Sunday, December 7, 2008

Shift in bearing may cap Bay storm strength

Vinson Kurian

Thiruvananthapuram, Dec 5 Thursday’s depression in the Bay of Bengal has intensified into a deep depression on Friday, just a churn away from being classified as a cyclone, but slight shift in coordinates could weigh in to alter storm strength and direction.

According to the Joint Typhoon Warning Centre of the US Navy, the storm, numbered 07B, has of late shown a tendency to move along a more northerly track than seen earlier, guiding itself into a region massed by vertical shearing winds.


These shearing winds can lop off the top of a building storm, undermining its strength. A JTWC update cited satellite imageries to assess that 07B may already have been affected in this manner, with the low level circulation centre around which the storm builds getting partially exposed along the western flank.

Convective bands should ideally wrap into a closed core for the system to grow in strength. But incursion of dry winds or close proximity to land could work against consolidation in this manner despite obliging surface waters.

07B is forecast to continue to move to the northwest-to-west-northwestward direction (a net northerly bias to the track) that would bring it face to face with shearing winds.


This may minimise 07B’s development as it moves in for a landfall which JTWC now figures could take place even along the southeast Indian coast (Tamil Nadu coast) by Saturday night or early Sunday morning.

In this context, the forecast by India Meteorological Department (IMD), the Regional Specialised Meteorological Centre for Indian Ocean (both the Bay of Bengal and Arabian Sea), is instructive.

The IMD update said on Friday that overnight’s depression over the southeast Bay moved west-northwestward and intensified into a deep depression at 5.30 a.m. It remained practically stationery about 650 km southwest of Port Blair, 1,100 km southeast of Chennai and 800 km east-southeast of Triconmalee.

The system is likely to intensify further and move in a west-northwesterly direction towards north Sri Lanka, Tamil Nadu and Puducherry coast during the next three days.


Rain or thundershowers are likely at many places over Andaman and Nicobar Islands and at a few places over coastal Tamil Nadu and Puducherry during the next 24 hours and at most places, thereafter.

Mainly dry weather is being forecast for the rest of Tamil Nadu, Kerala, Lakshadweep, Rayalaseema and coastal Andhra Pradesh during next 24 hours but scattered to fairly widespread rain or thundershowers thereafter. Isolated heavy to very heavy falls is likely over coastal Tamil Nadu and Puducherry commencing from Saturday night.

Squally winds speed reaching 55-65 km/hr in speed and gusting to 75 km/hr are likely along and off the Tamil Nadu and Puducherry coasts from Sunday morning.


Peak winds could gust to even 85 km/hr on Sunday and Monday. Sea condition will be very rough to ‘high’ along these coasts and fishermen have been advised not to venture out. Extended forecasts up to Wednesday spoke of fairly widespread rainfall activity over extreme south peninsula and the Andaman and Nicobar Islands.

Meanwhile, a moderately strong western disturbance is set to enter northwest India around the same time with a likely embedded cyclonic circulation. It is expected to cause scattered precipitation over the western Himalayan region and adjoining plains of northwest India.