A deep depression formed over the Bay of Bengal was likely to cross the Tamil Nadu-Andhra Pradesh coast on Monday night, the weather office in Chennai said.
The forecast warned of heavy rains in the next couple of days even as authorities took precautionary measures.
The system was likely to intensify and move in a westerly direction, the weather office said.
It added that heavy to very heavy rains were likely to occur during the next 48 hours in north coastal Tamil Nadu and Pondicherry.
The depression, lying about 300 kilometres off the coast, will cause winds with speeds reaching 50-60 km were likely to be experienced in the coastal areas of north Tamil Nadu, Pondicherry and southern Andhra Pradesh during the next 36 hours, it said.
A cyclone alert has been sounded in Cudallore and neighbouring districts.
Authorities in various districts took precautionary measures in view of the rains since Sunday.
Schools and colleges in Cuddalore, Kancheepuram, Chennai and Tiruvallur districts have been closed.