With weakening westerlies, Stratiform (Stratus or Nimbostratus clouds) rains are possible in Chennai tonight. You would have already noticed dark low clouds without any growth in height. It will be only moderate rains with sharp spells at some places. Not all places will get rains and rains may not last beyond 15 mins
So no thunderstorms rains possible, we need to wait for some more days as suppression is seen through high pressure in Bay of Bengal.
Courtesy : Tamilnadu weatherman