A sombre grey sky hung heavy on the city through Thursday, but the rain was light.

The Met office on Wednesday predicted heavy to very heavy rain from Thursday afternoon. But as a low-pressure area over the Bay of Bengal moved into the mainland, the system drifted towards Bangladesh.

The low-pressure system that had intensified into a deep depression by Thursday morning made landfall near Raidighi in South 24-Parganas’ Mathurapur in the afternoon. It then drifted towards Bangladesh, sparing Calcutta and many parts of south Bengal the onslaught of heavy rain.

The Met office said there was a possibility of heavy rain even late on Thursday, but Calcutta would experience only light to moderate rain on Friday.

“The system made landfall near Raidighi (in South 24-Parganas’ Mathurapur). It then started to move north-northeast towards Bangladesh. After landfall, any system loses strength. This one was moving further away from Calcutta,” said an IMD official.

Somenath Dutta, head of the Regional Meteorological Centre in Calcutta, said if the landfall had happened west of Raidighi, the south Bengal districts would have received far more rain on Thursday. “That the system drifted towards Bangladesh spared the south Bengal districts,” he said.

On Friday, Calcutta is likely to receive light to moderate rain. On Saturday, the sky over Calcutta is likely to be cloudy, but rainfall will go down further, said an IMD official.

A bulletin issued by the IMD said “light to moderate rain/thundershower” was “very likely to occur at most places” in Calcutta on Friday.

Up to 15.5mm of rain is classified as light rain by the IMD, and rainfall between 15.6mm to 64.4mm is moderate rain.

In Met parlance, rainfall between 64.5mm and 115.5mm is heavy rain, and between 115.6mm and 204.4mm is very heavy rain.

The low-pressure system that became a depression on Wednesday further intensified into a “deep depression” by Thursday morning.

In the morning, it lay centred over the Bay at 60km southeast of Sagar Island, 110km east-southeast of Digha, 160km east of Balasore, 180km south-southwest of Mongla in Bangladesh and 180km west of Khepupara in Bangladesh.

“It then moved slowly northwards and made landfall near Raidighi,” said an official.

Calcutta’s sky even saw flickers of the sun on Thursday morning. The sky started to turn grey from late morning, and it rained off and on from then on.

The Kolkata Municipal Corporation, which measures rainfall in its drainage pumping stations, recorded light to moderate rain.

The maximum rain recorded between noon and 8pm — 23mm — was at Jinjirabazar on Calcutta’s southwestern fringes. The station at CPT canal recorded 21.6mm, Jodhpur Park 15mm, Maniktala 18mm, Mominpore 19mm, and Behala Flying Club recorded 19.2mm rain in the period.

For Friday, the Met office forecast heavy rain at one or two places in North and South 24-Parganas, Nadia and Murshidabad districts. Squally wind speed reaching 40 to 50kmph, with gusts going up to 60kmph, was likely at one or two places in these districts.