A fresh low-pressure area over the Bay of Bengal is likely to trigger heavy rain in Calcutta over the weekend, the Met office said.

“There is a possibility of the formation of a low-pressure area on Friday over the west-central Bay and the adjoining northwest Bay,” said Somenath Dutta, the head of the India Meteorological Department, Calcutta.

On Saturday and Sunday, widespread rain is likely in all the districts of south Bengal and five districts of north Bengal.

The Met office has issued a heavy rain alert for East and West Midnapore, North and South 24-Parganas, Calcutta, Howrah and Hooghly on Saturday.

On Sunday, heavy rain is likely in South 24-Parganas, East and West Midnapore, Jhargram, Bankura, Purulia and West Burdwan in south Bengal and Kalimpong, Jalpaiguri and Alipurduar in north Bengal.

In Met parlance, more than 60mm in 24 hours qualifies as heavy rain.

Monsoon rain in lower Gangetic Bengal is dependent on low-pressure systems over the Bay of Bengal. A low-pressure over the Bay had on June 29 triggered 92mm of rain in Calcutta, the heaviest so far this monsoon on a single day. Until then, the rain deficit in the city was around 80 per cent.

On Monday, another low-pressure area took shape over the northwest and adjoining west-central Bay. But it was a relatively weak system and did not persist for long.

It moved to Chhattisgarh the next day. By Wednesday, it had moved further west and was over western Madhya Pradesh.

The system does not have any direct impact on Bengal but the monsoon trough is passing through that low-pressure area on its way to the Bay.

“The direction of the winds to the north of the trough is southeasterly. As a result, south Bengal is getting southeasterly winds. Which means moist winds from the Bay. That is why the sky is partly cloudy almost every day,” Dutta said.

The sky was cloudy on Wednesday with brief periods of sunshine. Many parts of Calcutta received a spell of light rain on Wednesday afternoon.