A "very severe" cyclonic storm lay centered about 250 km of Chennai, with the weather office forecasting winds touching up to 135 kph and advising fishermen against venturing into sea for the next two days.
"The very severe cyclonic storm 'THANE' over southwest Bay of Bengal moved west-southwestward and lay centered about 250 km east-southeast of Chennai, 270 km of Puduchery and 420 km north-northeast of Trincomalee (Sri Lanka).
"The system is likely to move westwards and cross north Tamil Nadu coast between Nagapattinam and Chennai, close to Puducherry around early morning of December 30th," the regional Meteorological department said.
It forecast rainfall at most places with isolated heavy rainfall over north Tamil Nadu, Puducherry and south coastal Andhra Pradesh, saying the intensity of the rainfall would increase from heavy to very heavy, with the likelihood of even recording 25 cm or more over North Tamil Nadu and Puducherry during the next 48 hours.
Squally winds speed reaching 55-65 kmph gusting to 75 kmph is likely along and off north Tamil Nadu, Puducherry and south Andhra Pradesh coasts. The windspeed will increase, gradually touching 110-120 kmph gusting to 135 kmph along and off north Tamil Nadu and adjoining south Andhra Pradesh coast from tonight onwards, it said.
Storm surge of about 1 meter height above the astronomical tide would inundate low lying areas of Chennai, Tiruvallur, Kanchipuram and Villupuram districts in the state at the time of landfall, the department said.
Sea condition was 'very rough' and it would gradually become very high to 'phenomenal' along and off north Tamil Nadu, Puducherry and south Andhra Pradesh coasts.
Fishermen along north Tamil Nadu, Puducherry and south Andhra Pradesh coasts are advised not to venture into sea during the next 48 hours, it added.
"Extensive damage" to thatched roof and huts and minor damage to power and communication lines due to uprooting of large trees and flooding of escape routes were the "expected damage" it said, adding standing crops could also be affected.
The weather office suggested total suspension of fishing activities besides asking coastal hutment dwellers to move to safer places.