Chennai rain: What caused the overnight deluge that flooded Tamil Nadu’s capital?

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Updated: November 07, 2021 6:00 PM

In one of Chennai’s heaviest rain episodes in 2015, the city recorded rainfall of 24.6 cm in 24 hours on November 15 and 16, breaking a November 2005 record of 14.2 cm.

chennai rainThe northeast monsoon brings 60% of the coastal regions’ annual rainfall, while the interior districts get 40-50%. (IE)

The overnight rain that lashed Chennai flooded at least 40 of the city’s residential and commercial neighbourhoods. The rain on Saturday night, the heaviest since 2015, was caused by a low-pressure formation over the Bay of Bengal.

Overnight impact
The heavy rainfall that began on Saturday night and continued till early this morning flooded several neighbourhoods such as Vyasarpadi, T Nagar, Velachery, Adyar, Mylapore, and Royapettah.

Madhavram, Jawaharlal Nehru Nagar, Tondiarpet High Road, Royapuram, Northern Trunk Road, Khader Nawaz Khan Road, Teynampet, the interior parts of Velachery, and stretches of Sholinganallur in the suburbs reported flooding or inundation early this morning.

The Tamil Nadu government said it released 500 cusecs of water from the Puzhal reservoir and alerted residents of low-lying areas on the surplus canal banks. It also released water from the Chembarambakkam reservoir.

Heaviest rain since 2015
Data from Nungambakkam in the city and Meenambakkam in the suburbs suggests that the city and the suburbs received 21.5 cm and 11.3 cm rainfall, respectively, by 8.30 AM.

In one of Chennai’s heaviest rain episodes in 2015, the city recorded rainfall of 24.6 cm in 24 hours on November 15 and 16, breaking a November 2005 record of 14.2 cm.

But the maximum recorded rainfall over a 24-hours period may date to November 1976, when Chennai received 45.2 cm. Another two-day record was registered on November 12 (25 cm) and November 13 (33 cm) 1985.

Northeast monsoon
Chennai is largely reliant on the northeast monsoon, which brings rainfall during October and December. Easterly winds start blowing from mid-October, with the onset between October 10 and 20.

The northeast monsoon is Tamil Nadu’s primary monsoon and brings sufficient rainfall to the state. All other states rely on the southwest monsoon, which usually sets in during May, to July. While the southwest monsoon helps Tamil Nadu maintain groundwater tables after a prolonged summer, the northeast monsoon elevates it.

The northeast monsoon brings 60% of the coastal regions’ annual rainfall, while the interior districts get 40-50%.

Low-pressure system
The India Meteorological Department said a low pressure was likely to form over the Bay of Bengal and move towards the northern Tamil Nadu coast. It has predicted moderate rainfall in the coming days ahead of the system’s formation.

The Regional Specialised Meteorological Centre – Tropical Cyclones said in a bulletin an upper air cyclonic circulation was lying over southeast Bay of Bengal and the adjoining equatorial Indian Ocean region. It is likely to lead to the formation of a low-pressure area over southeast Bay of Bengal around Tuesday.

The system is predicted to become more marked during the subsequent 48 hours and move towards the northern coasts of Tamil, the Regional Specialised Meteorological Centre – Tropical Cyclones said.

Heavy rainfall warning has also been issued for Wednesday. A thunderstorm is likely to bring heavy-to-very heavy rainfall at isolated parts of Chennai, Kancheepuram, Tiruvallur, Chengalpet, Cuddalore, Nagapattinam, Mayiladuthurai, Tiruvarur, Pudukottai, and Thanjavur districts of Tamil Nadu, Puducherry, and Karaikal.

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