The city has been under a twin assault of heat and humidity for the past couple of days as a system is brewing on the Bay of Bengal.

The Met office said the sweaty conditions are likely to persist for now.

“Partly cloudy sky. Possibility of light to moderate rain/thunderstorm,” stated the official forecast for Monday.

The conditions feel more tormenting because the city was under a long rainy spell till recently, said a Met official.

On Sunday, the maximum temperature was around 35 degrees Celsius, two notches above normal. The minimum relative humidity was close to 60 per cent.

That means even during the driest part of the day, the humidity was so high that even a brief while spent outdoors was very taxing.

Last Sunday (September 15), the maximum temperature was around 26.5 degrees Celsius, almost seven notches below normal. On the preceding Friday and Saturday (September 13 and 14), Calcutta had seen over 100mm of rain as a deep depression moved inland through south Bengal towards Jharkhand.

“The hot and humid conditions are being felt more because of the long west spell that preceded it,” said a Met official.

He said this heat and humidity was typical of late September, when the monsoon was nearing its end.

“The withdrawal of monsoon is due to start from Rajasthan very soon. This type of weather is typical of early June, the time of arrival of monsoon, and late September, when monsoon is set to withdraw,” he said.

Met radars are continuously observing a fresh system on the Bay.

“An east-west trough runs from Andhra Pradesh coast to south coastal Myanmar with two embedded upper air cyclonic circulations, one over Westcentral Bay of Bengal and another over south coastal Myanmar and neighbourhood. The later circulation is likely to move west-northwestwards. Under the influence of these two upper air cyclonic circulations, a low-pressure area is likely to form over west-central Bay of Bengal and neighbourhood around September 23,” said a Met report on Sunday.

“What the system becomes after turning into a low-pressure area and the likely course and impact on Bengal will be clear on Monday,” said a H.R. Biswas, head of the weather section at the Regional Meteorological Centre, Calcutta.