Northeast monsoon arrives early; heavy rain alert for South India

The northeast monsoon is a seasonal wind-and-rain system affecting southern peninsular India from October through December. The India Meteorological Department declared its onset this year over the region. It matters because it supplies a major share of rainfall to several southern states and territories.

It is also called the winter monsoon because it occurs during the cooler part of the year, after the southwest monsoon season. During this period, the affected region includes Tamil Nadu, Puducherry, Karaikal, coastal Andhra Pradesh, Kerala and southern Karnataka. Tamil Nadu receives nearly half its annual rainfall then.

The season is currently bringing heightened rainfall activity, gusty winds, thunder and lightning. The IMD expects these conditions through October 18 in several areas. Understanding the winter monsoon helps communities prepare for heavy rain, while also tracking whether rainfall meets the seasonal forecast of 88 per cent of average.

The northeast monsoon affects a broad belt of southern India and nearby territories. The IMD lists Tamil Nadu, Puducherry, Karaikal, Yanam, Rayalaseema, coastal Andhra Pradesh, Kerala, Mahe and south interior Karnataka among the affected areas. These regions depend on the season for important rainfall.

This year, the IMD declared the monsoon’s onset on Thursday, but the article does not provide the exact calendar date. Its normal onset is around October 20. Therefore, the arrival was early, although the number of days ahead cannot be calculated from the information given.

The early onset is already linked with active weather. Through October 18, Kerala, coastal and southern Karnataka, Tamil Nadu, Puducherry, Mahe, Karaikal and coastal Andhra Pradesh may see heavier rain, gusty winds, thunder and lightning. Local authorities and residents therefore face an active early-season period.

The IMD forecast normal northeast monsoon rainfall, defined in the article as 88 per cent of the long-period average. That average is 334.1 millimetres for the 1971–2020 period. The forecast matters because it gives communities and planners a seasonal benchmark for water supplies and rainfall expectations.

Calculating 88 per cent of 334.1 millimetres gives 294.008 millimetres. Rounded sensibly, the expected seasonal rainfall is about 294 millimetres. This is a whole-season estimate, not a prediction that every location or every day will receive the same amount.

Rainfall can still be intense over short periods even when the season’s total is near normal. The IMD has warned of heightened activity, gusty winds, thunder and lightning through October 18. A cyclonic circulation over southern Tamil Nadu may further increase intense rainfall in parts of the region.

Forecast heavy rain and powerful weather can affect daily life quickly. Water may collect on roads, enter homes and flood low-lying areas. Gusty winds can break branches, damage weak structures and disrupt transport. Thunder and lightning can threaten people outdoors and interrupt electricity services. These are general weather risks associated with the conditions described.

For example, intense rain can overwhelm urban drains, while wind-driven rain can reduce visibility on roads. Lightning may also force outdoor work, school activities or public events to stop. Flooded roads can delay buses, trains and emergency vehicles. The severity depends on local drainage, construction quality and the duration of the storm.

The IMD expects heightened activity through October 18 in Kerala, coastal and southern Karnataka, Tamil Nadu, Puducherry, Mahe, Karaikal and coastal Andhra Pradesh. A cyclonic circulation over southern Tamil Nadu may intensify rainfall. Residents should follow official warnings and avoid exposed locations during storms.

The northeast monsoon is especially important to Tamil Nadu because nearly half of the state’s annual rainfall arrives during this season. Rainfall from October to December therefore strongly influences the state’s overall water picture. A normal season can help meet needs after the southwest monsoon ends, while unusual rainfall can create shortages or flooding.

The key mechanism is concentration: a large share of the yearly total falls within only three months. The IMD expects normal northeast monsoon rainfall overall, at 88 per cent of the 334.1-millimetre long-period average, or about 294 millimetres. However, seasonal totals do not prevent intense rain on individual days.

This year, a cyclonic circulation over southern Tamil Nadu is expected to contribute to increased intense rainfall. The IMD also forecast gusty winds, thunder and lightning through October 18. Tamil Nadu therefore faces both the benefit of vital seasonal rain and the immediate need to manage severe weather.

A cyclonic circulation is a broad area where winds rotate and air converges. This movement can bring moist air together and encourage it to rise. Rising moist air cools, helping clouds grow taller and rain become heavier. This is a general meteorological explanation of how such a circulation can intensify rainfall.

Over southern Tamil Nadu, the circulation is acting as an added weather trigger during the northeast monsoon. Moisture-laden air converging near the circulation can support thick rain clouds. If the rising motion continues, rainfall may become intense over a shorter period rather than falling evenly across the day.

The IMD specifically said the circulation will further contribute to increased intense rainfall. It also warned of gusty winds, thunder and lightning across southern peninsular areas through October 18. The circulation does not determine the entire season’s total, but it can make local rainfall episodes more dangerous.

El Niño is a climate pattern involving warmer-than-usual surface waters in the tropical Pacific Ocean. It changes large-scale air pressure and wind patterns around the world. In India, these changes can weaken the moisture flow and rising-air patterns that support the southwest monsoon. This explanation uses established climate science; the article identifies El Niño as the main influence on this year’s shortfall.

When the Pacific warms, atmospheric circulation can shift in ways that reduce or disrupt monsoon moisture reaching India. The result may be fewer or weaker rain-bearing systems during the southwest monsoon season. The effect varies by event, but El Niño is widely associated with below-normal rainfall in India.

The article says intensifying El Niño conditions primarily influenced the southwest monsoon’s outcome in 2026. Seasonal rainfall ended 13 per cent below normal, making 2026 the fourth driest monsoon since 2001. The northeast monsoon forecast is separate and currently calls for normal rainfall at 88 per cent of its long-period average.