Cyclone Dana’s precursor curled up in the form of a depression over the Bay of Bengal around 750km from the Bengal coast on Tuesday morning.

It is set to hit the Odisha-Bengal coastline as a severe cyclonic storm between Thursday night and Friday morning, unleashing a peak wind speed of 120km an hour, the Met office said.

Evacuations are currently under way in coastal regions of Odisha and Bengal as preparations intensify. South Bengal is anticipated to experience rainfall starting Wednesday.

For Calcutta, the forecast is heavy rain on Thursday and Friday. Strong winds are predicted between Thursday night and Friday morning.

The clouds from the system will start triggering rain in south Bengal on Wednesday, according to the Met forecast. The intensity of the showers will go up as the storm nears land. Extremely heavy rain (more than 200mm) is expected in coastal areas of East and West Midnapore and South 24-Parganas. Calcutta, Howrah, Hooghly and North 24-Parganas are bracing for heavy to very heavy rain (70mm to 200mm).

A Met bulletin said the well-marked low-pressure area had intensified into a depression over the east-central Bay of Bengal on Tuesday morning.

The system moved west-northwest with a speed of 7kmph and at 5.30pm on Tuesday it was around 690km southeast of Paradip (Odisha), 740km south-southeast of Sagar Island (Bengal) and 710km south-southeast of Khepupara (Bangladesh).

“It is very likely to move west-northwest and intensify into a cyclonic storm over east-central Bay of Bengal by Wednesday.

“Thereafter, continuing to move northwest, it is very likely to intensify into a severe cyclonic storm over northwest Bay of Bengal by Thursday morning and cross north Odisha and the Bengal coastline between Puri and Sagar Island between Thursday night and Friday morning as a severe cyclonic storm with a wind speed of 100-110kmph gusting 120kmph,” said the Met bulletin issued on Tuesday afternoon.

Some private forecasting agencies suggested a possible landfall area between Bhadrak and Paradip in Odisha.

The forecast track projected by the Met office showed a similar path. But the accompanying cone of uncertainty was between Puri and Sagar Island, a stretch of around 450km.

“A cyclone has a far-reaching impact. It can cause rain up to 500km from the landfall area. The effect of winds is felt up to 200-250km,” said a Met official.

The core, or the eye of a cyclonic storm, is a relatively quiet zone, surrounded by spiralling bands of winds.

As a storm passes, an area experiences the peripheral winds in the front. As the relatively calm core passes through land, there is a brief lull. That is followed by the passage of the rear winds, which are just as strong as the ones at the head of the spiral.

Squally winds with a speed of 40-50kmph gusting to 60kmph are likely in coastal areas of East Midnapore and South 24-Parganas on Wednesday. The maximum wind speed is tipped to reach 80kmph in these areas from Thursday morning.

Between Thursday evening and Friday morning, during the time of landfall,the peak wind speed is expected to reach 120kmph in East Midnapore and 110kmph in Sagar Island and the Sunderbans.

West Midnapore, Jhargram and other areas in South 24-Parganas are likely to be buffeted by a peak wind speed of 90kmph. Calcutta, Howrah, Hooghly and North 24-Parganas will be struck by winds clocking 70kmph, according to the forecast.