The cyclone is now around 340 km south-southeast of Chennai and is moving, an official said.
"Nilam is expected to cross the coast between Cuddalore (Tamil Nadu) and Nellore (Andhra Pradesh) today (Wednesday) evening," the weather official said.
According to the weatherman, rainfall is expected at most places over coastal Tamil Nadu and Andhra Pradesh.
Wind speed would reach around 80 kmph and prevail along and off north Tamil Nadu, Puducherry and adjoining south Andhra Pradesh coasts as Nilam nears its landfall.
The sea condition will be rough along and off north Tamil Nadu, Puducherry and south Andhra Pradesh coasts during the next 36 hours.
Storm surge of about 1 to 1.5 metre over the astronomical tide is likely to inundate the low lying areas of Chennai, Kanchipuram and Tiruvallur districts in Tamil Nadu and Nellore district of Andhra Pradesh.
Standing crops - paddy, groundnut and maize - in coastal districts of Tamil Nadu and Andhra Pradesh are likely to be damaged, said a weather bulletin.
The Tamil Nadu government has announced holiday for schools and colleges in the coastal districts.