CHENNAI: The onset of northeast monsoon (NEM) in Tamil Nadu may be delayed by 10 days as most weather models have forecast that rain may start by end of the month. The normal onset date is October 18. Despite possible delay, the India Meteorological Department (IMD) said monsoon rainfall is likely to be normal to above normal this year. Tamil Nadu’s average rainfall during the northeast monsoon is 44cm.
As per IMD, there are four criteria for declaring monsoon onset --- withdrawal of southwest monsoon, onset of persistent surface easterlies over Tamil Nadu coast, depth of easterlies up to 850 hPa over Tamil Nadu coast, and fairly widespread rainfall over coastal Tamil Nadu, south coastal Andhra Pradesh and adjoining areas.
SW monsoon likely to fully withdraw by October 15
Sources in the regional meteorological centre told TNIE as of today, southwest monsoon has not fully withdrawn from south India. “Parts of south Karnataka and TN are still getting rains. Once it withdraws and northeasterly winds get established along with widespread rains, the NEM onset can be declared. The wind pattern is likely to change only in the last week of October even then without meeting rainfall criteria we can’t declare it.”
As per official records, the earliest onset date from 1977-2020 is October 5 in 1984. The most delayed onset is November 2 which occurred in 1988, 1992 and 2000.Forecast for the next five days shows heavy rainfall in Tenkasi, Tirunelveli, Kanniyakumari, while light to moderate rainfall till October 15 in places including Chennai, under the influence of a cyclonic circulation that lies over Rayalaseema and adjoining south interior Karnataka and a trough running from this cyclonic circulation to Comorin area in lower tropospheric levels.
Mahesh Palawat, vice-president, meteorology and climate change at Skymet, told TNIE there may not be a substantial delay in the onset of NEM. The southwest monsoon is likely to fully withdraw by October 15 and the northeast monsoon may take another one week to establish. “The onset may happen between October 20 and 25.”