Over the past few days as Monsoon made quick progress across the Indian sub continent the leeward areas of Tamil Nadu saw a reduction in thunderstorm activity. As we often mention subcontinental weather oscillates like a Pendulum often. The two large scale events, SWM & NEM, swing back and forth every year bringing associated rains with it. During Southwest Monsoon season the rains swing back and forth between West and East Coast during strong and weak phases. The west coast receives widespread rains when the monsoon is strong . The leeward east coast gets it’s rains through thunderstorm activity when the monsoon weakens.
The critical point to be noted here is the weakening westerlies during enhanced thunderstorm activity over the East Coast. When the winds weaken it allows for convection to build up over the interior areas, particularly around Eastern Ghats. When the winds are strong they shear off the convection build up preventing the forming of thunderstorms. Except for a narrow patch in UP, parts of Rajasthan, Haryana and Punjab monsoon has covered most of the country. Ironically this narrow patch got sandwiched on both sides, North & South by areas where monsoon has arrived.
oYesterday morning Delhi NCR and surrounding areas saw monsoon arrive with a bang. A morning which brought more than 23 cms of rains in 6 hours. Some models like UKMO / GEM etc indicated moderate to isolated heavy rains around Delhi NCR region. But no model indicated an event of such extreme proportions is about to unravel. This once again confirms disaster managers have their hands full with sudden urban flooding events in the days to come.
After a couple of days of widespread rains along the Peninsular coast and in the Ghats monsoon has started to weaken. Though Pandalur and Devala in Nilgiris came under an extreme spell of rains due to trapped moisture overall monsoon has weakened considerably over the southern parts of Peninsular India. This weakening now opens a window of thunderstorm activity for the leeward areas of Tamil Nadu. Sub seasonal models have been consistent about this weak phase of Monsoon over southern parts of Peninsular India
A northward propagating MISO will favour the Indo Gangetic plains for the next couple of weeks. This phase is likely to bring possible flooding events in the IGP, Terai Plains and some parts of Himalayas. At the same time the upcoming week is also likely to see enhanced thunderstorm activity for leeward areas of Tamil Nadu. The extended Extreme Rainfall Index from ECMWF show a weak probability of anomalous rains around North Coastal TN also confirm the enhanced thunderstorm activity.
From today we may see a gradual return of thunderstorms over the interior areas which will eventually move towards the coast. Weather models indicate tomorrow and day after we may see fairly widespread thunderstorms around North TN and South AP on the back of weakening westerlies. The average rainfall during the month of July is about 11 cms for Chennai and suburbs. There is increasing confidence the mean rainfall for the month may be achieved by the first week of July.