3-day rain forecast across India from Wednesday to Saturday mornings.

(TWC Met Team)

Wednesday, July 13: With the monsoon rains pouring in full force across India, the India Meteorological Department (IMD) has painted almost the entire country with yellow, orange and red alerts for the week. While this is expected during the monsoon season, one may wonder what changed, especially after some parts of India witnessed up to 70% rainfall deficit in June.

Here are a few reasons why the monsoon has turned active and vigorous over the last two weeks in multiple regions across India.

The monsoon trough

The monsoon trough — an elongated low-pressure area that drives monsoon rains across the country’s northern half — is usually responsible for such drastic changes in monsoon rainfall. The trough extends from low-pressure areas over Pakistan to the ones over the Bay of Bengal. It is said to be in its usual position when it passes roughly through Ganganagar, Allahabad, Kolkata and the head of the Bay of Bengal.

At times, this monsoon trough runs close to the foothills of the Himalayas — north of its normal position — and such phases are called the ‘break’ phases of monsoon. Rainfall decreases sharply over most parts of the country during this phase, which usually lasts around two weeks. But the monsoon rains get revived across northern and central plains as the trough moves southwards again.

At present, the monsoon trough is active and south of its normal position. As of early Wednesday, it was passing through Bikaner, Jaipur, Guna, Jabalpur, Raipur, the centre of a well-marked low-pressure area over south coastal Odisha and neighbourhood, and thence southeastwards to the east-central Bay of Bengal.

Moreover, a few other meteorological systems are interacting with the monsoon trough to create added precipitation over different regions, such as a monsoon low-pressure area.

Low-pressure over East India

As of Wednesday morning, a well-marked low-pressure area lingers over south coastal Odisha and the neighbourhood. As the name indicates, the atmospheric pressure in these areas is lower than that of surrounding locations, which is why they are commonly associated with inclement weather such as cloudy, windy, with possible rain or storms.

Research shows that such monsoon lows are responsible for up to 70% of the summer monsoon rainfall over eastern India and around 50% over the entire country. But on the flip side, these monsoon lows have been responsible for triggering several disastrous floods in the past, like the 2018 Kerala floods that killed over 400 people and displaced a million more.

According to RSMC New Delhi, the present system will intensify further and become more marked in the next 24 hours. It also has moderate chances of strengthening into a depression in the next 24-72 hours.

This on-land system will (directly and indirectly) result in widespread heavy to very heavy rains across East and adjoining Central and South India, impacting the likes of Odisha, Chhattisgarh, Madhya Pradesh, Maharashtra, Gujarat, Coastal Andhra Pradesh, Telangana, Karnataka and Kerala during the next 4-5 days.

Off-shore trough from Gujarat to Karnataka

Whenever a low-pressure system forms over the Bay of Bengal and passes along the monsoon trough, it activates an offshore trough across the west coast, resulting in strong moisture-laden winds lashing coastal districts of Maharashtra, Goa, Karnataka and Kerala. As the Western Ghats block further penetration, these regions also receive substantial rainfall.

Even this week, an offshore trough persists off the west coast, extending from the Gujarat coast to the North Karnataka coast. It will likely aid moisture intrusion from the Arabian Sea to India’s western coastal regions.

Accordingly, the IMD has predicted isolated heavy rains over Karnataka and Kerala, and isolated to scattered heavy to very heavy showers with isolated extremely heavy falls over Gujarat, Maharashtra and Goa over the next 4-5 days.

Overall, wet conditions are on the horizon for India’s northern, central, western and southern states, with parts of Gujarat, Maharashtra, Madhya Pradesh and Odisha particularly under intense rainfall alerts during this forecast period.

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