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Twin-engined reverse monsoon in top gear

Vinson Kurian

VIGOROUS START: Powered by active low-pressure areas on either side of the peninsula, the northeast monsoon triggered a soaking wet session in Kerala, especially the southern districts, on Wednesday. More rain has been forecast for the State in the coming days. - S. Gopakumar

Thiruvananthapuram , Oct. 12

THE northeast monsoon has undergone a dramatic transformation with the east-west shear zone, the hotbed of monsoon turbulence in the Southern peninsula, getting elongated and held point-to-point by active low-pressure systems on either flank.

The `low' over the south Bay of Bengal has persisted over the last three days and has remained more or less stationary. It is now joined by a concurrent system over the Arabian Sea, bringing the entire southern peninsula under a sheet of falling rain.

The latter system has had its genesis as an upper air pulse over the bay a few days back before crossing over land and into the Arabian Sea.

Speaking to Business Line, Dr K. J. Ramesh of the National Centre for Medium range Weather Forecasting (NCMRWF) said the elongated shear zone with two embedded `lows' provides the best available platform for the northeast monsoon to fire full-throttle. Under its influence, regions as far as Goa and the north Konkan region are tipped to receive widespread rainfall during the coming few days.

The shear zone on its own can hold sway for at least 2-3 days irrespective of the tenure of the embedded `lows.' The dragging effect of the zone is such that any incipient turbulence coming within range will be made to feed on available moisture to become potential `lows' or even further intensified systems. Prevailing weather features in the locality will have a great role in deciding how the reverse monsoon goes on to deliver, Dr Ramesh said. For the fourth consecutive day on Wednesday, parts of Tamil Nadu, Pondicherry, Lakshadweep, coastal Andhra Pradesh and south interior Karnataka came under scattered to fairly widespread rainfall. Significant rainfall was reported from Chennai — 9 cm; Paradip and Thiruvananthapuram — 8 cm each; Kavali — 7 cm; Ongole, Nellore and Thiruchirappalli — 5 cm each; and Puri, Bapatla and Karwar — 4 cm each.

The NCMRWF has extended by another day its watch for the virtually stationary `low' over the south Bay to become well-marked and move very slowly in a westerly/north-westerly direction. This would culminate in the system making a landfall over the peninsular coast earliest by Thursday. The more the bay system stagnates, better will be the rainfall realised. In tandem with the pulse that crossed over west into the Arabian Sea, the system promises to recharge groundwater levels, particularly in Andhra Pradesh and interior Karnataka.

With the likely strengthening of the north-easterly wind regime packed in the active northeast monsoon, rainfall activity over the coastal regions of Orissa, Gangetic West Bengal and even the Northeastern States would get a boost. On the other hand, the persisting good rainfall all over southern peninsula will expectedly create an above normal rainfall scenario for the season, the NCMRWF said.

Model predictions indicate that the hilly regions of Jammu and Kashmir, Himachal Pradesh, Uttaranchal and Sikkim are likely to have isolated to scattered rainfall activity for the next 2-3 days under the twin effect of a passing western disturbance and the eastward movement of an upper air cyclonic circulation.

In region-wise forecasts for the next 4-5 days, the NCMRWF said sub-Himalayan West Bengal and Sikkim are likely to experience only isolated spells of rainfall activity. It will be scattered to fairly widespread over Orissa, Gangetic West Bengal, Nagaland, Manipur, Mizoram, Tripura, upper Assam and Arunachal Pradesh.

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