.
January is normally one of the driest months for Tamil Nadu, with a long-term average of just 12.3 mm of rainfall. However, the State has already received 7.8 mm between 1 and 5 January this year. With another widespread spell forecast between 9 and 12 January, January 2026 is likely to end in the excess rainfall category once again.
Meteorologists say the current rainfall is partly compensating for subdued precipitation in December, when northeast monsoon activity weakened earlier than usual. December, typically the tail-end month of the NEM, saw fewer active weather systems and prolonged dry spells across large parts of the State.
An official from the Regional Meteorological Centre (RMC), Chennai, said the usual date for cessation of the northeast monsoon over southeast peninsular India is 30 December. “However, an analysis of rainfall data over the past 100 to 120 years shows that the northeast monsoon extends into January in nearly one-third of the years,” the official noted, adding that post-monsoon low-pressure systems over the Bay of Bengal often drive such delayed rainfall episodes.
Seasonal data from 1 October to 31 December indicate that the northeast monsoon was largely normal across Tamil Nadu, despite sharp spatial variations. Rainfall was deficient in 10 districts, normal in 27 regions, including 25 districts of Tamil Nadu, Pondicherry, and Karaikal, and excess in three districts, with one district recording large excess rainfall, covering a total of 40 units.
The upcoming rain spell is expected to be particularly intense over delta districts such as Mayiladuthurai, Nagapattinam, Thanjavur, Tiruvarur, Pudukkottai, Cuddalore, Villuppuram, and Chengalpattu, where an Orange alert has been issued. Chennai and surrounding areas may experience isolated heavy showers, with thunderstorms and lightning likely at scattered locations.
Weather experts say the combination of a weaker December and increasingly frequent January rain spells indicates greater intra-seasonal variability within the northeast monsoon, a trend with implications for agriculture planning, reservoir management, and flood preparedness across the State.