Mumbai got 43.6% of June’s average rain in nine hours

The south-west monsoon announced its arrival in Mumbai on Wednesday, two days prior to the official onset date of June 11, with 220.6mm rain between 8.30am and 5.30pm, according to data from the weather bureau’s Santacruz observatory — representative of the city.

The south-west monsoon announced its arrival in Mumbai on Wednesday, two days prior to the official onset date of June 11, with 220.6mm rain between 8.30am and 5.30pm, according to data from the weather bureau’s Santacruz observatory — representative of the city. The downpour within nine hours has already accounted for 43.6% of the city’s average rainfall for June (505mm).

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The India Meteorological Department (IMD) predicts that the city may get another 200mm or more of rain between June 11 and June 13, and that heavy rain is likely to persist till at least June 15. The high probability of heavy to very heavy rains prompted the IMD on Wednesday to issue an “extremely severe weather warning” for the entire Konkan coast and some parts of Madhya Maharashtra till June 13. The IMD placed Mumbai, Thane, Palghar and Raigad districts under a red category storm warning on Wednesday (upgraded from the initial yellow category warning issued a day prior), although the intensity of rain significantly reduced after 3pm. From Thursday until Sunday, all seven coastal districts are placed under an orange category warning, indicating that local authorities should “be prepared” for “heavy to very heavy rainfall”.

With rainfall accumulated due to pre-monsoon showers between June 1 and June 8, the city has already achieved 84% of June’s monthly average in just nine days. Mumbai has so far recorded 416.2mm of rain this month (until 5.30pm on June 9) against a monthly average of 493.1mm. For comparison, Mumbai received only 344.4 mm of rain in June 2020, its lowest average for the month in five years.

There are two significant weather systems driving these torrential showers, including an upper air cyclonic circulation over the Arabian Sea, and a low pressure zone forming over the Bay of Bengal. Experts described these simultaneous circulations as having a “push and pull” effect on the monsoon trough, which has now entered most parts of interior Maharashtra, some parts of south Gujarat, parts of Telangana and Andhra Pradesh, most parts of central Bay of Bengal, and some more parts of north Bay of Bengal today.

“Due to the strengthening of westerly winds along the west coast, along with the these circulations in the Arabian Sea and Bay of Bengal, dense moisture laden clouds are kept spinning over Mumbai resulting in heavy to very heavy showers,” said KS Hosalikar, senior scientist with the IMD in Pune. Weather models had not, until Wednesday, predicted the circulation in the north Arabian Sea, leading to heavier than expected rain on the day.

Officials were reluctant to comment on the outlook for the season beyond June 15. “It will depend on what weather systems develop and where. For Mumbai, such heavy rain this early in June is certainly an abnormality, but it is likely to be an isolated event due to prevailing weather systems. Once they pass, Mumbai may get a break from high intensity, short bursts of rain. Drawing links with broader climate change conditions is not as easy in this case as it is with cyclones, for example,” said one meteorologist with a government institute in Pune, asking not to be named.