CHENNAI: A low-pressure area has formed over the Southwest Bay of Bengal off the Tamil Nadu coast and it will trigger heavy to very heavy rainfall across the State. Under the influence of an upper-air cyclonic circulation, the system developed on Wednesday and continues to persist over the same region. The associated cyclonic circulation extends up to 5.8 km above mean sea level.

According to the Regional Meteorological Centre (RMC), heavy to very heavy rain is likely at isolated places over Thoothukudi, Tirunelveli, Kanniyakumari, Tenkasi, Virudhunagar, Ramanathapuram, Theni, Dindigul, Madurai, Coimbatore, Nilgiris, Sivaganga, Pudukkottai, Thanjavur, Tiruvarur, Nagapattinam and Mayiladuthurai and the Karaikal area on Thursday.

On Friday, heavy to very heavy rain is likely over Kanniyakumari, Nilgiris and Coimbatore, while heavy rain is expected at isolated places over several other districts, including Erode, Salem, Namakkal, Karur, Tiruchy, Madurai, Cuddalore, Villupuram, Tirupur, Perambalur and Puducherry.

VR Durai, Director and Senior Atmospheric Scientist at the RMC, said, “The rain, accompanied by thunderstorms and lightning, will continue across Tamil Nadu and Puducherry until October 26. A red alert is unlikely, as the present system is weak with low chances of intensifying.”

He added that another low-pressure area is expected to form over the southeast and east-central Bay of Bengal on October 23 and intensify into a deep depression and move west-southwestwards. A separate system over the southwest Arabian Sea is moving west-northwest towards the Somali coast.

“For Chennai, moderate rainfall is expected over the next three days, with heavy rain likely on October 25 as the current system moves northwards. Chennai, Tiruvallur, Kancheepuram and Chengalpattu are likely to receive moderate rain on Thursday,” added Durai.

On Wednesday, the city recorded its highest rainfall of 15.4 cm at MGR Nagar, followed by 13.7 cm at Vadapalani, 12.9 cm at Saidapet, 12.8 cm each at Alandur and Chennai Airport, 12.6 cm at Kolapakkam and 12.4 cm at Meenambakkam. Winds of 35-45 kmph, gusting up to 55 kmph, are expected, and fishermen have been advised to exercise caution in coastal areas for the next two days.

Over the past five years, the northeast monsoon has extended up to mid-January. El Niño tends to increase rainfall intensity rather than extend the seasonal duration. The monsoon rainfall for the State is normally 44 cm between October and December. From October 1-21, the State recorded 11.1 cm against the normal 10 cm, a 12% surplus.