Northeast Monsoon, or retreating monsoon as it is called technically, is the primary rainfall season for most of Tamil Nadu including Chennai. According to the Climatological Normal from IMD for the period 1991 to 2000 the seasonal mean rainfall during Southwest Monsoon (June to September) is 474 and 440 mm respectively for Chennai AP and Chennai Nungambakkam. This is roughly 1/3rd of the annual rainfall the two observatories receive with Northeast Monsoon contributing nearly 60% of the annual rains. With three weeks of September still to go the year’s seasonal Southwest Monsoon tally for Chennai AP and Chennai Nungambakkam stands at 774 and 597 mm respectively with both the observatories seeing large excess so far. The normal rainfall as on date is about 390 mm for Chennai AP, at 774 mm this year’s Southwest Monsoon is nearly twice of the normal rainfall so far.
One may wonder how Chennai is witnessing such a season. The weak monsoon conditions during June and August certainly provided a very good platform for the numbers to pile up. Interestingly September is showcasing a different dynamics once again aligning for some part of leeward Peninsular India to witness thunderstorms during active monsoon conditions. Most of us know by this time during peak monsoon conditions the leeward areas of coastal Tamil Nadu mostly remains dry on account of the wind shear created by the Monsoon Jet at lower levels. But this time around in a twist of irony the active monsoon conditions, albeit not strong enough to shear convection, is expected to play the role of remnant moisture from the west getting pushed all the way to Peninsular East coast allowing it to interact with air particles that climb up through day time heating over interior parts of Tamil Nadu creating favorable conditions for thunderstorms.
While the Strong / Not so Strong westerlies set the ball rolling for favorable conditions over the next couple of weeks weak circulations over Bay of Bengal and Central India is likely to provide wind convergence over parts of Peninsular India that will allow thunderstorms to sustain. One such scenario is currently underway with the remnant circulation from the Monsoon Low that weakened a couple of days back is expected to remain stationery over the next 2 / 3 days. This could potentially mean a window of back to back thunderstorms days during the weekend may open up over parts of Tamil Nadu and Andhra Pradesh. Depending on the real time wind convergence the same set of areas may witness moderate to heavy thunderstorms during the evening / night hours.
Going by the wind charts put by models it appears the two areas, Delta districts and AP / TN border areas, that witnessed thunderstorm activity could once again be the hotspot for thunderstorms during the weekend. With Chennai AP already recording 774 mm this year 2023 may become the first year in this millennium to cross 800 mm. Subject to verification this has happened only twice in the last 200 odd years, 1870 and 1996, of recorded history.
For the sake of music fans we hope the Marakkuma Nenjam concert happens this week. But there is a very high chance of disruption at least during the second half of the concert.