Context: After a gap of six years, heavy to very heavy overnight rains pounded Chennai and its suburbs, leading to water-logging everywhere, with the deluge entering houses in low lying areas on Sunday and the sluice gates of three reservoirs here being opened to release surplus water.

• The overnight rain, which was reportedly the heaviest since 2015, was part of a formation of a low pressure over the Bay of Bengal.

Possible Reason

• Chennai’s monsoon is largely about the Northeast Monsoon, rains during October to December, with easterly winds starting from mid-October, precisely the usual onset that begins between October 10 to 20. 
• It is the Northeast Monsoon, also known as the ‘primary monsoon of Tamil Nadu,’ that brings sufficient rains to the state when all other states depend on the South West monsoon for rains that sets in from May, June and July. 
• The South West monsoon, after a prolonged summer, helps Tamil Nadu to maintain the ground water tables, it is the North East monsoon that elevates the table.

Why does Chennai get rain during winters?
In India monsoons establish an entire season in which the winds reverse totally.

• Chennai is the capital city of Tamil Nadu. It lies on the Coromandel coast on the eastern coast of the Peninsula.
• During summer, Arabian Sea branch of South-west Monsoon ascends the Western Ghats and results in maximum rainfall on its western side.
• On the other hand, the Tamil Nadu coast on which Chennai is situated stays dry due to its location off the way of South-west Monsoon as moving towards east they shed their moisture.

Tamil Nadu receives rainfall from Northeast trade winds. 

• From early June to mid-September, between 100- 120 days is the duration of the monsoon. Some places like Tamil Nadu receive a major portion of its rainfall during October and November, but most parts of the country receive rainfall from June to September.
• During the winter season, the winds prevailing in the country are the Northeast trade winds.
• In major parts of the country, it is a dry season, as the winds blow from land to sea. 
• When the winds blow from sea to land, it picks up moisture from the sea and brings rainfall in the country.

The North-east dry Monsoon winds blow over the Bay of Bengal during winter.

• As they pass over the Bay of Bengal they develop moisture and bring rain to the Tamil Nadu coast. As now they are under the influence of on-shore winds, they are full of moisture. Chennai is located on the eastern coast. That is why; Chennai obtains rainfall more in winter than in summer.
• Due to the geographical location, the east coast receives rainfall as the north east trade winds blow from sea to land in this location. Hence the coast of Tamil Nadu receives winter rainfall.

Moreover, Tamil Nadu’s coastal districts get 60% of the annual rainfall and the interior districts get about 40-50% of the annual rainfall from the North East monsoon.

• Note: The rainfall in India is an outcome of the confluence of wind flow from the Bay of Bengal and reverse winds from the South China Sea. The rainy season remains from June to September with a regular annual rainfall between 750–1,500 mm across the region.