So far this Northeast Monsoon season Chennai and suburbs has been relatively stress free. Despite both IMD observatories recording more than 96 cms for the NEM season. Both Nungambakkam and Meenambakkam have recorded more than 175 cms for the year 2024. 2024 is now the 5th consecutive year where the average rainfall has been normal or above normal. One of the reasons why Chennai has seen a relatively stress fee monsoon despite above average rains is the long breaks between spells. Only once so far IMD observatory has seen 7 consecutive rainy days this NEM. Similarly Meenambakkam has seen only 6 consecutive rainy days. But all this could potentially change with a fresh low pressure in South Bay.
The cyclonic circulation over South Andaman Sea and adjoining South Bay is likely to trigger a fresh low in the next 24 hours. Most models are converging on the LPA becoming Well Marked over the next 48 hours. Models are also in convergence this Fresh Low may travel in a WNW direction towards Sri Lanka / TN Coast. As we have seen so far this season pretty much every disturbance has moved initially in a WNW movement. The upcoming Low is also likely to follow the same trend.
Except for GFS there is tight agreement among weather models about the intensity as well. Most models expect it to remain a WML with potentially a depression as it nears TN Coast. There is increasing alignment among the models about a potential stall / slow movement scenario as well. A high pressure zone that is currently sitting over Peninsular India is likely to influence the movement of the LPA. Just like Cyclonic Storm Fengal a couple of weeks models are converging on this stall / slow movement for a period of 24 to 48 hours.
There is tight alignment among models about the Low coming near Sri Lanka on 17th morning. But even as late as 20th morning most models expect the circulation to remain off North Coastal TN. GFS continues to remain an outlier as it strengthens the LPA and takes it further North. GEM is the only model which takes the circulation inland by Friday morning. But having observed GEM over the past couple of months it seems to have a bias pushing circulation inland well in advance. This becomes a crucial period potentially triggering an extreme rainfall event over parts of North Coastal TN depending on the location of WML.
Additionally there is also increasing convergence among weather models about potential interaction of moist Easterlies and drier westerlies. This area of interaction is likely to be near South Coastal AP / North Coastal TN depending on the location of Low. This adds a risk of Extreme Rainfall event over the region. Depending on the real time convergence the event could happen either off the coast or over land. Over the past couple of weeks the lake levels around Chennai have increased considerably. Additionally all the three rivers carrying water to the sea for the 1st time this season. This increases the risk of water logging during any sustained rainfall event much more than any time this season so far.
The Key take aways for those short on time.
- Increasing convergence among weather models about the Fresh Low forming today over South Andaman to become well marked
- The next 48 hours it is likely to move WNW to reach near Sri Lanka at which point widespread rains for Coastal TN may start
- Increasing convergence among models about potentially slow movement of the WML due to the presence of a ridge over Peninsular India.
- Any stall / slow movement could convert the rainfall event into a widespread heavy rainfall event for North Coastal TN from Tuesday night.
- The potential for a Westerly / Easterly interaction near South Coastal AP / North Coastal TN is increasing. This could potentially trigger an extreme rainfall event near South Coastal AP / North Coastal TN
- Depending on the real time convergence this extreme rainfall event could happen either off the coast or over land.
- As things stand the stretch between Marakkanam and Sriharikota needs to be cautious and alert fo this potential rainfall event.
- Between 17th and 20th December North Coastal TN and South Coastal AP can expect widespread rains. The stretch between Pondicherry and Nellore may see widespread heavy / very heavy rains during this period.
We will continue to update as things evolve. As things stand it is essential to be prepared but without a sense of panic. There is a possibility the eventual rainfall event may be manageable. But with a 50:50 chance for a widespread heavy to very heavy rainfall event it is essential to be aware and alert.