Thunderstorm hits Mumbai

MUMBAI: The city recorded a total of 253mm of rainfall in about 4.5 hours on Friday morning, according to data from the India Meteorological Department’s (IMD) monitoring station in Santacruz -- representative of the city and suburbs. This marks the heaviest rainfall event of the season so far, and the second largest quantum of daily rainfall (in July) recorded over the past 10 years. The city had earlier witnessed 375.2mm of rain on July 2, 2019, as per the IMD data.

Friday morning’s rain was concentrated largely over the western suburbs and parts of central Mumbai. Rainfall data for the day shows large discrepancies in spatial distribution, which experts attributed to easterly winds that pushed the cloud cover away from south Mumbai. As a result, IMD’S observatory in Colaba recorded only 13mm of rain as of 8:30am on Friday.

“Radar images from late on Thursday did not show any indication of thunderstorm formation. In the absence of largescale features such as a deep layer of winds from the west over Mumbai, a monsoon lowpressure system over east or central India, or a vigorous offshore trough, even weather models would not have been able to predict this,” said Akshay Deoras, independent meteorologist and PHD student at University of Reading.

It is, however, clear from available satellite and radar images that there was a rapid formation of cloud cover over Mumbai early on Friday, around 5am, due to intense convection over the region. As winds in the lower atmosphere quickly converged and moved to higher levels forming clouds, easterly winds in upper levels steered clouds towards the Arabian Sea, saving south Mumbai and Colaba from a torrential downpour. The storm system later blew past to the south of Gujarat.

Friday’s showers are likely to be the “first event of this magnitude in this century to have been triggered in the absence of any Bay of Bengal monsoon lowpressure system in the month of July,” Deoras added. There are a total of five instances between 2000 and 2020 when Santacruz received rain in excess of 250mm, all of which were attributed to low-pressure systems that formed over the Bay of Bengal. Deoras also explained that the mountainous terrain in Mumbai’s suburbs -- specifically in Sanjay Gandhi National Park and nearby green areas -- may have driven the formation of this intense thunderstorm earlier in the day. With a total seasonal rainfall of 1545mm, the city has now surpassed the normal rainfall amount expected till end July, which is 1332.5mm. The normal rainfall between June 1