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Vinson Kurian

Thiruvananthapuram , Dec. 21

THE National Centre for Medium Range Weather Forecasting (NCMRWF) maintained its outlook of a fresh low-pressure area brewing over the southeast Bay of Bengal on a day when Tropical Depression 07B virtually stagnated over the seas 500 km east-southeast of Chennai.

The new `low' will be a remnant of the Tropical Depression 25W migrating from the tropical western Pacific, which the Tropical Storm Risk (TSR) Group of London traced to south of the Cambodian landmass in its latest forecast.

The European Centre for Medium-Term Forecasts (ECMWF) also concurred with the NCMRWF outlook of the migrating system setting up a `low.'

The new system might tread the path of 07B in a pattern identified with cyclonic circulations developing during December, said Dr Akhilesh Gupta of the NCMRWF. His research into cyclone behaviour sometime back had revealed that 23 per cent of the systems straying outside of the 15 degrees north latitude (passing through Tadipatri in Andhra Pradesh and Hubli-Dharwad in Karnataka) during this time of the year tended to head north-northeast and dissipate over the waters.

The UK Met Office said 07B will become a storm of moderate strength from 5.30 p.m. on Wednesday and could even intensify slightly. It will retain the status until Friday when it will start weakening rapidly. It will reach below tropical storm strength by Saturday evening. The Joint Typhoon Warning Centre (JTWC) said the system would kick up speed in lateral movement as it tracked northeast.

The JTWC said in its 48-hour outlook that the system is likely to be consumed by the large-sized westerly trough, which was primarily responsible for causing it to drift away off the peninsular coast and head northeast. The maximum possible period to which the system can possibly retain its individual self is until it reaches a location 278 nautical miles (514 km) south of Kolkata.

The westerly troughs display the same atmosphere dynamics as conventional `lows' but are endowed with much larger amplitude, Dr Gupta said. What they lack is moisture, but even that is assured once they run into and devour relatively pint-sized tropical systems..

A `rain-bearer' payload thus gets added in the bargain and the extra-tropical system will now be able to trigger rains upfront. In fact, every third cyclone in the tropics is invariably impacted by these extra-tropical systems as they traverse the globe from west to east.

This goes to explain the forecast made by the European Centre for Medium-Range Forecasts (ECMWF) of a weather event in east India, even extending into Bangladesh, Dr Gupta said. The system might even proceed to converge over South China Sea towards further northeast. The northern flank of the same westerly trough is what triggers the advance of cold air into the plains of north India.

In its update, the NCMRWF said 07B remained practically stationary till 8.30 a.m. on Wednesday. It is likely to move further northeast and weaken gradually. Under this scenario, no adverse weather is likely to impact the east coast of India. But coastal Orissa, West Bengal and the Northeastern States are likely to experience isolated to scattered rainfall during the next two days. Isolated to scattered rainfall is also likely over coastal Tamil Nadu, Pondicherry, coastal Andhra Pradesh and the Andaman and Nicobar Islands during the period.

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Outlook on new low-pressure area over Bay maintained

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