The Hindu Business Line : Southwesterlies continue to drive rain

Roll of thunder and flashes of lightning have announced the monsoon transition phase in parts of the southern peninsula, but it's still the dominant southwesterlies that are driving the showers.

"Thunder activity results from increasing wind shear as the northeasterly winds run into prevailing southwesterlies," explained Dr Akhilesh Gupta, Advisor to the Department of Science and Technology.

The northeasterlies continue to be weak and disorganised over the Bay of Bengal, the breeding ground for weather events that herald the northeast monsoon. They are better established over the Arabian Sea but that's poor consolation.

Northeasterlies must get into their rut over the Bay for the northeast monsoon to flourish. The shear of zone of monsoon turbulence would also need to get suitably organised, and the Inter Tropical Convergence Zone of low-pressure accentuated in tandem along 13N latitude (Chennai).

Ideal condition

The Arabian Sea falling into line will help enhance the rain activity, but that's about all. The most ideal condition would have been established when both the peninsular seas break into a song, with a `low' in the North Bay and a mid-tropospheric cyclonic circulation over Gujarat's Arabian Sea coast.

A simple trough over the Bay is all what is required for the northeast monsoon, also called the retreating monsoon, to get going. There's no clear sign of this happening at least until October 20 (Friday). This means, southwesterlies will continue to hold sway.

Withdrawal process

Meanwhile, the National Centre for Medium Range Weather Forecasting (NCMRWF) said the southwest monsoon has further withdrawn from the Northeast, the north Bay, Gangetic West Bengal, Orissa, north interior Karnataka, madhya Maharashtra, Konkan, Goa and Telangana.

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