The system, named 'Nilam', as suggested by Pakistan under the cyclone naming procedure, would move northwestwards and cross the coast between Nagapattinam and Nellore by Wednesday afternoon or evening.
Under the influence of the storm, rainfall at most places, with isolated heavy to very heavy rainfall, would occur over coastal Tamil Nadu and Puducherry during next 12 hours, a bulletin issued by Area Cyclone Warning Centre said.
It forecasts heavy to very heavy rainfall and isolated extremely heavy rainfall (25 cm or more) in the region during the subsequent 36 hours.
"Squally winds speed reaching 45-55 kmph gusting to 65 kmph would prevail along and off north Tamil Nadu, Puducherry and adjoining south Andhra Pradesh coasts during next 12 hours. The wind speed would gradually increase thereafter as the system comes closer to coast," it said.
Sea will be "very rough to high" and fishermen in Tamil Nadu, Puducherry and adjoining south Andhra Pradesh coasts have been advised to stay off the sea.
Windspeeds can reach upto 90 km in Chennai and there could be some disruption in communication lines, the bulletin said.
Danger signals ranging from five to seven have been hoisted at Chennai, Ennore, Cuddalore, Nagapattinam and Puducherry ports. In Chennai, normal life was hit by incessant rains since morning.
A holiday was declared today for schools and colleges in Chennai and some other districts.