Vinson Kurian

Thiruvananthapuram, Nov 18

THE next round of northeast monsoon rains is expected to commence over Tamil Nadu and adjoining States earliest by Sunday.

According to the National Centre for Medium Range Weather Forecasting (NCMRWF), the leisurely pace allowed the `low' over Bay of Bengal to progress no farther than to a position immediately to the southwest during the past 24 hours.

The system is not seen undergoing rapid intensification, which could deny it the capacity to force the heavy precipitation witnessed during previous spells over the peninsula, says Dr K. J. Ramesh of the NCMRWF. But the presence of the east-west shear zone will ensure its survival over the next four-five days.

It is the seemingly hyperactive seas that is proving the system's undoing.

A procession of sea-based circulations has been causing intense precipitation, cooling down the seawaters in the process.

This deprives a prospective system of the `warming anomaly' to feed further on and thus intensify.

Ideally, sea-based systems are comfortably spaced one after the other, allowing for thermodynamic changes to work suitably in order to create a warming anomaly.

The coastal waters normally warm the most during the season. But colour-coded graphics posted by the US Naval Coupled Ocean Data Assimilation (NCODA) programme on Friday showed that the warming anomaly in the Bay was limited to within 1 deg Centigrade.

One other factor that is working against the system is the presence of Typhoon Bolaven in the western Pacific. Raging as a Category 1 storm already, it will retain the status until Sunday, says the London-based Tropical Systems Risk (TSR) Group. It will weaken on making landfall over the Philippine coast and prevail as a tropical storm until Wednesday in the South China Sea.

Being part of the same Inter Tropical Convergence Zone (ITCZ), the `low' over the Bay and Typhoon Bolaven will need to share available rain bands among themselves.

This could rob the Bay system of some of its punch, says Dr Ramesh. But the shear zone will ensure that the system holds good, even enabling it to cross over into the Arabian Sea by Thursday next.

Meanwhile, in its weather update, the NCMRWF said isolated rainfall has been reported from the Northeastern States, the Andaman and Nicobar Islands and parts of southern Kerala during the 24 hours ending 8.30 a.m. on Friday.

The `low' over southeast Bay moved slightly westwards and lay to the west of 90E to the east of Sri Lanka. Associated upper air cyclonic circulation extended up to 2.1 km above mean sea level.

The system is likely to move slowly in west-northwestward direction in the coming days. The fact that it is embedded within the east-west shear zone covering Bay of Bengal, southern parts of the peninsula and the Arabian Sea will ensure that it remains active over the coming few days.

Under the expected scenario, fresh rainfall activity over Tamil Nadu and adjoining States may begin around Sunday.

Scattered to fairly widespread rains with isolated heavy falls can be expected over the regions of coastal Tamil Nadu initially, possibly extending later to the south coastal districts of Andhra Pradesh and Kerala.

(This article was published in the Business Line print edition dated November 19, 2005)
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