The statistical NEM season according to IMD is between 1st October and 31st December. This has been done with maintaining long term rainfall data in mind. A rigid season allows better comparison of long term rainfall data and trends. But this does not take into actual onset and withdrawal of monsoons. Similar is the case with SWM season which extends from 1st June to 30th September though withdrawal is not complete until 2nd half of October. During La Nina years we have often seen extended NEM bring rains for Tamil Nadu even during January.
In recent years 2021 is an example that stands out. A year when the NEM withdrawal post from COMK was put up on 9th January. Places in Cuddalore district recorded more than 25 cms for 24 hours ending on 13th January 2021. On Pongal day that year the extended NEM brought in 6 to 9 cms of rains over many places in Delta districts playing spoilsport. Even South TN saw widespread rains leading to Thamirabarani seeing 50000 cusecs flow during Pongal time.
The last few days saw a weak rainfall activity happen over many parts of the state. While this year’s Pongal did not see rains like 2021 it did rain in many places in South TN. Today also parts of South TN could see moderate rains continue. From tomorrow we can see some cessation in rains over most parts of TN. But this break is likely to be only for a couple of days before rains return. The cyclonic circulation that brought in the rains and currently over SE Arabian sea is expected to revive rains again. Under the influence of the Westerly wind burst created by the movement of MJO over Indian Ocean will push this circulation back into Equatorial South Bay. This in turn is likely to trigger what may be one last rainfall episode from extended NEM.






Rains are likely to be widespread across the state on 19th and 20th January. Places like Chennai and North TN may see light / moderate rains similar to the earlier rains last week. Models indicate Parts fo Delta and South TN may see widespread moderate / heavy rains around the weekend. Places in South Delta and adjoining Pudukottai districts have a later harvest window compared to North Delta. This could potentially mean these areas can bear one more round of rains. The earlier rainfall episode for Delta turned out to be lower than model estimates. Hopefully the upcoming one turns out in similar way for North Delta which is closer to harvest window.
South TN may see moderate / heavy rains over many places which could potentially be good news for some of places in Tirunelveli / Thoothukudi district. Particularly villages that still have tanks that are not yet full. Additionally some places along the Ghats in Virudhunagar and Tenkasi district may also benefit from a spell or two of rains before summer. For places like Chennai in North TN it is business as usual with some rains expected around Saturday / Sunday. This is unlikely to disrupt normal routine. Ensembles are fairly in confidence extended NEM could gradually wind down after this rainfall episode.
Starting from today the COMK ECMWF ensemble based rainfall range is tweaked to give 7 day daily rainfall range for better clarity. Please note this is an exercise which involves learning / tweaking as we continue to fine tune this. But there is a fair confidence that the range could give a good idea on what to expect in terms of overall rainfall pattern.