Most weather observers would accept when one says Sun / Heat is the driver of Rainfall. From small scale events like thunderstorms to large scale synoptic events like Monsoon everything owes something to Sun / Heat. But time to time the favourable presence of tropical waves plays a crucial role in enhancing the rainfall. Many a time we have seen favourable passage of MJO trigger cyclones under right conditions in North Indian Ocean. We have also seen constructive interplay between tropical waves create conducive atmospheric conditions. Occasionally interference between tropical waves could alter the dynamics drastically even in the short term window.
Over the past week or so after a long delay summer thunderstorms picked up over Peninsular India. In particular interior areas and places along the Ghats saw thunderstorms regularly triggered by LWD conditions. While South coastal areas saw some moderate rainfall activity so far North Coastal TN has remained a spectator mostly. This is par for course if one looks at wind pattern over places like Chennai where Easterlies hold sway.
Weather models indicate Peninsular India may see an enhanced episode of Summer thunderstorms. A combination of Kelvin / MJO and ER may in turns help to enhance rainfall activity this week. With a cyclonic circulation seen off South Kerala over the Comorin Sea / Arabian Sea area conditions remain favourable. The Easterlies / Southerlies due to the circulation may favour places along the Ghats in Kerala & South TN mostly. Just like NEM time though North coastal areas may see light / moderate rains during morning hours. For North Coastal TN to get a good spell of rains during summer one may need a strong circulation.
But the presence of multiple waves competing with each to other to take control may potentially limit the possibility of a strong circulation. This is possibly the reason why models are struggling to identify the potential evolution of the current cyclonic circulation. While some expect it to move further East a few like ECMWF expect it to remain quasi stationery. Nevertheless most models are in sync about possibly a good week of rains ahead for South TN, Kerala and a few places along the Ghats in Karnataka. Depending on how they estimate the circulation rains for Chennai are estimated from few mm to few cms among models. Most models are in sync about above average rainfall for most parts of Peninsular India this week. Similarly there is high consistency about Summer coming to a halt this week due to the circulation induced rains.
If one looks a few days further models indicate a possible genesis of an LPA near Andaman during May 3rd week. Once things fall in place for this LPA there is a very high chance it may move towards North Bay. This would also mean strengthening of the westerlies over Peninsular India. It remains to be seen if the LPA strengthens into potentially a cyclone just before Monsoon onset window. Given the abnormally hot ocean surfaces over Bay of Bengal one cannot rule out a cyclone during end May.
This essentially means the rainfall window for Chennai and rest of North Coastal TN is effectively a couple of days. This window will be available when the cyclonic circulation is either East of Sri Lanka or stays over GOM. It is pertinent to keep in mind the mean rainfall for Chennai during the month of May is 5 cms. As things stand there is very less indication the upcoming rainfall episode may exceed the May average rainfall.