Advancing faster than expected, Monsoon 2018 covered entire country on June 29 that was over 15 days in advance. The official date of onset of Monsoon over the last post of the country is July 15. Not only this, the advancement of Northern Limit of Monsoon also established Monsoon trough, which can be touted as the backbone of the four-month long season.
Usually, this trough that is established when the Monsoon covers the country, forms by mid-July. However, this time it came into existence well before the r theIt runs all across the Indo-Gangetic plains till head Bay of Bengal.
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Monsoon trough’s inherent characteristics makes it the most important Monsoon driver. Not only the trough governs the rain over large parts of the country, it also establishes Monsoon patterns. Apart from this, the other two important drivers for the season are off-shore trough that runs all along the West Coast from Coastal Gujarat to Kerala and Monsoon systems that mostly form in Bay of Bengal and move inland. Arabian Sea very rarely see any weather system forming.
The Monsoon trough is responsible for rains across Indo-Gangetic plains including Madhya Pradesh. Usually, the trough is seen running through Sri Ganganagar, Delhi, Allahabad, Jharsuguda, Kolkata and North Bay of Bengal. However, Monsoon trough is oscillating in nature that keeps moving north or south from time to time.
To whichever direction it is moving, rain would also concentrate over that region, particularly over the areas falling in southern region.
The trough also has a tendency to move closer to the foothills of Himalayas when moving northward. During this time, it runs along the foothills of Punjab, Haryana, Uttar Pradesh, Bihar, Sub-Himalayan West Bengal, Assam and Arunachal Pradesh.
According to weathermen, whenever any active Western Disturbance passes through Jammu and Kashmir, it pulls the western end of the trough. This trough is a divide between is the westerly winds and easterly winds. As the trough moves northwards, we can expect heavy to extremely heavy rainfall all along the foothills. Especially places such as Sub Himalayan West Bengal, Arunachal Pradesh and Assam are likely to witness extremely heavy rainfall till the time trough is running across the foothills. However, parts of Punjab, Haryana, West Uttar Pradesh and Bihar would see rains in pocket.
Nepal also witnesses extremely heavy rainfall during this time that usually triggers flood like situation over parts of Bihar and Uttar Pradesh.
However, if the Monsoon trough persists over the foothills for a longer time, it is termed as break-Monsoon conditions. During such situation, the pan India witnesses westerly wind flow that generally reduces the rainfall over most parts of the country. The only region to recorded rainfall during this time are West Coast, foothills of Himalayas, Rayalaseema and Tamil Nadu.
Any information taken from here should be credited to skymetweather.com