Mumbai’s heaviest one-day downpour since 2019: How it eluded weather models and left weathermen puzzled

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As heavy showers lashed Mumbai in the intervening night of Sunday and Monday, data furnished by the Brihanmumbai Municipal Corporation (BMC) showed that the heaviest downpour was recorded in the eastern suburbs of Mumbai between 1am and 7am with its Automatic Weather System (AWS) recording 315.6mm rain in Vikhroli, followed by Powai where 314.5mm rain was recorded, Andheri East (292 mm), Chakala, (272 mm) and Aarey (259 mm).

Meanwhile, IMD data showed that the Santacruz recording station registered 268mm rain, even as the Colaba centre registered only 84mm between Sunday and Monday morning. This paved the way for the city setting the record for the highest 24-hour period rainfall recorded since 2019, when the suburbs recorded 375.2mm rain within 24 hours on July 2.

According to scientists, even as a nowcast warning was issued around midnight, the intensity of rains gathered pace after 2.30am. Sushma Nair, scientist at IMD Mumbai, said, “Our records show that the Santacruz station registered only 40.9mm rain until 2.30am. After that, rain intensified and 210.9mm was recorded between 2.30am and 4am.”

IMD officials attributed the heavy showers to the strengthening of an offshore trough around midnight. “The offshore trough along north Gujarat and Kerala coast, which was feeble earlier, strengthened along north Konkan around midnight, leading to heavy rains,” added Nair.

The heavy showers came even as the IMD had issued only a yellow warning on Sunday in Mumbai, stating the likelihood of ‘heavy rainfall at isolated places’, while no warning was issued for Monday and the rest of the week.

That the weather bureau failed to flag the extreme weather event that unravelled in the late hours drew flak from citizens, who were left reeling under an inundated city throughout Monday.

An official from IMD Mumbai said, “The weather event overnight was likely caused by an offshore vortex. Our models could not capture such an intense yet small-scale weather system and the resulting rains.”

Meanwhile, weather experts across the city echoed that all models, alongside the IMD’s, failed to foresee the extremely heavy downpour in the city.

Subimal Ghosh, IIT-Bombay professor and convener, interdisciplinary program in climate studies said, “None of the models could forecast the extreme rains. The IMD model placed no warnings. The ECMWF model forecast low rainfall. Even the NCEP-GFS model showed low rainfall. Since our model uses NCEP data to statistically downscale at local level, even our models failed to forecast the rains.”

“We are yet to determine what caused the extreme rain event in Mumbai. We are now trying to understand the factors by looking at the circulation as well as the moisture flow, where it came from, the wind directions etc,” added Ghosh.

According to Raghu Murtugudde, professor of climate studies at IIT-Bombay, who attributed the heavy showers to northward expansion of the southwesterly jet, the forecast models often fail to capture the edge effects of the jet.

“Mumbai has been at the edge of the southwesterly jet for weeks, resulting in deficit rain. But yesterday, this jet expanded slightly northward and started pumping moisture directly, causing heavy rains,” said Murtugudde.

“The models have not been doing very well in capturing the edge effects of this jet. Slight off jet means you get a wrong forecast. Unfortunately, this happens as it is a tough system, especially to forecast at local levels,” added Murtugudde.

Red alert for Mumbai

After the overnight downpour, heavy showers continued to batter the city. While the rain activity in the suburbs diminished, the island city division registered over 100mm rain in a span of nine hours on Monday.

Data procured by the IMD showed that while the Santacruz observatory recorded 14.1mm between 8.30am and 5.30pm, the Colaba coastal observatory recorded 101.8mm rain during the same period.

In light of the continued downpour, the IMD — on Monday evening — placed Mumbai under a red alert until Tuesday morning, indicating that heavy to very heavy rainfall will continue.

Sunil Kamble, director of IMD Mumbai, told The Indian Express, “We have received over 100 mm rain in the past few hours and are expecting over 100mm more rainfall in the next 12 hours. Therefore, a red alert has been issued.”

For Tuesday, the IMD has sounded a yellow alert in Mumbai as well as its neighbouring districts of Thane, Raigad and Palghar.