The monsoon is expected to hit Kerala on May 29 – three days ahead of its normal date of onset on India’s mainland.
“The south-west monsoon is expected to set in over Kerala on May 29 with a model error of ± 4 days,” said India Meteorological Department (IMD) on Friday while announcing possibility of early arrival of the monsoon.
The south-west monsoon sets over Kerala generally on June 1 with a standard deviation of about seven days. The event marks the start of the rainy season over the region and as the monsoon progresses northward, relief from scorching summer temperatures is experienced over the areas.
The country’s national weather forecaster, IMD, has been issuing operational forecasts for the date of monsoon onset over Kerala since 2005. Its operational forecasts of the date of monsoon onset during the past 13 years (2005- 2017) were proved to be correct, except in 2015.
The forecast of the onset date comes a month after the IMD’s first prediction about this year’s monsoon. It had on April 16 predicted a ‘normal’ monsoon this year by pegging the season’s rainfall in the lower end of the normal range, at 97% of the long period average (LPA).
This is the third consecutive year that the IMD had predicted a ‘normal’ monsoon, although rains ended below normal last year. Rainfall between 96% and 104% of LPA during the June-September monsoon period is considered normal. The IMD will update its forecast, along with predictions for different regions and months, in early June.

Stating about advance of monsoon over the Andaman Sea, the IMD on Friday said, “The south-west monsoon advances over the Andaman Sea normally around May 20 with a standard deviation of about one week. Conditions are likely to become favourable for the advance of south-west monsoon into some parts of Andaman Sea and south-east Bay of Bengal around May 23.
Specifying the modalities of its forecast, the IMD said, “An indigenously developed state of the art statistical model with a model error of ± 4 days is used for this purpose.”
It uses six predictors in the model. Those six predictors are: (i) Minimum Temperatures over North-west India, (ii) Pre-monsoon rainfall peak over south Peninsula, (iii) Outgoing Long wave Radiation (OLR) over south China Sea, (iv) Lower tropospheric zonal wind over southeast Indian ocean, (v) upper tropospheric zonal wind over the east equatorial Indian Ocean, and (vi) Outgoing Long wave Radiation (OLR) over the south-west Pacific region.

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