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As the southwest monsoon starts its retreat process, the wind pattern and change of airmass together increase the chances of cyclone formation in the Indian seas. The monsoon stream is a stable airmass, and therefore, tropical storms do not form in the core monsoon months of July and August over the Indian seas. Between the Arabian Sea and Bay of Bengal, the latter is more conducive to storm formation in the month of September. Also, Arabian Sea storms largely track away from the Indian coastline and do not have adverse impacts.
Monsoon withdrawal, as such, is a slow and long-drawn process. The normal date for commencement of withdrawal from West Rajasthan is 17th September. On many occasions, the withdrawal gets delayed. In a sizeable number of cases, the monsoon starts retreating at the fag end of September or even in October, as it happened in 2007, 2017, 2019, 2020, and 2021. On all such occasions, the chances of storm formation become the least in September.
No cyclonic storm has formed over the Bay of Bengal in the last three years during the month of September. The last cyclone over the Bay of Bengal was ‘Gulab,’ between 24th–28th September 2021. The storm crossed the Andhra Pradesh coast near Kalingapatnam. Incidentally, this cyclone traversed across the South Peninsula and entered the Arabian Sea. It picked up intensity to become a severe cyclonic storm, named ‘Shaheen,’ in the Arabian Sea. Ultimately, Shaheen tracked westward and struck Oman.
From historic data, it is inferred that storms were more frequent in the month of September during the 1950s, 60s, and 70s. In fact, in 1955 and 1968, two cyclones formed each over the Bay of Bengal in September. After 1990, storms have become few and far between in September. Only one storm each formed over the Bay of Bengal in 1997, 2005, 2018, and 2021.
Southwest monsoon 2025 has already started withdrawal (14 Sep 2025) from northern parts of the country. The Bay of Bengal has been an active basin. There was a low-pressure area earlier this week, and another one is forming shortly over the central and northern Bay of Bengal. This system, at best, may become a depression and cross the coastline of Odisha on 27th Sep. It is likely to reach North Konkan and South Gujarat at the fag end of this month. In the meantime, another remnant circulation will enter the Bay of Bengal from the Myanmar region on 29th/30th September. Both these systems will stand a chance to intensify in their respective regions. As model accuracy drops after a lead time of 5 days, it will be premature to prognosticate beyond these timelines. Notwithstanding, the Indian seas on either side of the coastline will stand a chance to host major synoptic systems, including tropical storms, during the first week of October 2025.
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