The last few days have seen the return of convective thunderstorms over the leeward areas of Tamil Nadu. With monsoon winds slowing down over Peninsular India it has allowed a period of enhanced convective thunderstorms along the East Coast. This Southwest Monsoon season has seen active phases of convective thunderstorms on a regular basis. The seasonal average for the TN & PDC stands at an excess of 65% as on date.
Aided by frequent circulations / LPAs the monsoon trough has parked itself over the IGP. This has resulted in reduced rains over Peninsular West Coast and enhanced rains over the Northern plains. The current cyclonic circulation over Gangetic West Bengal will strengthen into an LPA over the next 24 hours. The LPA will gradually move along the trough in a WNW direction keeping the trough closer to the foothills. Though not a classic break in monsoon conditions which results in the monsoon trough parking itself along the foothills. The current position of the trough has certainly weakened the monsoon flow over Peninsular India.
In addition to mid level wind instabilities which allows thunderstorms to linger longer the presence of North South Trough at lower levels bring additional layer of atmospheric instability. The weak steering results in slow moving storms dumping a lot of rains to places they pass through. On a larger scale the presence of MJO over the Western Hemisphere has meant unfavourable set up for Monsoon over Peninsular India. This is expected to continue for another week to 10 days as MJO continues to move West to East. For the 1st time in the last few months MJO will possibly make a near global circuit.
Weather models indicate a fresh MJO pulse to emerge over Indian Ocean during the 2nd fortnight of August. When a fresh MJO pulse moves across East Indian Ocean it will bring about a revival of Monsoon. There is increasing indication a period of 2 to 3 weeks of active monsoon conditions is likely from end August. Until then interior Tamil Nadu will continue to seen enhanced convective thunderstorms resulting in rains during evening / night hours.
Coastal places of Tamil Nadu like Chennai are often at the mercy of the movement of thunderstorms. Very rarely thunderstorms develop within the vicinity of a coastal place like Chennai. Occasionally we see thunderstorms develop due to sea breeze front during transition seasons (Pre SWM / Pre NEM). On majority of the occasions it is the interior thunderstorms that give rains while crossing the coast. What works in favour of Chennai in addition to the remnant sea breeze front is coastal friction which enhances thunderstorms as they pass through. This results in coastal areas getting much heavier spells than interior areas despite seeing less frequent thunderstorms.