On Thursday, the India Meteorological Department (IMD) officially declared the arrival of the southwest monsoon in Kerala, signaling the commencement of the four-month rainy season.

The typical date for the monsoon’s onset is June 1, allowing for a variation of up to seven days.

The IMD has also issued a ‘yellow’ alert for Kerala, cautioning residents about the likelihood of heavy rainfall, with expected precipitation ranging from 70 to 110 mm over a 24-hour period across all districts until June 7.

This year, the meteorological agency anticipates below-average rainfall nationwide. The forecast predicts rainfall to be about 90 percent of the Long Period Average, which stands at 870 mm based on data from 1971 to 2020. This projected decrease in rainfall is largely attributed to the emergence of El Niño, a naturally occurring climatic phenomenon in the equatorial Pacific that typically leads to warmer ocean surfaces. This condition is known to hinder the summer monsoon and contribute to extreme temperatures in India.

While Kerala has experienced substantial rainfall in recent days, the IMD applies specific criteria to confirm the monsoon’s arrival. These criteria include (i) the occurrence of at least 2.5 mm of rainfall over two consecutive days at a minimum of 14 designated stations in Kerala, Lakshadweep, and Karnataka after May 10; (ii) the enhancement and deepening of westerly winds to speeds of 15-20 knots and at least 600 hectopascals; and (iii) the Outgoing Long Range (OLR) value remaining below 200 watts per square meter in the region between 5-10ºN latitude and 70-75ºE longitude.

Following the monsoon’s onset in Kerala, it typically progresses northward in several surges, influenced by monsoon wind pulses and other environmental factors. Generally, by mid-July, the monsoon covers the entire country, entering periodic active and break phases thereafter.

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