The Hindu Business Line : Rains along TN coast to intensify

Scattered rainfall activity has occurred over coastal Tamil Nadu and south Andaman and Nicobar Islands, while being isolated over south Tamil Nadu and Lakshadweep during the 24 hours ending Wednesday morning, an India Met Department (IMD) update said on Wednesday.

BUILD-UP ON

In what is seen as a build-up to the northeast monsoon, a warning issued by the Centre and valid for the next two days said that isolated heavy rain is likely over coastal Tamil Nadu and Puducherry.

Rain or thundershowers are likely to occur at many places over coastal Tamil Nadu and Puducherry and at a few places over interior Tamil Nadu, south coastal Andhra Pradesh, Rayalaseema, south Interior Karnataka, Kerala and Lakshadweep.

Isolated rain or thundershowers have been forecast over coastal Karnataka. Updated forecasts from international models suggest that the best, if not worst, of the seasonal rains over the southern Indian peninsula may be triggered from this weekend.

This is based on the forecast values of outgoing long-wave radiation (OLR) from the surface - negative values indicate presence of cloud cover and, by extension, possibility of rains.

All leading models tracking OLR are unanimous in their outlook that the OLR values would stay moderately to strongly negative for at least two-week period from November 1 or 2.

Some of these are of the view that the southwest Bay of Bengal may not yet have come out from the residual impact of a preceding dry phase of the Madden-Julian Oscillation (MJO) wave with positive OLR values.

The MJO wave is an upper level phenomenon transiting from west to east over the Indian Ocean and has alternating dry and wet phases.

SIGNIFICANT ACTIVITY

The impending wet phase is invariably linked to negative OLR values, which sparks monsoon onsets and other significant atmospheric disturbances over the region falling under its footprint.

The OLR-tracking models - Wheeler, Jones and Extended Wave Propagation technique by the US National Centres for Environmental Prediction (NCEP) - predict that the negative values would stay strong until November 16 up to which forecasts were available.

The OLR values decay over time and it is expected that the wet phase over the peninsula would take that much longer to spend itself out.

The precipitation models too seem to have tweaked their forecasts to reflect the emerging trend in the OLR values. The NCEP says that heavy rains would be confined largely to coastal Tamil Nadu between October 28 and November 4.

But the week starting November 5 would see the rains being propelled further westward across the peninsula into interior Tamil Nadu and adjoining Kerala.

NOT RULED OUT

The NCEP does not also rule out the possibility of some storm activity in the Bay. This is even as the Round-Albany tracking model continues to keep a vigil over the Bay waters for signs of storm development during the first week of November.

The IMD outlook said that easterly and northeasterly winds over Bay of Bengal and peninsular India would gather further strength over the next few days.

Satellite imagery on Wednesday showed convective clouds over parts of south Andaman Sea, southwest and southwest Bay of Bengal. Low or medium clouds were seen over parts of south peninsular India.

The east-west trough from southeast Arabian Sea to south Andaman Sea through extreme south peninsular India and south Bay of Bengal persisted with an embedded cyclonic circulation over southeast Arabian Sea.

A western disturbance is likely to affect western Himalayan region during the next two days, the IMD added.

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