The monsoon has left Calcutta for this year but the rain has not.

The city was lashed by a spell of thunderstorm on Wednesday evening after intermittent showers since early afternoon.

Around 5.45pm, the rain was almost blinding in pockets such as Ballygunge, Park Street, Theatre Road and Kasba. The showers were accompanied by strong gusts of wind, frequent streaks of lightning and claps of thunder.

Around 3.50pm on Wednesday, the Met office in Alipore issued a warning. “Thunderstorms with lightning and light to moderate rainfall are likely to affect some parts of Kolkata and North 24-Parganas districts during the next 2-3 hours,” the bulletin said.

The warning did come true. Around 5.45pm, Salt Lake and New Town in the north, Esplanade, Park Street and Theatre Road in the centre and Gariahat, Kasba and Naktala in the south were among the places that were struck.

The Met office in Alipore recorded 38.2mm of rain on Wednesday.

Similar thunderstorm activities were reported from East Midnapore, South 24-Parganas, Howrah and Hooghly as well.

The southwest monsoon retreated from Bengal on October 13 and from the entire country on October 15, the Met office said.

A Met official said the withdrawal of the monsoon from a place does not necessarily mean a total stop to rainfall there.

He attributed the rain to an anticyclonic system, or a high-pressure area, over coastal parts of south Bengal.

“There is a fair amount of moisture incursion from the system into the mainland. The moisture is teaming up with the heat to form thunderclouds. These thunderstorm activities are typical in the run-up to the monsoon. They are not unusual in the post-monsoon period as well,” said H.R. Biswas, head of the weather section at the Regional Meteorological Centre, Calcutta.

Scattered rain and thunderstorms will persist in Calcutta and other parts of south Bengal for the next three to four days, he said.

The moisture incursion is happening in the lower levels of the atmosphere. “Higher up, the winds from the direction of the Bay of Bengal have been replaced by northerly winds and some northeasterly winds,” said Biswas.

Weather scientists generally treat three to four rain-free days during this time of the year as a prerequisite to announcing the withdrawal of the southwest monsoon. A dip in the moisture content in the air is another marker of the end of the monsoon.

But the most telltale sign is the reversal in the flow of wind. The end of the monsoon is marked by the arrival of the winds from north India. The winds from the direction of the Bay gradually stop flowing into the city.

November is also the time for cyclones over the Bay.

A system over the southwest Bay is hurtling towards the southeastern coastline but is unlikely to turn into a cyclone, according to the Met forecast.

“A well-marked low pressure area over the central part of south Bay of Bengal intensified into a depression over southwest Bay of Bengal around 5.30pm on Tuesday. It moved west-northwestwards with a speed of 15kmph during past six hours...” said a Met bulletin issued on Wednesday.

Biswas, of the Met office in Alipore, said the depression did not have any noticeable impact on Calcutta.

“The anticyclone that is sending moisture into south Bengal lies on the periphery of the depression,” he said.