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Rain belt moves north as Bay system turns into a depression

Vinson Kurian

Thiruvananthapuram , Oct. 26

TUESDAY'S low-pressure area over southwest Bay of Bengal jumped at least two notches in scale to turn into a depression overnight, with prospects of further intensification into a more severe weather system.

The National Centre for Medium Range Weather Forecasting (NCMRWF) has extended its heavy rain alert for north Tamil Nadu and south coastal Andhra Pradesh for the next two days. The longer the system plays out with its centre off the coast and into the seas, as it is doing presently, the more active it will remain.

According to Dr K.J. Ramesh of the NCMRWF, the rain bands that brought large areas of south interior Karnataka (Bangalore included) and north Tamil Nadu under a massive sheet of water have since shifted to the north. The Bay of Bengal system and a counterpart located over the Arabian Sea have been feeding each other bringing about some of the heaviest showers that these regions have ever witnessed.

But the depression is moving at so slow a pace that these regions, along with the contiguous south coastal Andhra Pradesh to the north, will continue to experience some really wet weather over the next two days. The system is expected to weaken after making a landfall over coastal Andhra Pradesh, according to Dr Akhilesh Gupta of the NCMRWF. But the landfall itself is some days away.

Coastal waters are much warmer than the relatively cool open seas where the system is centred and is generating the maximum rainfall. But the system thrives by feeding on warmer waters, which explains its movement towards the coast.

On Wednesday, the depression lay centred around 12North and 84.5East over the southwest Bay of Bengal that housed some of the warmest waters in the tropics this season. In fact, the seas around the globe straddling latitudes 10N to 20N have seen the Sea Surface Temperatures (SSTs) spurting by the order of one-to-two deg C (normal SSTs range from 27deg C to 28deg C). It's the warmer seawaters that give rise to severe weather systems, including violent storms marked by high winds and heavy precipitation.

According to unclassified data put out by Navy Coupled Ocean Data Assimilation (NCODA), a US military programme, the Bay of Bengal and the Central American waters (including the Caribbean Sea and rest of the North Atlantic which spawned some of the deadliest hurricanes in history) have both seen SSTs rise in the abovementioned range and, in some cases, even beyond.

For instance, the colour-coded NCODA graphics show the Bay waters around South Andamans (90deg E), as also parts of the comparatively vast and open tropical Atlantic, having warmed beyond the range. What however prevents the Bay systems from growing into monstrous storms in the league of a Hurricane Katrina or Hurricane Wilma is perhaps the relatively less distance they travel over sea.

A Bay of Bengal system travels smaller distance to make a landfall, denying itself the feed from the warm waters, and weakening immediately thereafter.

Meanwhile, region-wise forecast by the NCMRWF said isolated rains are likely over Chhattisgarh, east Madhya Pradesh and Vidarbha over the next two days. Scattered to fairly widespread rains are expected over Chhattisgarh, Vidarbha and east Madhya Pradesh subsequently.

In the south, fairly widespread to widespread rains with isolated heavy to very heavy rains are expected over Tamil Nadu, Pondicherry, south coastal Andhra Pradesh, Kerala, coastal and south interior Karnataka and the Andaman and Nicobar Islands during next two days. Thereafter, the rain activity is likely to taper off in these regions.

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