Tropical Cyclone Ward has spooked forecasts by real-time cyclone tracking models by changing course overnight on Sunday and heading west-southwest from its previous day’s bearing over the southwest Bay of Bengal.

It has since ceased to become a tropical cyclone, having weakened a notch lower into a deep depression with further prospects of weakening before it encounters the north-eastern coast of Sri Lanka by Monday morning for the first landfall.

India Meteorological Department (IMD) and other real-time tracking models such as the US Navy’s Joint Typhoon Warning Centre (JTWC) expect the weakened ‘Ward’ to later emerge into the Gulf of Mannar and crawl into the south-east Tamil Nadu coast between Kanyakumari and Pamban for a second landfall by Tuesday morning.

In contrast, the cyclone phase evolution and tracking models that put out forecasts even weeks ahead had been suggesting the approach of a cyclonic circulation over Sri Lankaand its passage into the Gulf of Mannar for a second landfall over mainland India.

But on Sunday, these models seemed to rule out the possibility of the remnant of ‘Ward’ making its way into mainland India (Kanyakumari-Pamban as suggested by IMD and others).

Instead, it would cross Sri Lankawith a west-southwest orientation and head into the adjacent equatorial Indian Ocean before getting dissipated. Only one model suggested the prospect of the system taking a re-curve from the Gulf of Mannar and heading back into the open waters of the Bay of Bengalskirting the Tamil Nadu coast.

Sea-surface temperature

An analysis of the meteorological parameters determining the course of ‘Ward’ suggested that the sudden drop of sea-surface temperatures (SSTs) may have caused the system to decelerate and weaken overnight.

Sea-based weather systems have a penchant for seeking warmer waters and this is what ‘Ward’ is doing by heading west-southwest into south-west Bay of Bengal and equatorial Indian Ocean surrounding central Sri Lanka.

The Gulf of Mannar happens to be cooler, and if the ‘Ward’ remnant were to enter these waters as forecast by the other agencies, it would lead to further weakening of the system en route to Kanyakumari-Pamban.

In its update on Sunday evening, the IMD said that the deep depression over south-west Bayof Bengal slightly moved southward and lay centred about 200 km east-northeast of Trincomalee (Sri Lanka), 400 km southeast of Nagapattinam and 400 km east of Pamban.

The Chennai Met Centre said in its update that the cyclonic storm had helped launch the north-east monsoon back into a vigorous phase over coastal Tamil Nadu.

Rainfall occurred at many places over coastal Tamil Nadu and at a few places over interior Tamil Nadu. Isolated rainfall occurred over coastal Andhra Pradesh and Rayalaseema.

Among centres recording heavy rainfall (in cm) are: Sriperumbudhur and Vallam-8 each; and Poonamalle-7.

Forecast for the next two days said that rain or thundershowers are likely at most places over coastal Tamil Nadu and Puducherry; at many places over interior Tamil Nadu andat a few places over south coastal Andhra Pradesh, Rayalaseema, Kerala and Lakshadweep.

Isolated rain or thundershowers are likely to occur over north coastal Andhra Pradesh, coastal and south interior Karnataka.

An IMD warning has said that squally winds with speed reaching 55 to 65 km/hr gusting to 75 km/hr are likely along and off the south Tamil Nadu coast during next two days.

Squally winds with speed reaching 50 to 60 km/hr gusting to 70 km/hr is likely along and off north Tamil Nadu and Puducherry coast during the next 24 hours.

Sea condition will be very rough to high along and off south Tamil Nadu coast and very rough along and off north Tamil Nadu and Puducherry coasts.

International models suggested that another wave of disturbed weather would descend over south-east and adjoining south-west Bay of Bengalfrom December 21 onwards.

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Posted on Monday, December 14, 2009 @ 08:28:34 LKT by SaNDun

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