The hyperactive trough that is drenching Chennai and the rest of Tamil Nadu will stay ‘live’ until the weekend to extend the wet spell for as long and beyond.

The rains will continue to be heaviest around Chennai, Tiruvallur and Kanchipuram districts and adjoining south coastal Andhra Pradesh, say global forecasts.

Heavy rain forecast The Regional Met Centre, Chennai, agrees, and has put out the following warning:

Wednesday: Widespread showers over Tamil Nadu and Puducherry with isolated extremely heavy rainfall over Chennai, Tiruvallur and Kanchipuram districts.

Heavy very heavy rain over coastal Tamil Nadu and Puducherry. Isolated heavy rain over interior Tamil Nadu.

Thursday: Widespread showers with heavy rain and isolated heavy to very rain over Coastal Tamil Nadu and Puducherry.

Isolated heavy rain over interior Tamil Nadu.

Friday: Widespread showers with isolated heavy to very heavy rain over Coastal Tamil Nadu and Puducherry. Isolated heavy rain over interior Tamil Nadu.

Saturday: Isolated heavy to very heavy rain over Coastal Tamil Nadu and Puducherry.

Isolated heavy rain over interior Tamil Nadu. There is no significant change in weather seen until Tuesday.

Potent trough The causative trough (precursor of a low-pressure area) is expected to weakening after North-Easterly flows rush through the Palk Straits and the Gulf of Mannar to form another trough in the Lakshadweep Sea.

Even this trough will continue to attract the moisture-laden flows from the Bay of Bengal to Gulf of Mannar, raining it down on the Tamil Nadu and Puducherry coasts along the way.

These rains will gradually weaken from the North from Tuesday (December 8), and concentrate more on south Tamil Nadu and adjoining Sri Lanka.

The movement of the trough into the Lakshadweep Sea will in turn set off thunderstorms along the West Coast as also the interior of the peninsula.

Meanwhile on Tuesday, heavy rains lashed parts of Tamil Nadu, including Chennai, and neighbouring Puducherry, disrupting flight movement and leaving several areas inundated.

There is apparently no respite on Wednesday either since the Met Office has forecast heavy to very heavy rainfall for Tamil Nadu, especially over the northern districts as well as Puducherry.

The havoc wrought by the heavy downpour brought home the realisation how a fully-charged trough – no full-fledged low-pressure area here or depression here – can hold a metropolis to ransom.

Some of the troughs can drop anchor for much longer than slowly-paced low-pressure areas and produce sustained heavy to very showers, which is on view right now.

Tamil Nadu’s coast is always vulnerable to the whims of the Bay of Bengal which stays more active than the Arabian Sea through the monsoon seasons – South-West or North-East.

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