Kolkata and its suburbs on Tuesday woke up to the third-wettest September day since 1978, recording a rainfall of 251.4mm in 24 hours.

This marked the city’s third-highest single-day rainfall in September, followed by 369.6 mm on September 28, 1978, and 259.5 mm on September 26, 1986.

Considering all-time records throughout the year, this spell of rain is the city’s sixth-highest single-day total ever recorded.

The heaviest spell was recorded between 2.30am and 5am, when the state capital recorded close to 185mm. The city narrowly missed being hit by a cloudburst (100mm in an hour) in the wee hours on Tuesday. Multiple favourable weather factors locally led to short but an intense spell.

“The city of Kolkata received 251.4 mm of rainfall, a figure recorded at 8:30 am on September 23,” said an official of Indian Meteorological Department (IMD).

“Although the highest hourly rainfall was 98 mm, it did not meet the criteria for a cloudburst, which typically requires more than 100 mm per hour,” said a senior official of the IMD.

Causes and current forecast

The heavy rainfall was triggered by a low-pressure area that formed on Monday which moved towards the coastal areas of Gangetic West Bengal and adjoining regions, leading to a strong convergence of moisture over Kolkata. “Doppler weather radar images showed high moisture content with clouds reaching heights of 5 to 7 km, contributing to the intense downpour,” said the IMD.

The low-pressure system is expected to persist until Wednesday before becoming less marked, officials said.

However, another low-pressure area is predicted to form over the Bay of Bengal around September 25, which could intensify into a depression and cross the South Odisha-North Andhra Pradesh coasts around September 27.

This developing weather pattern is expected to bring widespread light to moderate rainfall, with isolated heavy to very heavy rainfall possible in some districts of South Bengal over the next seven days.

Kolkata Mayor and state Cabinet minister Firhad Hakim described the flooding as unprecedented, noting that even neighborhoods near the Adiganga, which normally drain well, were overwhelmed. The overflowing of the Adiganga, a canalised river, prevented natural water runoff.

In response to the flooding, authorities have opened several school buildings to provide shelter to people from low-lying areas and those living on the streets. The Corporation and the police have been tasked with arranging food for those displaced. The Mayor expressed hope that the water would recede by the night if no further rain occurred.

The heavy rain has also led to a warning for fishermen, advising them not to venture into the sea along the West Bengal-Odisha coast until September 27 due to rough sea conditions and squally weather. “Residents are also advised to avoid taking shelter under trees or electric poles and to be cautious of lightning strikes,” read an IMD alert.

The Met department has warned of continuing rainfall over Odisha till September 26 but the rain may take a break for now over West Bengal and possibly not dampen the upcoming Durga Puja festivities.

This monsoon, there have been multiple incidents of cloudbursts striking Uttarakhand and Himachal Pradesh.

One such incident was reported in Chennai, another Indian metro city, on August 31, when it recorded 270mm in 24 hours.