Independence Day weekend is likely to be rainy in the city.

A low-pressure area is expected to form over the Bay of Bengal from Thursday, the Met office has said. Under its influence, there is likely to be a rise in rainfall in south Bengal between Friday and Sunday.

The monsoon trough, passing over south Bengal, has been triggering rainfall in many places, including in Calcutta. The intensity is likely to go up because of the low-pressure system, the Met office has said.

Several parts on the southern fringe of the city and South 24-Parganas received a sharp spell of rain on Wednesday afternoon.

“The system is a cyclonic circulation on the north-west Bay of Bengal as of Wednesday and is expected to intensify into a low-pressure area on Thursday. The next three days are likely to see an increased amount of rainfall,” said G.K. Das, the director of the India Meteorological Department, Calcutta.

“The rainfall could be heavy if the system is strong. But it is too early to predict. Things will become clearer on Thursday,” he said.

August has so far received over 130mm of rain. The normal quota for the month is around 345mm.

July ended with a marginal rain deficit in the city. Most of the showers over the month were because of local cloud formation. They were quick, sharp spells unlike slow and steady monsoon rain. A system over the Bay of Bengal — that could have dragged the monsoon trough down from the foothills of the Himalayas — was absent.

A low-pressure system over the northern Bay of Bengal had been tipped to bring heavy rain in the city and the rest of south Bengal last week. Calcutta received 40mm of rain last Tuesday and around 25mm last Wednesday. Heavy rain, in Met parlance, is over 60mm in 24 hours. The system’s route was farther from the city than expected, the Met office had said.

The monsoon trough is now passing over Santiniketan and Canning to the north-east Bay of Bengal, a Met official said.

“If there are a couple of more such systems after this weekend, the August rain will be normal or above,” said the official.

The cumulative volume of rain in the city so far — from June 1 to August 11 — has been over 850mm compared with the usual quote of around 800mm, the official said.