Open easterly trough (left) represents a bend in the easterly flows while a low-pressure area is (right) is a closed circulation. A trough is forecast to bring in the north-east monsoon for the South Peninsula. | Photo Credit: (nil)

The keenly-awaited north-east monsoon could set in over the South Peninsula on Thursday, with piloting moderate to heavy rain already lashing parts of the Tamil Nadu coast through Wednesday, and thunderstorms, lightning and gusty winds signalling a gathering change in weather over the past few days. 

India Meteorological Department (IMD) expects the onset of the October-December rainy season to coincide with withdrawal of preceding south-west monsoon. Driving the transition is a cyclonic circulation over the Comorin area, about 1.5 km above the surface, linked to a trough stretching from the Gulf of Mannar across the south-west Bay of Bengal to the Odisha coast.

By Wednesday afternoon, the trough extended along the coasts of Tamil Nadu, Puducherry, Andhra Pradesh and Odisha, ready to channel moisture-laden easterlies from the Bay towards the South Peninsula.

Isolated very heavy rain

The IMD has forecast isolated very heavy rain over Kerala and Mahe, and Tamil Nadu, Puducherry and Karaikal on Thursday. Heavy rain is may persist across several southern and adjoining regions over the following five days, with thunderstorms, lightning and gusty winds adding to the unsettled conditions. Strong surface winds are also forecast over Coastal Andhra Pradesh, Yanam and Rayalaseema from Thursday.

The North-East monsoon is set to announce arrival over Tamil Nadu in a deceptively subtle fashion, not with a well-defined storm spinning over the Bay of Bengal, but with an elongated trough in the easterlies that may organise clouds and trigger rain gradually over the larger South Peninsula as well. This will hold true for the island nation of Sri Lanka just to the south-east, which has come out with outlook for heavy rain not just for Thursday but following days also along its east and south-east coast that open out to the south-west Bay.

The Sri Lanka Meteorological Department said a ‘low-level atmospheric disturbance persisting in the Bay may bring cloudy skies and increased rainfall over the island. Showers or thundershowers will occur at times in the Northern, Eastern, Southern and Western provinces. Heavy falls above 7.5 cm are likely at some places in Western, Sabaragamuwa, Central and North-central provinces and in Trincomalee and Kurunegala districts.

Why does a trough matter?

Unlike a cyclonic circulation, a trough, while precipitating the north-east monsoon this time round, does not need to develop into a closed system or reach the coast before rain begins. Cloud-building (convection) can develop ahead of its axis, along it or behind it, depending on the wind structure, moisture supply and atmospheric instability. Nor is a trough necessarily a weaker rain producer.

An easterly wave is a disturbance embedded in the prevailing easterly flow, without the closed circulation associated with a low-pressure system. As winds approach a trough, they may slow, change direction or converge. This concentrates moisture and forces air upwards, allowing clouds to grow and rains to gather when the atmosphere is sufficiently moist and unstable. Instead of one organised storm system, Tamil Nadu coast could experience successive bands of showers and thunderstorms separated by deceptively clear intervals.

What happens next?

An approaching trough may initially be difficult to recognise from the ground. The coast could remain partly sunny, with scattered cumulus clouds gradually growing into taller towers. Humidity may increase without any marked fall in temperature, while surface winds fluctuate as the disturbance approaches.

As the more active part of the trough draws closer, cloud cover could expand rapidly. Separate convective clusters may merge into broader rain bands, darkening the horizon in patches, rather than uniformly. Gusty winds and short, intense showers may alternate with quieter spells.

Watch for heavy rain

But the heaviest rain need not arrive before the trough axis passes. Research on easterly waves over South India has found that rising motion and convergence can be concentrated over and behind the trough, while sinking air associated with higher pressure and a comparatively drier environment, may prevail ahead of it. Rainfall could consequently persist or intensify on the trailing side, depending on the wave’s structure and movement. 

Published on October 14, 2026