For nearly 80 years the traditional onset and withdrawal dates for Southwest Monsoon was 1st June over Kerala and 1st September over Northwest India. A couple of years back IMD decided to realign the onset and withdrawal dates taking into account the monsoon dynamics seen over the past few decades in terms of onset and withdrawal. Since the start of the 2020 monsoon season the new dates came into place for both onset and withdrawal. While the traditional onset date over Kerala did not change the onset dates over West Rajasthan, the last check in point for Monsoon was pulled back by a week from 15th July to 8th July. Similarly West Rajasthan which is the first check out point as well for withdrawal saw the dates revised from 1st September to 17th September.
Similarly the monsoon onset and withdrawal dates for various parts of the country has been revised. Just like how the monsoon onset date over Kerala at the starting point did not change from 1st June the last withdrawal phase over Peninsular India was not changed from 15th October. But if one were to look at the withdrawal of monsoon over the past decade 7 out of 10 years the monsoon has started withdrawing after the revised withdrawal date over Rajasthan. 2019 and 2021 saw Southwest Monsoon start its withdrawal only in October. While 2019 saw Peninsular withdrawal by 15th October, 2021 saw Southwest Monsoon complete its withdrawal over Peninsular India only by 23rd October.
With the first check out point in the monsoon withdrawal schedule, 17th September, behind us it is time to look at what is in store for us over the next couple of weeks. Keeping in trend over the past few years the start of monsoon withdrawal is likely to be delayed this year as well with a train of pulses from Maritime Continent coming into Bay of Bengal keeping the monsoon dynamics going. Currently there is a Low Pressure Area over Rajasthan and adjoining areas and is expected to persist over the next couple of days. A cyclonic circulation over North Andaman Sea is expected to drift towards Odisha coast and possibly deepen into a Low Pressure Area later this week and will drift in the same path as the earlier LPA once again keeping the monsoon dynamics going for the next week to 10 days.
Looking at long term weather models there is a fair chance the start of monsoon withdrawal could happen towards end of September but as mentioned in our earlier post consequent to MJO moving into an unfavorable phase during October the withdrawal process will quicken and we might possibly see Southwest Monsoon complete its withdrawal by middle of October. This process could potentially be influenced by a possibly Depression / Cyclone during the 2nd week of October. But more on this when we get closer to the event.
In the meanwhile while the monsoon withdrawal process may start in a delayed manner the persisting westerlies in a way is likely to favor coastal places like Chennai in leeward Tamil Nadu. As we get to the transition season the winds slow down leading to more thunderstorms over the interior areas of Tamil Nadu which on many occasions may not reach the coastal areas. But with monsoon remaining active over the next couple of weeks thunderstorms are likely to continue over interior Tamil Nadu and bring rains to coastal places like Chennai.
Long term weather models continue to show the remaining days of September to bring more thunderstorms to many parts of Tamil Nadu, particularly to North Coastal TN and adjoining South AP, though it may not rain daily.
Today once again we are likely to see thunderstorms over many places of Tamil Nadu, with Tiruvannamalai, Salem, Namakkal, Kallakurichi, Villupuram and Cuddalore dts likely to come under intense thunderstorms during the evening hours. The steering may not favor Chennai and suburbs today and tomorrow though moderate rains over a few places during the midnight hours cannot be ruled out.
COMK wishes all a Happy and prosperous Vinayagar Chathurthi