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It is often remarked as far as weather goes you are never in isolation. Events that happene many thousands of kms away tend to influence weather at your place. Southwest Monsoon obviously is a classic example of how events from afar can trigger large scale weather phenomenons. Similarly even a mesoscale event like convective thunderstorms could come under the influence of weather factors that are many hundred kms away.

While the background behind break in monsoon period thunderstorms for the leeward areas of Tamil Nadu has often been discussed it is interesting to also observe one another phenomenon which happens during a narrow window of opportunity. Thunderstorms that happen during the phase when one circulation sits embedded in the monsoon trough to the NW of Tamil Nadu longtitude while a fresh pulse is under evolution over North Bay area over the eastern end of the monsoon trough. This window is normally available for 3 / 4 days and depending on localized factors like lower level wind convergence / sea breeze intrusion etc thunderstorms pop up over pockets of Tamil Nadu.

Depending on the location of monsoon trough the wind instabilities also start showing up over the leeward areas of Tamil Nadu in the form of weakening westerlies and at times wind convergence as well along the coast. This along with the day time heating sets up the template for what IMD remarks as Veppasalanam Rains in their daily forecast bulletin. It is without doubt a set of various factors which need to fall in place at the right time and at the right place for thunderstorms to happen during the evening / night hours.

Weather models indicate over the next 3 / 4 days possibly until the weekend this window of thunderstorm activity to benefit Tamil Nadu. With westerlies expected to remain weak during this period coastal places like Chennai may require some luck in the form of fairly intense thunderstorms to happen around 50 to 75 kms off the coastline which eventually cross coast in a generic West to East direction under their own steerig mechamism rather than wait for atmospheric steering to guide them towards the coast.