The department also revealed the timings when the cyclone may hit Tamil Nadu and Andhra Pradesh where emergency alert has already been issued.
According to Met sources, the cyclone will hit the two states by Wednesday afternoon. MR Ramanan, the IMD director, was quoted as saying, "Cyclone Nilam will hit the area between Nellore and Nagapattinam by Wednesday evening."
Chennai, Kancheepuram, Cuddalore and Villupuram received heavy rains in the morning, inconveniencing office-goers.
The Met department official earlier had informed, "The deep depression over southwest Bay of Bengal moved westwards, and lay centred at 1800 UTC of yesterday over southwest Bay of Bengal near latitude 9.00N and longitude 82.00E, about 500 km south-southeast of Chennai and 100 km east-northeast of Trincomalee (Sri Lanka)."
"The system would intensify further into a cyclonic storm and move westwards for some more time and come very close to Sri Lanka coast and then move northwestwards and cross north Tamil Nadu and adjoining south Andhra Pradesh coast between Nagapattinam and Nellore by tomorrow afternoon," added the official.
It forecast "heavy to extremely heavy rainfall" up to 25 cm or more besides squally winds reaching up to 45-55 kmph of speed and gusting up to 65 kmph off north Tamil Nadu, Puducherry and adjoining south Andhra Pradesh coasts during next 12 hours, it said.
"The wind speed would gradually increase thereafter as the system comes closer to the coast. Sea condition will be very rough to high along and off north Tamil Nadu, Puducherry and adjoining south Andhra Pradesh coasts during next 48 hours. Fishermen along north Tamil Nadu, Puducherry and adjoining south Andhra Pradesh coasts are advised not to venture into the sea," the department said.
(With PTI inputs)