The cyclone is expected to cross between Puducherry and Nellore in Andhra Pradesh close to Chennai this evening.
"The system would intensify further into a severe cyclonic storm and move north-westwards and cross north Tamil Nadu and adjoining south Andhra Pradesh coast between Puducherry and Nellore close to Chennai by this evening, it said in its latest bulletin.
Gusty winds with a maximum surface wind speed of 45 kmph to 110 kmph is expected, it said.
Extremely heavy rainfall of over 25 cm or more is expected over north Coastal Tamil Nadu and Puducherry in the next 24 hours.
Rainfall at most places with isolated heavy to very heavy falls would occur over south coastal Andhra Pradesh, Rayalaseema and north interior Tamil Nadu in the next 48 hours with gale speeds of 90-100 kmph, gusting to 110 kmph along and off north Tamil Nadu, Puducherry and adjoining south Andhra Pradesh coasts.
The sea condition would be "high to very high" along and off North Tamil Nadu, Puducherry and South Andhra Pradesh coast, the Met office said.
Storm surge of about 1-1.5 metre over the astronomical tide is likely to inundate low lying areas of Chennai, Kancheepuram, Tiruvallur and Nellore districts, it said.
The Met office also advised total suspension of fishing operations and for coastal dwellers to move to safer places.
Danger signals ranging from seven to five have been hoisted at Chennai, Ennore, Cuddalore, Nagapattinam, and Puducherry ports.
Local cautionary signal number three has been hoisted at Pamban and Thoothukudi ports, it said.
As cyclone nears TN coast Kalpakkam Nuke plant goes on alert
With cyclone 'Nilam' intensifying into a severe storm, Madras Atomic Power Station at coastal Kalpakkam today said the situation was being "closely watched" and asserted the plant is designed to withstand a wind velocity of 160 kmph.
Presently, both the reactors at MAPS were operating "safely", its Director K Ramamurthy said in a statement as 'Nilam' cyclone was expected to cross Tamil Nadu-Andhra Coast close to Chennai this evening.
The Met office has forecast gusty winds with a maximum surface wind speed of 45 kmph to 110 kmph.
"Kalpakkam being a coastal site (about 55 km from Chennai), all the structures and equipment were designed to withstand even the wind velocity of 160 km per hour," he said.
Ramamurthy said all agencies have been instructed to be on standby alert. Additional personnel have been deployed to keep vigil on "important systems" and "the situation is being watched closely for any developments and to take immediate appropriate action," he said.