Friday was consistently cloudy and rainy in phases in Calcutta as a system kept getting stronger over the Bay of Bengal.

The forecast was for "heavy to very heavy rain (70mm to 200mm)" on Friday. The forecast was for 24 hours between 8.30am on Friday and 8.30am on Saturday. Till 8.30pm on Friday, the Met office had recorded around 35mm of rain in Alipore.

A Met official said heavy rain was not ruled out in Calcutta on Saturday.

"Generally cloudy sky. Heavy rain/thundershower with one or two intense spells very likely to occur over the next 24 hours," said the Met forecast for the city, issued around 5.30pm on Friday.

As it rained on Friday afternoon, the protesting doctors camping near Swasthya Bhavan huddled under tarpaulin sheets. Many of them got drenched but stayed put.

Dark clouds hovered over the city from forenoon. The rain came in multiple phases. The showers were often accompanied by gusts of wind.

The swaying trees and slamming window panes suggested that it was not just another spell of passing monsoon showers.

What was a cyclonic circulation over southeast Bangladesh on Thursday morning had by the night intensified into a low-pressure area, said a Met official.

"It moved west-northwestwards and was a well-marked low pressure area over the northeast Bay of Bengal and adjoining southeast Bangladesh coast around 8.30am on Friday," he said.

"It is likely to move west-northwestwards and intensify into a depression over coastal Bengal and the adjoining northwest Bay of Bengal by Saturday," he said.

"The system would extend to 50km to 60km inland from the coast," said the Met official.

A Met bulletin predicted "extremely heavy rain (more than 200mm)" in Bankura and West Midnapore on Saturday as the depression moved northwest. Heavy to very heavy rain- fall is likely in East and West Burdwan, East Midnapore, Jhargram, Birbhum and Purulia and heavy rainfall is likely in the remaining districts of south Bengal on Saturday.

On Sunday, heavy rain is likely in Jhargram, Purulia, West Burdwan and Birbhum, the bulletin said.

On the sea, a depression can keep intensifying, turning into a deep depression first and a cyclone next. But this system is expected to lose steam after becoming a depression, said Met officials.

The Met bulletin said surface wind speeds reaching 30 to 40kmph, gusting up to 50kmph, are very likely in the districts of North and South 24-Parganas, Hooghly, Howrah, Calcutta, East and West Midnapore and Jhargram on Saturday.

The Met bulletin advised fishermen not to venture into the sea.

"Squally weather with surface wind speed reaching 35-45kmph gusting to 55kmph is very likely along and off West Bengal, Bangladesh and Odisha coasts and North Bay of Bengal till Sunday. Sea conditions are likely to be rough to very rough. Fishermen are advised not to venture into the sea along and off West BengalBangladesh and Odisha coasts and North Bay of Bengal till Sunday," it said.