Northeast Monsoon as often remarked is not a large scale phenomenon like the Southwest Monsoon which is pretty much a global scale weather event. While it does have active and suppressed phases even though the monsoon possibly lasts about 60 to 75 days in all, often modulated by the presence of favorable tropical waves, the seasonal performance is largely driven by favorable disturbances than just tropical waves. Even the current year if one were to look at the numbers slightly deeply except for the Michaung track which brushed North TN coast pretty much all the cyclones took a Northward track making it unfavorable for Tamil Nadu.
Nowhere this gets reflected better than comparing the numbers of Nagapattinam, Thiruvarur and Thanjavur district seasonal performance so far. Nagapattinam standing on the coast is healthy with a +8% seasonal performance, Thiruvarur which was carved out of the western areas of Nagapattinam along with parts of Thanjavur district stands at -17% while Thanjavur district, to the west of Thiruvarur, is even more bad at -39%. Little more west the numbers of Tiruchirappalli district looks very bad at -42% though ask any serious blogger he would say even at -42% the numbers are skewed by the rainfall received by Thenparanadu sitting over the Eastern Ghats in Thuraiyur taluk.
With Cyclone Michaung over the next few days a dry spell is likely to prevail over most parts of Tamil Nadu. One of the reasons for the active phase of monsoon between 20th November and 5th December was the passage of MJO from West to East across the Indian Ocean. Weather models indicate MJO has further moved East to the edge of Eastern Maritime Continent and is likely to push into West Pacific shortly. This is likely to bring about unfavorable conditions over the Indian Sub continent leading to a spell of dry weather in the absence of any disturbances like cyclonic Circulation or Low Pressure Area etc. The last time MJO went into West Pacific it took about 3 weeks to come back into Indian Ocean and become favorable again. With both El Nino and IOD weakening one can expect MJO to once again complete the global circuit in about three weeks to reach Indian Ocean. By then we might have a case of the ITCZ moving further south altering the area of tropical wave influence to favor south of Equator.
Despite tropical waves playing a huge role in enhancing rainfall prospects during Monsoon time, the biggest driver is Inter Tropical Convergence Zone (ITCZ) a band of convective clouding that decides where the monsoon remains active. During Southwest Monsoon time the Monsoon trough over the Indian Sub continent remains part of ITCZ. As the ITCZ moves South as SWM withdraws during its retreat over the Peninsular India it morphs as Northeast Monsoon before heading South of Equator to herald monsoon over the Southern Hemisphere. Currently the ITCZ is seen straddling the Equator and with weather models indicating less possibility of any strong disturbance to hold the ITCZ to the North of Equator it may not come as a surprise if the ITCZ makes a jump across the Equator to suddenly switch off Northeast Monsoon. El Nino years in the past has seen Northeast Monsoon switch off all of a sudden. The current location of ITCZ is likely to favor South TN mostly along with Sri Lanka.
Nevertheless there is a spell of dry weather ahead of most parts of Tamil Nadu for the next few days under the influence of drier Northerly winds coming in. While South TN and parts of West Interior TN may see rain under the influence of the cyclonic circulation over Arabian Sea, as the circulation moves west the rains would taper off here as well until 14th possibly. Weather models indicate an area of disturbance currently off Sumatra coast is expected to move closer to Sri Lanka by then bringing back the rains to coastal areas of Tamil Nadu between Pondicherry and Kanyakumari and gradually to the interior areas of South TN. Whether the rest of the state gets any rains from this spell will depend a lot on whether this circulation climbs up or not. Considering the current location of ITCZ and an lack of support from tropical waves for the next couple of weeks there is increasing confidence NEM may have wound up over places like Chennai and North interior TN.