According to IMD the Northeast Monsoon season starts on 1st October and ends on 31st December. As we mention often, this is for statistical convenience rather than a reflection of the actual season. The mean onset date for Northeast Monsoon season according to this study by Dr YEA Raj and Dr B Geetha is 18th October. The mean withdrawal date is 23rd December. But Northeast Monsoon season has come to a close well into January on many occasions.
The above study also highlights the mean withdrawal date between 1911 and 1940 was above 30 days. This indicates during this period on an average Northeast Monsoon season ended in January rather than December. 1933-34 season saw NEM withdraw on 28th January making it one of the longest seasons historically. While we wait for the official announcement from IMD on withdrawal atmospheric factors indicate monsoon could wind down gradually.
Interestingly the above study has also captured a spatial variation in Northeast Monsoon season withdrawal within Tamil Nadu. While the mean withdrawal date for North Coastal TN is 17th December for South and Central Coastal TN it is 31st December. The study also highlights the fact Central and Coastal TN continues to see some rainfall activity even after the withdrawal. This also explains why on occasions the Manjolai Ghats gets heavy rains even during February.
Today and tomorrow we may see moderate rainfall activity over parts of Central and South TN. Once again there is a high chance Manjolai Ghats may see heavy rains today. Gradually from tomorrow rains are expected to fade. The Ensembles also indicate except for South TN and parts of Delta coast most parts of the state may see dry weather. The satellite image also indicates bulk of the convection starting to push south of Equator indicative of the changing dynamics.
The West to east moving MJO played a major role in enhancing the last rainfall episode over Tamil Nadu. There are indications as it moves further east it’s interaction with the base La Nina state could weaken it over MTC. As we get closer to February further weakening and slow eastward movement will make conditions unfavourable for NEM to continue. There is a chance parts of South TN may see one more spell of rains around month end. But this could be mostly light / moderate and restricted to extreme southern parts of TN.
As mentioned above while parts of South TN may continue to receive spells of rains the monsoon dynamics is expected to wind down gradually. In a couple of days time we may gradually start seeing the 2nd phase of mild winter conditions establish over most parts of Peninsular India. This is indicative of the overall dry weather that is expected to prevail. The COMK Ensemble rainfall range chart will now have the headquarters of all the districts in TN & PDC. Over the next few days gradually the daily rainfall range charts will also start reflecting this change.