Thunderclouds over the skyline on Sunday afternoon as a cyclonic storm headed towards the city. Picture by Sanat Kumar Sinha

The deep depression over the Bay of Bengal developed into a cyclone on Sunday but the weatherman said the storm, which was advancing towards the coast at 70-80km per hour, might miss Calcutta.

At 5.30pm, cyclone Aila (name chosen from a list following a convention) was 500km south of Calcutta.

“Even if Aila does not hit Calcutta, it would cause heavy rainfall,” said a senior Met official.

Under Aila’s influence, rainfall is likely at most places over Gangetic Bengal, with heavy to very heavy rain at a few places. Isolated “extremely heavy rain” (over 250mm) is likely in Gangetic Bengal, including Calcutta and its adjacent areas, over the next 48 hours, said the official.

On Sunday, the city received 4.6mm of rain till 5.30pm. The humidity ranged between 91 and 58 per cent. The discomfort index touched 60 degree Celsius, five degrees above normal.

The temperature remained lower than usual — the maximum, 31.7 degree Celsius, was four degrees below normal. The minimum temperature was 26.2 degree Celsius, a degree below normal.

“There is a cyclone alert for the Bengal coast for the next 24 hours. The cyclone is moving north and will cross the Bengal-Bangladesh coast, 100km east of Sagar Island, early on Monday,” said G.C. Debnath, the director of the Regional Meteorological Centre at Alipore.

“The sea condition will be high to very high along and off the Bengal coast and very rough along and off the Orissa coast. Fishermen are strongly advised not to venture into the sea along and off these coasts,” added Debnath.

On Sunday, the south-west monsoon advanced over more parts of south Arabian Sea, some parts of south Kerala and south Tamil Nadu, most parts of south-west & east-central Bay of Bengal and some parts of west-central & north-east Bay of Bengal.

“The India Meteorological Department declared onset of southwest monsoon in Kerala on Saturday, about a fortnight earlier than usual,” said a senior official of the Alipore Met office. Over the next 24 hours, the southwest monsoon is likely to further advance over the Bay of Bengal.