Chennai has not even seen a single day of minimum temp below 20 C this year due to persistent cloud clover. Remember cloud cover at night will trap the heat escaping atmosphere making the nights warmer and this happens when easterlies are present. With NEM coming to an end, we might see high pressure taking control over peninsular India and the clear skies with nil winds.
Why not next few days, the next 5-6 days southerlies are present this might bring some cloud cover and keeping the nights warm at 22 C in the coastal areas. However, Fog will be seen in Tamil Nadu in the coming days due to lack of winds and warm winds from southerlies blowing over cooler land areas.
From 31st Jan to 4-5 days, the night temp across Tamil Nadu will dip a bit and we can see Fog returning too. Coastal districts normally remain warm being close to the sea, but this time we might see places like Chennai, Pondy and Cuddalore all dipping below 20 C. Actually for coastal places like chennai there is no actual winter, so lets even rejoice these 4-5 days of sub-20 C weather.