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Many parts of Tamil Nadu, especially Chennai, are experiencing a torrent of rainfall since October 14 which may continue for the next several days. The major reasons for the rainfall are an intensifying low pressure area off the coast of Tamil Nadu, the onset of the Northeast monsoon rains and what seems like a sudden withdrawal of the Southwest monsoon on October 15.
Looking at the intense rains on the intervening night of October 14-October 15 and the India Meteorological Department’s (IMD) alert for the next four days, the chief minister of Tamil Nadu, M K Stalin ordered the shutdown of schools and colleges in Chennai, Tiruvallur, Kanchipuram and Chengalpattu districts from October 15 to October 18.
Parts of Chennai have received more than 100 mm of rainfall between the mornings of October 14 and October 15.
IMD has declared a red alert for Tamil Nadu on October 15 and October 16, predicting intense rainfall. The weather agency has also declared a red alert for Andhra Pradesh on October 16.
Around 8:30 am on October 15, a well-marked low pressure area was present in the central part of the Bay of Bengal, according to the IMD. The low pressure is expected to move west-northwestwards and strengthen into a depression in southwest Bay of Bengal in the following 24 hours.
“It is likely to continue to move west-northwestwards towards north Tamil Nadu, Puducherry and adjoining south Andhra Pradesh coasts during subsequent 24 hours,” said IMD in its press release.
Apart from this, a cyclonic circulation also exists in west central Bay of Bengal off the coast of Andhra Pradesh which may also influence the rainfall, according to IMD.
The IMD also announced the complete withdrawal of the Southwest monsoon on October 15, which coincided with the normal date for the withdrawal of the major monsoon season for India.
In its press release on October 14, the weather agency had said that the withdrawal of the southwest monsoon from the remaining parts of Gujarat, Madhya Pradesh, Chhattisgarh, Odisha and some parts of Maharashtra would take place in the following two days.
IMD had predicted the complete withdrawal of the southwest monsoon in another two days. But this entire process of withdrawal seems to have happened within a day.
Simultaneously, IMD also declared the onset of the Northeast monsoon which is the major rainfall season for Tamil Nadu and also causes rainfall in other southern states such as Kerala, Andhra Pradesh and Karnataka.
For the declaration, IMD cited the existence of the low pressure area, the setting in of easterly and northeasterly winds over southern Peninsular India and light to heavy rainfall over southeast of the Peninsula.
The coming depression may also bring intense rainfall to parts of Karnataka and Kerala, apart from Tamil Nadu and Andhra Pradesh between October 15 and October 17.
The Windy weather analysis and visualisation platform shows intense rainfall over Chennai on October 18 as well, though IMD has declared only a yellow alert for rainfall on the date as of October 15.