Yesterday was the 3rd day this July Chennai saw temperature exceeding 40⁰C with both the observatories recording above 40⁰C for the first time this July. Pondicherry continues to be under extreme heat wave conditions with second consecutive day of 40.9⁰C. Yesterday also saw Cuddalore touch 40⁰C for the first time this July.
The conditions are expected to continue for a further couple of days at least as South Andhra Pradesh & adjoining Coastal Tamil Nadu possibly become the hottest areas in India (excluding the desert regions of Western India) with the places along the coastal region from Chennai to Pondicherry possibly crossing 40⁰C at a few places. There is unlikely to be any thunderstorms in Tamil Nadu today, all in all it is going to be an extremely hot day move most parts of Tamil Nadu especially in North Tamil Nadu.
Chennai Weather Update: The temperatures in Chennai are expected to cross 40 Degrees and is likely to be higher than yesterday with a good possibility of today’s max coming close to the all time record of 41.1⁰C. Sea Breeze is expected to set in late in the evening after 4 PM and is possibly going to be very weak until about 7 or 8 PM thereby making today possibly one of the most uncomfortable day. As you can see in the graph below today early morning temperatures are almost a degree higher than yesterday and the minimum is expected to be around a very uncomfortable 31⁰C. 
Please avoid going out in the afternoon, if your job involves moving out around the afternoon time ensure adequate fluid intake is done to prevent heat stroke. Today will be Chennai Weather at its worst.