Over the past few days parts of Peninsular India, particularly Tamil Nadu, has been seeing fairly widespread thunderstorms. As the monsoon trough became less prominent and moved closer to Himalayas westerlies weakened over Peninsular India. This opened a window of opportunity for thunderstorms over the leeward plains of TN. Though Chennai may feel disappointed with the rains some parts of TN has seen seasonal average in a few days.
Villupuram recorded nearly 500 mm in a few days with 220 mm on Saturday and 180 mm on Friday night. Pondy recorded 155 mm on Friday breaking the highest 24 rainfall recorded during the month of August. In both the extreme rainfall instances at Villupuram and Pondy bulk of the rains happened in a couple of hours. For the 7th straight day the daily rainfall average for TN & PDC is in excess and will continue for the next few days. The seasonal average for TN & PDC according to RMC Chennai now stands at a whopping 81% excess as of yesterday.
In addition to the weakening of Westerlies a Cyclonic Circulation over the Comorin Sea area has altered the wind pattern. While the southern parts of Peninsular India is under Easterlies northern parts are seeing Northerlies / Westerlies. This has created a zone of wind instability all the way from South AP to South TN. This zone of instability provides the platform for thunderstorms to develop and sustain for long hours. Additionally the weak steering means storms dump a lot of rains over the areas it passes through.
As the cyclonic circulation moves west across Comorin Sea this zone of instability will also shift inland into Peninsular India. Over the next few days interior areas of Peninsular India will continue to see daily thunderstorm activity. This is expected to continue for the next week to 10 days before an incoming MJO pulse switches things. Monsoon dynamics on the back fo MJO influenced North propagating MISO will pick up during 2nd fortnight of August. As the MISO moves up Peninsular West coast is expected to see increased rainfall. MJO will possibly move in a more traditional manner from now on for the remaining SWM season.
Over the next couple of days parts of West TN and Central TN may see intense thunderstorm activity. This will push into South Interior Karnataka gradually during the week as the circulation shifts west. Weather models indicate temporary Easterlies to prevail over southern parts of Peninsular for most of this week and early next week. During this period we may see West TN come under thunderstorms similar to the transition season. Question marks will remain over rainfall prospects for coastal areas like Chennai until the winds are from East. One thing that is fairly sure, afternoons will remain very hot over Chennai under fairly clear skies.