Academically Southwest Monsoon season starts from 1st June and ends on 30th September according to IMD. As we explain often this seasonal calendar is to ensure long term data availability is consistent. It is essential data remains consistent to understand long term changes in rainfall pattern. Prior to 2020 the withdrawal date for Southwest Monsoon over Northwest India was 1st September. After an extensive deliberation IMD decided it was essential to realign the monsoon arrival and withdrawal dates. The new scheduled the withdrawal over Northwest India to 17th September. Since the new dates were announced the earliest withdrawal from NW India was 20th September in 2022.
Just like a robust onset criteria IMD also has a fairly objective criteria for withdrawal too. Looking at sub seasonal models there is a very high chance this year also may see withdrawal after 20th September. The depression that dumped extremely heavy rains over AP & TS states is now heading towards Central India. This may remain near stationary for the next couple of days over the region leading to potentially very heavy rains over parts of Gujarat and Rajasthan. This year’s SWM has been characterised by bouts of extreme rains interspersed by long dry spells.
The daily rainfall chart for Kerala sums up the nature of Monsoon this year. 3 / 4 episodes including the Wayanad episode has driven the overall numbers. Ironically Wayanad still is in deficit category with 29% less rains than normal as on date. MJO influenced Monsoon intra-seasonal oscillation is likely to remain unfavourable for Peninsular India for the next couple of weeks. This could mean effectively the core monsoon zone in Peninsular India may see a weak end to SWM season. It is indeed a strange situation considering Monsoon has remained active with two depressions over the past couple of weeks. There is a very high chance for two more Monsoon lows coming up.
The first one is likely to form in West Central Bay on 5th September. This will gradually move NW towards Central and subsequently NW India. With MJO remaining in the MTC there is a chance another pulse may push into Bay of Bengal next week once again keeping the monsoon trough around Central India / Gangetic Plains. This would mean the earliest window for monsoon withdrawal from Northwest India may be around 22nd September +/- 3 days. The good news though is with MJO expected to move into West Pacific and possibly western hemisphere we can expect a fast withdrawal subsequently.
Over the next 3 / 4 days as the circulation in Bay of Bengal strengthens into a Low Pressure thunderstorm window opens up for TN. Predominantly South AP and adjoining North TN may be better placed considering the location of the circulation and associated wind convergence. The midnight rains over parts of Chennai and suburbs is an indication of this with good chance for late night rains this week. Which area sees rains will depend on real time storm genesis and movement.