Monsoon reached north Bengal on Tuesday, the Met office said.

It is expected to cover the rest of Bengal by the end of this week, according to a Met bulletin.

A Met official said the monsoon is likely to set in over Calcutta within the next two to three days. “The southwest monsoon advanced to Kalimpong, Cooch Behar and Alipurduar districts; most parts of Jalpaiguri and some parts of Darjeeling districts on Tuesday. The northern limit of the monsoon now passes through Siliguri. Conditions are favourable for further advance of southwest monsoon into the remaining parts of southwest Bay of Bengal, some more parts of westcentral and northwest Bay of Bengal and West Bengal...during the next 4-5 days,” said the bulletin.

Monsoon is declared to have arrived at a district if more than 50% of its Met stations receive rain and there is a change in the wind pattern, said a Met official.

North Bengal is receiving widespread rainfall.

The Met bulletin predicted thunderstorms and rain across south Bengal from Wednesday. “In Calcutta, there is a 40% chance of rain on Wednesday and 70% on Thursday,” said the official.

There are multiple weather systems at play.

“The upper air cyclonic circulation over east Uttar Pradesh and neighbourhood extends up to 1.5km above mean sea level. A trough at the mean sea level runs from Punjab to Bihar across Haryana and Uttar Pradesh. A trough now runs from east Bihar to the coastal parts of Gangetic West Bengal at 1.5km above mean sea level. An upper air cyclonic circulation lies over sub-Himalayan West Bengal and Sikkim and neighbourhood at 0.9km above mean sea level,” said the bulletin.

The usual date of monsoon’s arrival is June 5 for north Bengal and June 10 for south Bengal.

Calcutta spent another sweltering day, waiting for rain. The maximum temperature was 35° Celsius, normal for this time of the year. But the minimum relative humidity was as high as 66%, adding to the discomfort.