A low-pressure area is expected to take shape over the north Bay of Bengal in the third week of May, heralding the monsoon into parts of northeastern India, the Met office has said.

Usually, the monsoon reaches Bengal four to five days after it arrives in the Northeast, said Met officials.

“A low-pressure area will take shape over the north Bay and adjoining central Bay of Bengal between May 22 and 28. The southwest monsoon is likely to set over Kerala during that period. The monsoon is also likely to set foot in the Northeast at the same time,” said an official at the Regional Meteorological Centre in Alipore. He spoke based on the “extended range forecast” (for the next two weeks from May 15 to 28) issued by the India Meteorological Department on Thursday.

The monsoon set foot in the Andaman and Nicobar islands on Tuesday, a week ahead of schedule.

The usual date of the arrival of the monsoon in Kerala is around June 1. The usual onset date for north Bengal is June 5, and south Bengal June 10.

The low-pressure area is likely to trigger uniform and widespread rain across northeastern parts of India and north Bengal, the official said. South Bengal is also likely to get some rain.

For now, the day temperatures have dropped in south Bengal. A north-south trough runs from north Bengal and Sikkim to southeast Madhya Pradesh across north Chhattisgarh in lower tropospheric levels, a Met report said.

The system, aided by the heat trapped near the earth’s surface, is playing the catalyst in triggering thunderstorms. Bankura, Birbhum, East and West Burdwan were among the districts that got thunderstorms on Thursday.

The rain was heavier in north Bengal. In neighbouring Sikkim, roads have been damaged by the incessant showers.

The cyclonic circulation near Andaman Sea, which had been the subject of relentless speculation, is unlikely to intensify, the Met office said.

“The circulation will last for a couple of days and then fizzle out. It will not intensify into anything,” said a Met official.

Calcutta is likely to get at least a couple of thunderstorms by the end of this week, said the Met official.

On Thursday, the maximum temperature in Alipore was 35.8 degrees, normal for mid-May. But the minimum relative humidity was close to 60 per cent, making the conditions sweaty.