Saturday was consistently cloudy and rainy in phases in Calcutta.

According to the Met forecast, the city and most other parts of south Bengal will continue to be rainy. Gangetic Bengal districts will likely get light to moderate rain with thundershowers over the next few days.

"The spread and intensity of the showers may dip on Sunday and Monday. The flow of the monsoon currents may weaken a little. But they are expected to gain steam from Tuesday again," said H.R. Biswas, head of the weather section at the Regional Meteorological Centre, Calcutta.

"Another cyclonic circulation may take shape after August 15, over the sea or Gangetic Bengal. Under its influence, the volume of rain is expected to go up," he said.

The favourable position of the monsoon trough and a cyclonic circulation over eastern Uttar Pradesh contributed to the overcast conditions, the Met office said.

The trough's position is expected to trigger more moisture incursion in the coming days.

Another system is expected to take shape over the Bay of Bengal or Gangetic Bengal around Independence Day, said Met officials.

For almost all of June and a substantial part of July, the trough — an imaginary line connecting the low-pressure points from west to east, it is not called monsoon trough in Met parlance unless the southwest monsoon covers the entire country — was passing through the foothills of the Himalayas.

But since late July, the trough has mostly been passing through south Bengal.

"The monsoon trough at mean sea level now passes through Bikaner, Rohtak, Fatehgarh, Churk, Purulia, Contai and thence east-southeastwards to northeast Bay of Bengal," said the daily weather report on Saturday.

The normal position of the trough is via Allahabad, Gaya and Sagar Islands into the Bay, said Biswas. Sagar Island is just over 100km from the heart of Calcutta. Contai is around 150km from the city.

The monsoon deficit in Calcutta is now around 26 per cent.

"From June 1 to August 10, the city has received 587.1mm of rain. The usual volume it should have received in this period is 796.9mm," said Biswas.