Mumbai delayed monsoon withdrawal: On May 26, Mumbai witnessed its earliest monsoon onset in 75 years, and the city has since been experiencing above-normal rainfall. Now, the India Meteorological Department (IMD) has forecast more spells of showers starting Monday (October 6).
The weather bureau has sounded a yellow alert in the neighbouring Thane and Palghar districts for Tuesday, while Mumbai is slated to experience moderate showers alongside thundershowers during the week. The forecast indicates that the withdrawal of the monsoon, which Mumbai typically experiences around October 8, is set to be deferred this year.
Why is Mumbai set to experience more rain in October?
According to IMD scientists, the city is likely to experience enhanced rain activity between October 6 and October 8 owing to a system of western disturbances. These are east-moving, rain-bearing wind systems that originate beyond Afghanistan and Iran, close to the Mediterranean Sea.
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As they travel, they pick up moisture from the Mediterranean Sea, the Black Sea, the Caspian Sea and the Arabian Sea. Primarily, they affect the weather systems of north India during the winter months. However, in recent decades, they have featured during summer months as well and have seen variations in the areas covered.
In Mumbai’s case, they have contributed to the formation of a trough. The monsoon trough is an elongated low-pressure area which extends from the heat low over Pakistan to the Bay of Bengal. It is a semi-permanent feature of monsoon circulation, and its southward movement results in heavy downpours in regions like Mumbai.
According to the inference charts, a trough from the south interior Karnataka to the Comorin area across interior Tamil Nadu persists currently, and the southwest monsoon is strong over the Central Arabian Sea.
Speaking to The Indian Express, a senior scientist from IMD Mumbai said, “While the city is experiencing warm and sunny days at present, by October 6 and 7, it may get cloudy and start experiencing enhanced rainfall. This is because a western disturbance will be affecting the region over the next couple of days. This western disturbance is presently dipping, and it will bring in moisture. This moisture availability will lead to enhanced showers, even as the intensity is likely to remain moderate.”
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However, IMD officials maintained that Cyclone Shakthi, which is currently impacting western India, poses no threat to the coast of Maharashtra. Their regional inference records indicate that the severe cyclonic storm was expected to reach the northwest and adjoining west central Arabian Sea after October 5, following which the system is likely to recurve and move “east – northeast wards by October 6 morning and weaken gradually”.
What is the trend of monsoon withdrawals in Mumbai?
The IMD declares the withdrawal of monsoon (usually around October 8) after observing factors such as spatial continuity, reduction in moisture as per imagery, change in wind regime, and persistence of dry weather for at least five consecutive days.
While the city is projected to witness a delayed withdrawal, scientists maintained that late withdrawals are a common phenomenon observed with persisting systems. Records show that for the past six years, Mumbai has experienced delayed monsoon withdrawals.
For instance, last year, the southwest monsoon withdrawal in Mumbai was declared on October 15, while in 2023 and 2022, withdrawals were delayed by over two weeks, with the IMD announcing them on October 23. In 2021, the withdrawal happened by October 14, while in 2020, it was on October 28. In 2019, too, monsoon withdrawal was announced on October 14.
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How much rainfall has Mumbai recorded this season?
Since June, Mumbai’s Santacruz observatory in the suburbs received over 3,100 mm of rainfall till September, which is significantly higher than its typical seasonal average of 2,319 mm of rain. The above-normal showers were spurred by heavy downpours in August and September, while the city recorded below-normal rainfall in June and July.
This year, the city received 512 mm of rain in June, 798 mm of rain in July, 1,184 mm of rain in August and 606 mm of rain in September. The Santacruz station has received 31 mm of rain in October so far, while Colaba station has logged 59 mm of rainfall.
