In the revised Southwest monsoon calendar, which will be in effect from the upcoming season, the monsoon will cover the entire country one week in advance, and begin its retreat two weeks later than the usual dates. This was stated in a first-of-its-kind study performed to derive monsoon onset/progress and withdrawal dates over India, a report of which was released on Friday by the India Meteorological Department (IMD).
The revision is done based on rainfall information gathered from about 2,500 weather stations, providing daily observations for the period 1961-2019 (onset) and 1971-2019 (withdrawal).
The monsoon onset and withdrawal dates that IMD referred to, until 2019, were based on the rainfall information recorded from merely 149 stations during the period 1901-1940.
The revision was inevitable after significant variations were observed in the arrival and withdrawal of monsoon over various parts of the country during recent decades.
As per existing dates, the monsoon onset over Kerala was June 1, and the system would make northward progress. Simultaneously, the Bay of Bengal branch of the monsoon would advance from the east, towards the west. Climatologically, the monsoon covers the entire country by July 15.
However, the onset date over Kerala has been retained as June 1, even though this year, it is expected to arrive late by four days, that is, on June 5.
As per the revision in dates, monsoon arrival over Andaman and Nicobar will be by May 22, after which it will cross the Bay of Bengal by May 26, and advance to Kerala by June 1. Following its onset over Kerala, it will progress into some parts of Tamil Nadu and the northeast by June 5, before entering Karnataka, Goa, Rayalaseema, Telangana, and the entirety of the northeast. This year onwards, the normal date for monsoon progress over Maharashtra will be June 10, and during this period, it will have covered most parts of West Bengal, Odhisha, Jharkhand and Bihar.
It will be the end of June, by the time monsoon arrives, over central and western India regions, covering Madhya Pradesh, Gujarat, and a few northern states like Jammu and Kashmir, parts of Punjab, Haryana, Delhi and east Rajasthan. The monsoon advance shall, from now on, be complete by July 9, this is, a week in advance from the present dates.
A fortnight’s delay will be the new normal in the monsoon withdrawal, which commences from extreme northwestern parts of the country, that is, parts of Rajasthan and Gujarat. The revised date of withdrawal is now September 17, instead of the present date of September 1.
IMD officials said the retreat, once it commenced, was found to be progressing rapidly. The monsoon will retreat from parts of Jammu and Kashmir, Punjab, Haryana, Delhi, Himachal Pradesh, Uttarakhand, Ladakh, west Uttar Pradesh and west Madhya Pradesh by October 1.
Maharashtra, along with east Madhya Pradesh, north Chattisgarh, West Bengal and Odhisha, will see a nearly ten-day delay in monsoon withdrawal, which will commence by October 10.