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Updated: Sep 16, 2019 06:24 IST

With another 15 days still to go till the end of this year’s rains, the city has already recorded its wettest-ever monsoon, with 3,458.2mm rain between June and September 15, despite a 15-day delay in the season’s onset.

Prior to 2019, the all-time record was 3,451.6mm in 1954, according to reports from the disaster management cell of the Brihanmumbai Municipal Corporation (BMC) and the Maharashtra government. The reports are based on data recorded by the India Meteorological Department (IMD).

A 2017 flood-preparedness guidelines report by the BMC, a relief and rehabilitation for disaster management report by the Maharashtra government, and five research papers (all of which HT has a copy of) stated the same — “The maximum annual rainfall ever recorded was 3,451.6mm in 1954 (as per IMD records).”

The IMD, however, did not confirm that the record had been broken and said they needed to check their entire data repository. “Rainfall during this monsoon is the highest since 2001, but for records from further past, we will have go through historical data at our office,” said KS Hosalikar, deputy director general, western region, IMD.

A senior BMC official, however, said if any rainfall figures have been published in our official document, it has been given by the IMD. “We have been using the 1954 figure for all our official rainfall-related datasets.” “This is definitely one of the highest. However, we need to check the 100-year rainfall data to be sure. This will take time,” said SG Kamble, scientist, IMD Mumbai.

The IMD data over the past 13 years shows the highest seasonal rainfall, prior to 2019, was recorded in 2010 — 3,327.9 mm over four months (June to September). The annual average rainfall for Mumbai is 2,514mm, while the seasonal average is 2,317mm. The southwest monsoon period for Mumbai is from June to September, with the normal onset date on June 10. However, this year, the monsoon onset was declared by IMD on June 25.

Mumbai also surpassed the 900-mm rainfall mark for September on Sunday. The city has so far recorded 904.3mm from September 1 to September 15 (8.30pm), which is just 0.3mm short of the September 1993 record of 904.6mm — the second-highest all-time September rainfall. The all-time record is in 1954 when 920mm was recorded over 30 days.

Independent meteorologists confirmed the record. “Even after the late onset, this has been a record year. Moreover, all the monthly averages were achieved within the first 4-5 rainy days. This shows that extreme events are on the rise owing to climate change,” said Sridhar Balasubramanian, associate professor, department of mechanical engineering and associate faculty, IDP Climate Studies, IIT- B.

Record-breaking rainfall this season was because of sustained weather factors, said Akshay Deoras, independent meteorologist and PhD researcher, University of Reading, UK. “The offshore trough remained active for a longer period and was sensitive to low-pressure systems forming in the Bay of Bengal. Nearly all of the meteorological sub-divisions along the west coast and west-central parts of India have received excess rainfall this season,” he said.

The Mumbai Metropolitan Region (MMR) received surplus rains during the monsoon primarily owing to the Indian Ocean Dipole (IOD) effect — an ocean-atmosphere phenomenon — with a difference in sea surface temperatures characterised by cooling in parts of Indian Ocean leading to enhanced rain, said Balasubramaniam.

“IOD has been in its positive phase since July as 2019 is likely to go down as a strong IOD year. The influence of IOD has been very active and has negated all ill-effects of El Nino,” he said.

Deoras added that frequent monsoon low-pressure systems had helped overcome the rainfall deficit across India. “Mumbai will witness more rainfall in the remaining days of September, so there’s a good chance that the record of the highest rainfall in September (920mm set in 1954) will be broken this year,” he said.

Moderate overnight rain was recorded in the suburbs (31.4mm) and light rain (3.4mm) in south Mumbai between Saturday and Sunday. Palghar district recorded very heavy rains overnight owing to a localised weather system north of Mumbai, said Kamble. Dahanu recorded the maximum rainfall at 216mm, while the majority of the district received over 100mm rain. Bhiwandi in Thane district recorded 150mm.

“We issued a yellow alert (be updated) for Thane and Mumbai for Sunday and orange alert (be prepared) for Palghar. Rain intensity will reduce all across these three districts on Monday, but isolated heavy showers cannot be ruled out,” said Kamble. On Sunday, 10.2mm rain was recorded in south Mumbai, from 8.30am to 5.30pm, while the suburbs recorded 4.8mm during the same time.