Heavy rain is on its way despite a clear sky and bright sun for most of Monday’s first half.

A low-pressure area has formed over the north Bay of Bengal and the system is gaining momentum, a Met official said. Its effect is likely to be most pronounced in the city between Tuesday afternoon and Wednesday morning, according to the forecast.

“A low-pressure area has formed over the north Bay of Bengal and its neighbourhood on August 24. It is likely to become more marked during the next 48 hours. Under its influence, widespread rainfall is likely to occur over the districts of south Bengal between August 24 and 27,” said a Met bulletin on Monday afternoon.

The system is likely to travel westwards passing over the northern fringe of Calcutta between Tuesday evening and early Wednesday. “Between Tuesday evening and Wednesday morning, at least one spell of heavy rain is due in the city,” a Met official said.

The city was bracing for heavy rain on Monday as well, but the morning was dry and sunny. The sky started becoming overcast in the afternoon and showers started by evening in many parts of the city. The Met office recorded around 30mm of rain in Alipore till 8.30pm on Monday.

The system is expected to travel westwards from the Bay. Bengal’s western districts are likely to get more rain from Wednesday, the official said.