Moisture incursion delays monsoon withdrawal but season will officially end in 9 days

The monsoon season will officially come to an end on September 30 but with the development of a fresh low-pressure area over the north-east Bay of Bengal, there are no signs yet of its commencement of withdrawal from north-west India, India Meteorological Department (IMD) said on Sunday.

“The low-pressure area will become more marked in the next 24 hours. The system is likely to move west-northwest wards leading to easterly winds at lower levels, which is likely to bring in a lot of moisture even to the north-western region. We are not seeing any parametres of monsoon withdrawal being met in the next two days. Rainfall should stop completely but it rained in western Rajasthan on Saturday and there is water vapour in the region; anti-cyclonic wind pattern is also not established yet. We have to monitor if conditions are becoming favourable for withdrawal after the low-pressure area weakens. But the monsoon season will end on September 30. There are no changes to the season,” said K Sathi Devi, head, National Weather Forecasting Centre (NWFC).

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The normal date for commencement of monsoon withdrawal from north-west India is September 17 and for complete withdrawal from the country is October 15 as per the new onset and withdrawal dates of the season issued by IMD in April. Until last year the normal date for commencement of withdrawal was September 1 and the process was complete by October 15.

IMD’s new onset dates are based on an analysis of monsoon data from 1961 to 2019 and withdrawal dates are based on data from 1971 to 2019.

Last year monsoon started withdrawing on October 9 against the normal date of September 1 and prolonged rains led to deluge in parts of Maharashtra, Kerala and Bihar in August. Monsoon withdrew completely in eight days by October 17.

Because of the development of the low-pressure area, there is an increase in rains across peninsular India.

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According to IMD’s Sunday bulletin, Brahmavar in Karnataka recorded 39 centimetres (cm) of rainfall; Vadakara (21 cm); Karkala (28 cm); Mulki (27 cm); Agumbe in Mangalore (22 cm); Valprai (12 cm)in Tamil Nadu. The rainfall in these places were in extremely heavy to very heavy rain categories.

The low-pressure area over north-east Bay of Bengal is very likely to move west-northwestwards during the next two-three days and become more marked over northwest Bay of Bengal and neighbourhood during the next 24 hours.

Under the influence of this low-pressure area widespread and extremely heavy rain is very likely over Odisha on Monday; over Gangetic West Bengal, Jharkhand, Bihar on Monday and Tuesday; Very heavy rain is likely over coastal Andhra Pradesh (AP), Yanam in Puducherry, Rayalaseema and Telangana on Monday.

An east-west shear zone is running across peninsular India in the middle and upper troposphere. Strengthening of lower level winds and its convergence along the west coast is very likely during the next three days, according to the IMD bulletin.

Due to convergence of strong moist winds from the Bay of Bengal, widespread rainfall with heavy to very heavy falls also likely over sub-Himalayan West Bengal and north-eastern states from Monday to Wednesday.

There is 12.5% deficiency in rainfall over the country in September. It is 53.9% deficient over northwest India, central India (32.4%), east and northeast India (4.4%).

Peninsular India has reported 77.4% of excess rainfall. During the monsoon season since June 1, there is 6.6% excess rainfall over the country, including 5.5% deficiency over northwest India.

Rainfall is 13.5% and 1.5% excess over central India and east and north-east India, respectively.

Peninsular India has reported 28.4% of excess rainfall.

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