Key Points

  1. The IMD expects the North-East monsoon to set in over the South Peninsula on Thursday, with rain already affecting parts of Tamil Nadu’s coast.
  2. A trough and a circulation near Comorin are drawing moisture-laden easterly winds, which can generate rain without a single organised storm.
  3. Isolated very heavy rain is forecast in parts of Tamil Nadu, Puducherry, Karaikal, Kerala and Mahe, with unsettled weather expected to continue in several areas.

How the North-East Monsoon Could Reach Tamil Nadu

India’s Meteorological Department (IMD) expects the North-East monsoon to begin over the South Peninsula on Thursday, as the preceding South-West monsoon withdraws. Rain has already been reported along parts of the Tamil Nadu coast, alongside thunderstorms, lightning and gusty winds.

The weather change is linked to a cyclonic circulation over the Comorin area and a trough extending from the Gulf of Mannar across the Bay of Bengal. The trough is helping draw moisture-rich easterly winds towards southern India. The IMD forecast isolated very heavy rain on Thursday in Tamil Nadu, Puducherry and Karaikal, as well as Kerala and Mahe. Heavy rain could affect several southern and neighbouring areas over the following five days.

This onset may not look like a single large storm. Instead, successive clusters of clouds and showers could form, bringing intense rain and gusty winds in some places, with clearer intervals in between. The timing and location of the heaviest rainfall will depend on how the trough and surrounding weather conditions develop.

The forecast also includes thunderstorms, lightning and gusty winds. Strong surface winds are expected in Coastal Andhra Pradesh, Yanam and Rayalaseema from Thursday. Sri Lanka’s meteorological department has separately warned of showers or thunderstorms across several provinces, with heavy falls possible in some areas.

Why a Trough Can Bring Heavy Rain Without a Storm

Why the trough matters

A trough is an elongated zone where winds may converge or change direction; it does not have to become a closed storm system before rain develops. Where moist air is pushed upward, clouds can build and produce showers if the atmosphere is sufficiently unstable. That helps explain why rain can start before the trough reaches the coast and why there may be no single, clearly defined storm.

Rainfall may vary over short distances

The article describes a pattern of separate showers and thunderstorms that could combine into broader rain bands. It also notes that the heaviest rain may occur over or behind the trough rather than ahead of it. These features make the precise timing and location of intense rainfall difficult to infer from the onset forecast alone; the IMD’s area-specific updates remain important.

What Residents in the Forecast Areas Should Expect

Residents in the forecast areas should plan for periods of heavy rain, thunderstorms and gusty winds rather than assuming conditions will remain steadily wet. Follow local IMD updates, especially for changes to rainfall warnings, and take particular care during lightning and intense downpours. In Coastal Andhra Pradesh, Yanam and Rayalaseema, also account for the forecast strong surface winds from Thursday.