After a gap of over six years, rain pounded Chennai and parts of Tamil Nadu on Sunday and is likely to continue till November 11 as the Indian Meteorological Department issued an alert. The National Disaster Response Force (NDRF) has deployed teams in three regions to help in rescue and relief operations.

“A fresh spell of heavy to very heavy rainfall with isolated extremely heavy rainfall likely over Tamil Nadu and south Coastal Andhra Pradesh likely during 9-11 November 2021 under the influence likely formation of a fresh low-pressure area,” IMD said in a statement, adding that a depression over the east-central Arabian Sea is also taking shape.

At least four people have died in rain-related incidents and one was injured and over 250 houses have been damaged. The government has opened 160 relief centres in Chennai.

WHY IS IT RAINING IN TAMIL NADU?

The rain in Tamil Nadu at this time is primarily caused by the northeast monsoons, which takes shape between October to December in the region. The IMD had predicted the onset of the northeast monsoon by October 26, delayed by nearly a week due to the delayed withdrawal of southwest monsoon that is responsible for rains in north India.

India receives most of its precipitation from two monsoon seasons, while 75 per cent of rains come from the southwest monsoon, the northeast monsoon covers the remaining. The northeast monsoon rains are triggered by changes in the wind patterns from south-westerly to the north-easterly as the southwest monsoon recedes.

The northeast monsoon dominates the parts of Tamil Nadu, Puducherry, Karaikal, Yanam, coastal Andhra Pradesh, Kerala, north interior Karnataka, Mahe and Lakshadweep. The 2021 northeast monsoon is coming on the back of a subdued 2020 season due to the La Nina conditions, which are also developing this year. However, the monsoon has been known to have unpredictable behaviour and is intensifying so far.

WHY IS IT RAINING SO HEAVILY IN TAMIL NADU?

While Tamil Nadu is receiving rainfall thanks to the northeast monsoon, the intensity and frequency have increased due to the creation of a low-pressure area in the Bay of Bengal and a depression in the Arabian Sea. Since the start of the Northeast monsoon in October, Tamil Nadu and Puducherry regions have experienced about 43 per cent excess rainfall.

Dr S Balachandran, deputy director, Regional Meteorological Centre in Chennai said that the duration between October to December is the peak for cyclonic formation in the North Indian Ocean due to the development of low-pressure areas in the Bay of Bengal and depressions in the Arabian Sea. This affects the intensity of rains and wind speeds in the adjoining regions.

According to the latest forecast by IMD, the depression over the east-central Arabian Sea moved west-northwestwards with a speed of five kilometres per hour during the past six hours and is about 840 km southwest of Mumbai and 790 km west-southwest of Panjim. “It is likely to move the west-northwestwards and maintain the intensity of a depression during next 24 hours.”

“A cyclonic circulation over the Southeast Bay of Bengal and adjoining the equatorial Indian Ocean off the Sumatra Coast persists and now extends up to 4.5 km above mean sea level. Under its influence, a low-pressure area is likely to form over the Southeast Bay of Bengal and the neighbourhood around 9th November. It is likely to become more marked and move west-northwestwards towards north Tamilnadu coast during the subsequent 48 hours,” IMD said in a release.

WHAT IS THE LATEST FORECAST?

Dr Balachandran said that overall there is going to be moderate rainfall in Tamil Nadu in the upcoming 48 hours. However, the intensity of rains will vary from one region to another. The Regional Meteorological Centre in Chennai has predicted heavy rainfalls in Chennai on November 10, 11 along with a few coastal regions.

The IMD in its forecast said there will be an increase in rainfall activity with heavy to very heavy rainfall at a few places and extremely heavy rains at isolated places likely over north coastal Tamilnadu and south coastal Andhra Pradesh. Meanwhile, heavy to very heavy rainfall is also expected at isolated places over Kerala & Mahe and Rayalaseema on November 10, 11.