Northeast Monsoon 2024 so far has flattered to deceive. People from most parts of the state may have something to complain about this year. The only exception possibly may be West interior TN which has seen a very good year so far. South Tamil Nadu continues to remain the worst performing region this NEM so far. Thoothukudi, Tenkasi and Tirunelveli are the 3 districts to receive least rains compared to its seasonal average so far. But for seasoned weather bloggers the pattern of patchy rains, breaks during season are nothing new. Most often than not Northeast Monsoon picks pace when MJO becomes favourable.
A couple of years back 2020 saw a very weak first half monsoon season. After 2015 NEM 2024 saw four consecutive weak seasons. 2020 also saw NEM start off on a very weak wicket with a weak onset around end October. Rainfall for October was only 101 mm against an average of 171.9 mm leading to genuine worries. Chennai was no different either Meenambakkam IMD observatory saw no 10 cm spell during the 1st half of NEM. The arrival of MJO during mid November changed the equation completely. The 2nd half of monsoon brought nearly twice the rains as 1st half for Chennai AP observatory.
Despite 2016 becoming the 2nd worst year overall for Tamil Nadu Cyclone Vardah with support from MJO tilted the scales slightly for Chennai during December. As this post in October 2020 will highlight arrival of MJO augurs well for Northeast Monsoon even if there was a long dry phase preceding it.
After passing through Indian Ocean during mid October MJO went on a global circuit leading to lesser than optimum dynamics for NEM since onset. After a slow travel around the globe MJO is now entering Western Indian Ocean. Over the next few days it will push into Eastern Indian Ocean bringing it with the most favourable period for NEM 2024. Ensembles picked up the mid October LPA that brought the best rains for Chennai so far well in advance. Ensembles picked up the November 2nd week LPA as well to influence coastal Tamil Nadu well in advance.
Similarly for the past couple of weeks Ensembles have been consistent about the November last week disturbance. Operational models continue to waver in terms of eventual landfall zone on account of westerly trough influence. Nevertheless the initial movement is expected to be WNW favoring widespread rainfall over Coastal Tamil Nadu. With westerly trough influence there is always a possibility of stall / recurve scenario closer to the coast. This could potentially create a repeat of Michaung scenario bringing stationary rain bands over a part of Peninsular Coast. It needs to be mentioned here though as things stand models are not picking up this scenario for Coastal TN.
Over the past few years Chennai has slowly started seeing a secondary rainfall peak around end November / early December. This year also we may see a similar situation aided by the disturbance expected to develop next week. There is fair confidence 2024 will also add to the list of positive NEM years in recent times. Similarly South TN which has seen weak Northeast Monsoon will get its due share as well. Not to forget the coastal TN stretch between Pondicherry and Ramanathapuram which benefits from WNW moving low latitude disturbances.
To summarise.
- MJO is expected to move into Eastern Indian Ocean by next week. This phase will coincide with the best period of NEM 2024.
- This peak period for Coastal TN is likely to sustain until 2nd week of December
- The upcoming weak may see Easterly wave driven rains benefit mostly Delta districts and South TN. North TN including Chennai may see relatively lesser rainfall
- The development of a LPA around 20th / 21st over South Bay will bring a spell of widespread rains over Coastal Tamil Nadu including North TN during the last week of November
- Supported by MJO this LPA is likely to intensify into a Deep Depression. There is a 50:50 chance to become a cyclone as it nears Peninsular Coast. As of now potential landfall zone is less clear. But the initial movement is expected to be WNW towards Sri Lanka / Tamil Nadu coast bringing good rains along with it.
- 1st week of December is likely to see another fresh Depression with initial movement once again expected to be WNW favouring coastal TN.
- As things stand NEM 2024 is likely to be remain activefor Coastal TN until mid December, Interior TN may receive widespread rains if either of these disturbances cross Peninsular India towards Arabian Sea.
- While there is no specific reference to any individual area of coastal TN there is a very high chance Delta to Chennai coastal stretch will end with another above normal NEM year after this phase of monsoon.