Why extreme rainfall in Mumbai is getting harder to predict

So, why do weather forecasts sometimes miss the mark? And, are there unique factors that influence rainfall patterns in Mumbai?

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In 2022, a study published in River Dynamics and Flood Hazards analysed the monthly and yearly precipitation patterns in Mumbai and Pune from 2000 to 2020. It revealed an increasing trend in annual rainfall and during the non-monsoon, southwest monsoon, and northeast monsoon seasons in both cities.

But, while Mumbai experienced consistent rainfall throughout the monsoon season earlier, it now has more days of heavy rainfall (defined as more than 2.5 inches in 24 hours) interspersed with long dry spells. A 2023 study in the international journal 'Theoretical and Applied Climatology' confirmed this shift. Analysing data from 1985 to 2020 for two IMD stations in Santacruz and Colaba, it found a significant change in Mumbai's rainfall patterns, with rainfall becoming more erratic and individual events more severe.

According to it, the frequency of heavy rainfall events (more than 120 mm/day) and extreme heavy rainfall events (more than 250 mm/day) increased in Santacruz after 1994, while Colaba saw an increase after 2005—the year Mumbai experienced a historic deluge with 944 mm of rain in a single day.

“There’s no denying that a drastic change has occurred in Mumbai’s monsoon rainfall patterns. Especially in the last decade, with climate change, there’s been a significant rise in frequency and intensity of extreme rainfall events. A decade ago, monsoon meant steady moderate to heavy rains, but now it’s more akin to dumping half the monthly average in a few days’ time,” says Athreya Shetty, independent meteorologist.

Mumbai Metropolitan Region (MMR)’s small hills, lakes, surrounding creeks, green cover, and increasing skyscrapers significantly influence weather patterns, making it difficult to pinpoint which areas will experience extreme rainfall. (File image)

Mumbai rain patterns: factors, 

But it isn’t just climate change. Several other factors are involved. Rainfall prediction in the tropics is inherently complex, and the dynamic nature of the monsoon only adds to the challenge, making it tough to forecast extreme events. Mumbai's location on the Konkan coast, combined with the highly variable local topography of the Mumbai Metropolitan Region (MMR), complicates predictions further. The area’s small hills, lakes, surrounding creeks, green cover, and increasing skyscrapers significantly influence weather patterns, making it difficult to pinpoint which areas will experience extreme rainfall.

There’s more. “A low-pressure region to the north or northeast of Mumbai can strengthen monsoon winds from the Arabian Sea, causing extreme rainfall. Secondly, a low-pressure system over the Bay of Bengal can similarly intensify monsoon winds. In both scenarios, the rainfall tends to be persistent. The third pattern involves localised and short-lived thunderstorms, such as those witnessed on 8 July 2024 and 17 July 2021,” says Dr Akshay Deoras, research scientist at the National Centre for Atmospheric Science & Department of Meteorology, University of Reading, UK, with over 15 years of experience in weather forecasting for the Indian subcontinent.

In fact, a 2023 study of 15 extreme rainfall events in Mumbai (from 1980 to 2020) looked at the key factors influencing these events. They included multiple rain-bearing systems (such as an offshore trough, mid-tropospheric circulation, or a Bay of Bengal depression moving north-northwestward), high sea surface temperature gradients along Mumbai’s latitude, and the orographic effect of the Western Ghats, which localises heavy rainfall over the city.

From low-pressure regions to the north and northeast of Mumbai, to high sea surface temperature gradients along Mumbai’s latitude, there are multiple factors affecting Mumbai rain patterns.

Looking ahead

That’s not all. Different weather models, such as ECMWF (European Centre for Medium-Range Weather Forecasts), GFS (Global Forecast System), and IMD GFS are commonly used to forecast rainfall. However, many of these global models are designed for temperate climates and do not handle the complex dynamics of tropical monsoons well. “While they excel at predicting broad weather patterns days in advance, they often fall short in accurately forecasting specific rainfall amounts. One reason for this is their low resolution, which limits their ability to account for local topography and detailed weather features,” says Shetty.

To address these challenges, forecasters must rely on their expertise and experience to complement model outputs. “A crucial step forward is developing a high-resolution weather prediction model tailored specifically to India's geography and monsoon dynamics, rather than relying on existing global models. Such a model could significantly enhance forecasting accuracy, particularly for extreme weather events,” adds Shetty.

Most global models are designed for temperate climates and do not handle the complex dynamics of tropical monsoons well. (Image credit: PTI)

In the end, improving weather predictions will require work on two fronts. “First, we need to better understand the triggers and processes of extreme monsoon precipitation events. It would help improve weather prediction models. Second, running these models at finer spatial scales and more frequently will capture crucial small-scale weather processes, thereby improving accuracy. Model development is a gradual process. However, enhancing real-time weather surveillance with tools like Doppler radars and issuing impact-based alerts, such as text messages and wireless emergency notifications, can help mitigate the impact of severe weather and better prepare communities,” says Dr Deoras.