The southwest monsoon is likely to set in over Kerala on May 27, earlier than the usual onset date of June 1, the India Meteorological Department (IMD) said on Saturday.

If the monsoon arrives in Kerala on the predicted date, it will mark the earliest onset over the Indian mainland since 2009, when it began on May 23, according to IMD data.

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As per IMD records, last year the monsoon arrived on May 30, a day before the normal date, while in 2023, it set in on June 8.

The weather department stated that the advance of the southwest monsoon over the Indian mainland is marked by its onset over Kerala and serves as a key indicator of the transition from the hot and dry season to the rainy season.

As the monsoon progresses northwards, it brings relief from scorching summer temperatures across the regions it covers.

The southwest monsoon typically sets in over Kerala on June 1, with a standard deviation of about seven days. It usually covers the entire country by July 8. The withdrawal begins from northwest India around September 17 and completes by October 15.

Since 2005, IMD has been issuing operational forecasts for the monsoon onset date over Kerala using an indigenously developed, state-of-the-art statistical model with a model error margin of ±4 days.

In April, the IMD forecast above-normal cumulative rainfall for the 2025 monsoon season, ruling out the likelihood of El Niño conditions, which are generally linked with below-normal rainfall in the Indian subcontinent.

The southwest monsoon contributes about three-fourths of India’s total annual rainfall. During this period, the average rainfall over the plains is approximately 87 per cent.

Its significance lies in its crucial role in replenishing water resources and supporting agriculture across the country.