Mumbai: 2nd highest 24-hour rain for July in a decade, but only in suburbs

MUMBAI: At 253 mm, the India Meteorological Department’s (IMD) Santacruz observatory recorded the second-highest 24-hour rainfall for July in a decade between July 15 and 16 (8.30 am-8.30 am). Previously, between July 1 and 2, 2019, the Santacruz observatory had reported 375.2 mm of rainfall, which has been the highest one-day July rain recorded so far in the past ten years. The rain recorded by the Santacruz observatory falls in the extremely heavy category, as per IMD’s classification.
However, no warning of extremely heavy rain was issued, and experts said it was because it was a localized phenomenon, with rain restricted only to the northern suburbs of Mumbai. Colaba observatory recorded a meagre 12.8 mm of rainfall. Following the overnight and early morning downpour, the IMD, which had also given a yellow alert for Mumbai for July 16, went on to upgrade it to orange, indicating heavy to very heavy rain at isolated places.
Dr Jayanta Sarkar, scientist and head, IMD Mumbai, giving a forecast for the next five days, said, “For Friday we have given an orange alert (heavy to very heavy rain), while for Saturday, a yellow alert has been issued for Mumbai.” Rainfall data showed that rain started building up from 2.30 am on Friday, with the maximum rainfall recorded in the window between 5.30 am and 8.30 am of over 150 mm.
With the heavy overnight rain, the month of July has so far recorded 583 mm of rainfall, making it 70% of the average monthly rainfall, i.e., 827.5 mm. Meanwhile, the total rainfall recorded so far for the season is also 70% of the average July rain. In the 12 hours ending 8.30 pm on Friday, however, the IMD Santacruz observatory recorded light rain of 15.7 mm and the Colaba observatory recorded 4.8 mm.
Officials at private forecasting agency Skymet attributed the heavy rain to the fact that the monsoon surge was very strong. “Weather is not likely to go completely dry, even though the surge will weaken over the weekend. While such massive rainfall of 253 mm was not anticipated, conditions appear to have become conducive for it. We expect the next heavy spell around July 21, with a low-pressure area likely to form over the northwest Bay of Bengal and the neighborhood around that period,” said Mahesh Palawat, vice president, meteorology and climate change at Skymet.
A few experts said current operational weather models would not have been able to forecast an event like this.
Akshay Deoras, an independent meteorologist who is a PhD student at the department of meteorology, University of Reading, UK, said existing large-scale weather conditions were not favorable for heavy, widespread and sustained rainfall. He said a strong thunderstorm got triggered over suburbs early in the morning and as it matured, winds from the east pushed it away from the city, resulting in very little rainfall over Colaba and south Mumbai. “Such thunderstorms are more common in June before the monsoon arrives. There could be many factors responsible for formation of the thunderstorm only over suburbs, but the role of mountains (in areas of suburbs, namely Sanjay Gandhi National Park), cannot be ruled out. Whilst weather models can well predict large-scale rainfall events, they have limitations in simulating such localised and transient rainfall events,” Deoras said.