A depression along Bengal’s coast is expected to bring “heavy to very heavy rain” in the city and its neighbourhoods on Thursday and Friday, the Alipore Met office warned on Wednesday.

Winds clocking up to 50km an hour will likely accompany the rain.

The Met office said a low-pressure area, which had formed over the northwest Bay of Bengal off the Odisha coast, became “well-marked” on Wednesday. It is slowly progressing northwards, towards the Odisha-Bengal coast.

“It is expected to turn into a depression by Thursday and bring in a lot of moisture and clouds that will trigger heavy to very heavy rain,” said the Met office.

In Met parlance, rainfall between 64.5mm and 115.5mm is classified as heavy rain, and rainfall between 115.6mm and 204.4mm is very heavy rain.

The weather office said Calcutta and its neighbourhoods are likely to receive maximum rain between Thursday afternoon and Friday afternoon.

If the forecast holds, it will be the first spell of such heavy rain this year.

Sources in the Kolkata Municipal Corporation (KMC) said they had deployed additional personnel in the drainage pumping stations and the lock gates to drain out water as quickly as possible.

“Such heavy rain, if it happens, is bound to cause waterlogging for some time at least. But we have deployed additional personnel to deal with the situation,” said a KMCofficial.

The additional personnel, usually deployed from June 1 every year as part of monsoon preparedness, will be deployed between Thursday and Saturday.

“The personnel will ensure that the pumps run to their full capacity and that the lock gates are lifted quickly after high tide in the Hooghly recedes, to ensure that the water is drained out fast,” said the official.

A bulletin issued by the Alipore Met office on Wednesday mentioned heavy to very heavy rain “was very likely to occur” in one or two places in East Burdwan, East Midnapore, North- and South-24 Parganas, Calcutta, Howrah, Hooghly, Nadia and Murshidabaddistricts of south Bengal.

In the remaining districts of south Bengal, heavy rain has been forecast.

“The low pressure over the northwest Bay of Bengal that had formed earlier became well-marked on Wednesday. It is likely to develop into a depression by Thursday. Calcutta and the rest of south Bengal will receive maximum rain between Thursday afternoon and Friday afternoon,” said an official of the India Meteorological Department (IMD), Calcutta.

A low-pressure area forms when the wind pressure over a large zone is lower than its neighbouring areas.

The low pressure intensifies into a depression when the gap in wind pressure between two neighbouring zoneswidens.

“There will be squally winds between 40 and 50kmph accompanying the rain,” said the official.

Fishermen were warned not to venture into the sea till Saturday morning.

“North Bengal will experience maximum rain on Friday,” the official said.

The maximum and minimum temperatures on Thursday are likely to be around 30 degrees and 26 degrees Celsius, respectively, the Met office said.

Wednesday’s maximum and minimum temperatures were 31.4 degrees and 25.1 degrees, 3.7 and 2.1 notches below normal, respectively.In its bulletin, the IMD warned that people should be ready to move to safe places if the weather worsens.

“Take shelter in a safe place during thunderstorm activity” and “avoid taking shelter under a tree or an electric pole, and avoid contact with water bodies” during lightning and thunderstorms, the bulletin mentioned.