Absolute ripe conditions are developing over Bay of Bengal (BoB) for arrival of northeast monsoon over South India. Much awaited monsoon is expected to make onset within next 24hr and commence the rainy season for South India, lasting till December 2021. Also, it is likely to coincide with the complete withdrawal of southwest monsoon from the country, concluding 4 month old rainy season on a satisfactory note.
Northeast monsoon is the main rainy season for the state of Tamil Nadu, bringing nearly 50% of its annual rainfall. The season also termed as ‘Indian Monsoon’, covers five sub divisions of South India : Tamil Nadu & Puducherry, Kerala & Mahe, South interior Karnataka, Coastal Andhra Pradesh and Rayalaseema. The variability of northeast monsoon remains very high and the coefficient of variation stands at 27%.
A set of meteorological conditions need to be in place for announcing arrival of northeast monsoon including Withdrawal of Southwest Monsoon up to 15°N latitude, Onset of persistent surface easterly winds over Tamil Nadu coast, Depth of easterly winds up to 5000’ over Tamil Nadu coast and fairly widespread rainfall over Coastal Tamil Nadu & South Coastal Andhra Pradesh
Northeast Monsoon is not declared before 10thOctober, even if conditions described exist. Normal date for onset of easterly winds over Tamil Nadu coast is 14thOctober. The average timeline for onset of northeast monsoon is 20thOctober, with a standard deviation of +/- one week.
Presently, a cyclonic circulation is marked over Southwest BoB, off Tamil Nadu Coast. Another broad cyclonic circulation is lying over Comorin area and Lakshadweep Sea. Under the influence of these two features, northeasterly stream is getting established over BoB, Coastal Tamil Nadu and South Peninsula. Cloud cover with embedded convective clusters is extending from the Southwest BoB to the coastline. These are ideal conditions for an emphatic onset of northeast monsoon.
As a precursor to the onset, Chennai has received scattered moderate showers in the past 24hr. This activity will build up further during the progress of this week. Heavy to very heavy rainfall is expected between 28thand 30thOctober. There are bright chances for this wet spell to extend in to opening days of November. The commencement of Northeast monsoon is invariably linked with intense rains for the capital city . Localized flooding, lightening, thunderstorm and gale winds is quite likely, with the monsoon making a loud and energetic start.