Northeast monsoon set in over South Peninsula on 25thOctober, with delay of 5 days from the normal date of 20th October. Unlike the Indian Summer Monsoon, Northeast monsoon spans for 3 months (Oct-Nov-Dec) and covers merely 5 sub divisions of South India : Kerala & Mahe, Tamil Nadu, Puducherry & Karaikal, Coastal Andhra Pradesh & Yanam, Rayalaseema and South Interior Karnataka. This is the chief rainy season for Tamil Nadu and the state experiences 48% of its annual rainfall. The season has invariably been making a late start and also clocking less than normal rains , with few exceptions. Unlike southwest monsoon, here the seasonal rainfall is considered normal when it falls within +/- 11% of long period average (LPA).
Tamil Nadu and Kerala are the rainiest pockets and Rayalaseema, the least of all. Seasonal and sub divisional rainfall variation, both remain high and occasionally exceed the coefficient of variation of 25%, more so for the state of Tamil Nadu.
Irrespective of the onset time, rainfall for the month of October is counted against the season. During October 2021, except for the sub division of Coastal Andhra Pradesh which bears a large deficit, rest of the four sub divisions are placed comfortably with adequate surplus. Global climate parameters like El Nino/ La Nina, IOD and MJO influence seasonal performance. Growing La Nina conditions are considered favorable for good northeast monsoon activity during 2nd half of the season. IOD is also expected to stay within the threshold value of +/- 0.4°C.
A low pressure area is marked over southwest Bay of Bengal, off Tamil Nadu coast. Also, a trough is extending from this system up to North Coastal Andhra Pradesh. Heavy rainfall is expected over Tamil Nadu and Coastal Andhra Pradesh between 29th Oct and 31st Oct. Heavy rainfall is likely over Kerala and South Interior Karnataka during the 1st week of November. Intense weather activity over the next one week will result above normal rains over the region. Overall, northeast monsoon season 2021, is expected to be normal.