Vardah lay 220 kilometre east-northeast of Chennai and 290 kilometre east-southeast of Nellore, the Meteorological office said.
"The system is very likely to move nearly westwards and weaken gradually while moving towards north Tamil Nadu and adjoining south Andhra Pradesh coasts," the office said.
"It is very likely to cross north Tamil Nadu and south Andhra Pradesh coasts, close to Chennai as a cyclonic storm with a wind speed of 80 to 90 kmph gusting to 100 kmph by 12th December 2016 afternoon," it said.
Heavy rains lashed Chennai, Tiruvallore and Kancheepuram districts since early morning even as squally winds were witnessed in these districts. Power supply was suspended in many parts of these regions as a precautionary measure.
The armed forces have been asked to be on standby with the amy, navy and air force prepared to be deployed anytime as and when required.
People in low-lying areas have been asked to move to safer zones even as various arms of the government were prepared to meet any eventuality.
Flight services were so far operating smoothly, airport officials said.