The deep depression, which has been categorized as a 'very severe cyclonic storm', was "practically stationary" at 2.30 pm on Wednesday at about 470 km east-northeast of Chennai and 290 km south-southeast of Vishakapatnam, it said.
It has cautioned about extensive damage to thatched houses, uprooting of trees, partial damage to telecommunication lines and flooding due to Helen.
Heavy to very heavy rainfall has been forecast from Wednesday along coastal Andhra Pradesh and in Tamil Nadu under the influence of the system.
"Helen, which spreads over a geographical area of 300 to 400 km, is currently moving at a speed of 5kmph, is stationery till now, the speed will increase after the next 12 hours. The system would further intensify into a severe cyclonic storm during the next 24 hours.
"It would move west-northwestwards for sometime, then west-southwestwards and cross south Andhra Pradesh coast between Nellore and Machillipatnam close to Ongole around morning of November 22," M Mohapatra, a scientist at the met department, told reporters in New Delhi.
Storm surge of about 1 to 1.5 m height would inundate low lying areas of Nellore, Prakasham, Guntur and Krishna districts of Andhra Pradeh at the time of the landfall, he said, adding wind speed would reach up to 120 kmph.
He, though, made it clear that its intensity would be "much less than" cyclone Phailin which had hit the Odisha coast last month.
"The intensity of Helen could be compared to that of cyclone Laila which hit Andhra Pradesh in 2010", he said.
While the met department has suggested total suspension of fishing operation, dwellers along the coast have been suggested to move to safer places.
He said they are constantly updating the state government and disaster management agencies about the movement of the cyclone.