The city reeled under the twin assaults of heat and humidity on Wednesday.

The Met office said heavy rain was on its way.

A cyclonic circulation is expected to take shape over the north bay of Bengal by Thursday and intensify into a low-pressure area by Friday, said a Met bulletin.

The system is a remnant of tropical storm Wipha, which pounded Southeast Asia before making its way to the Bay of Bengal in a depleted form.

A west spell is due in Bengal from Thursday to Monday, the bulletin said. Calcutta is most likely to get heavy rain on Friday.

South 24-Parganas, North 24-Parganas, Jhargram, East Midnapore, West Midnapore, Bankura, Purulia, Howrah, Hooghly, and Calcutta are among the districts expected to get drenched by the system.

“Very heavy rain (7-20cm)” is expected in some districts.

On Thursday, North and South 24-Parganas and East Midnapore are expected to get “very heavy rain”, according to the forecast.

Thunderstorm with lightning and gusty winds (30-40kmph) are expected across south Bengal on Wednesday.

The monsoon trough now passes through Uttar Pradesh, Jharkhand and south Bengal (Digha) into the Bay of Bengal.

The strong monsoon currents and the system are likely to team up to trigger widespread rain, said a Met official.

In north Bengal, districts like Kalimpong, Jalpaiguri, Darjeeling, and Alipurduar are likely to get drenched as well.

Wednesday was taxing. The maximum temperature was 35.2 degrees Celsius, three notches above normal. A minimum relative humidity of 62 per cent meant the discomfort index was high for most of the day.

Tropical storm Wipha made landfall in northern Vietnam on Tuesday, bringing strong winds and heavy rain to parts of the country’s north and central regions. Southern parts of the Chinese mainland and Hong Kong were also battered by rain under the influence of the storm.

“After landfall, the storm is moving through land. Facing friction in the form of poles and trees, it is losing strength,” said a weather scientist.