The city is likely to get heavy rain over the next few days, a forecast that holds the promise of making up for the shower deficit in June and July.
Satellite pictures show a cyclonic circulation over the Bay of Bengal by the Bengal coast, which weather scientists say will boost moisture flow from the sea and might trigger heavy rain over the next few days.
It has rained every day since August 1, adding up to 80.4mm, which is 14.1 per cent more than normal. In contrast, June and July had run up a deficit of 57 per cent and 17.5 per cent, respectively. The combined deficit of the two months was 32 per cent.
An overcast sky over the Bypass on Sunday afternoon. Picture by Sanat Kr. Sinha
"The deficit in July, usually the year's rainiest month, would have been much higher but for the 100mm of rain the city had received in a few hours on 25th," a Met department official said.
"A cyclonic circulation is forming over the Bay of Bengal near the Bengal coast and we expect heavy rainfall in Calcutta and the rest of the state. The rain will continue for two-three days," said Gokul Chandra Debnath, deputy director general, India Meteorological Department, Calcutta.
According to the IMD website, the monsoon trough, around which rainfall occurs, is passing close to the city. The monsoon trough is an imaginary line connecting the low-pressure points across the breadth of the country.
A cyclonic circulation as well as a monsoon trough have an atmospheric pressure lower than their surroundings. So, it draws air towards itself.
Meteorologists pointed out that weather systems like cyclonic circulations and low-pressure areas have formed away from the Bengal coast so far this monsoon.
Some have formed closer to the Odisha-Andhra Pradesh coast, some near the Bangladesh coast and the rest over land, in Madhya Pradesh and Jharkhand.
"For the first time this monsoon, a system that can facilitate heavy rainfall has formed close to Calcutta. That is why heavy and uniform rainfall is expected in the city", a senior Met official said.