The India Meteorological Department (IMD) on Wednesday said the southwest monsoon is likely to be "slightly delayed" and is expected to hit Kerala on June 6.

"This year, the statistical model forecast suggests that the monsoon onset over Kerala is likely to be slightly delayed. The southwest monsoon onset is likely to set over Kerala on June 6. But with a model error of four days," the IMD said in a statement.

The IMD has been issuing operational forecasts for the date of monsoon onset over Kerala from 2005 onwards. The weather agency has an indigenously developed state of the art statistical model with a model error of four days.

The model uses six parameters to get the probable onset date. They are -- minimum temperatures over northwest India, pre-monsoon rainfall peak over the south peninsula, outgoing long wave radiation (OLR) over the South China Sea, lower tropospheric zonal wind over the southeast Indian ocean, upper tropospheric zonal wind over the eastern equatorial Indian Ocean, and OLR over the south-west Pacific region.

"IMD's operational forecasts of the date of monsoon onset during the past 14 years (2005-2018) were proved to be correct except in 2015," the statement said.

"Conditions are becoming favourable for advance of southwest monsoon over the southern part of Andaman Sea, Nicobar Islands and adjoining southeast Bay of Bengal during 18-19th May 2019," it further said.

The monsoon is critical to India's agriculture as it accounts for more than 70 per cent of the country's annual rainfall.

The IMD defines average, or normal, rainfall as between 96 per cent and 104 per cent of a 50-year average of 89 centimetres for the entire four-month season beginning June.

On April 4, private weather forecaster Skymet had said that India will face a below normal rainfall this June-September monsoon season on account of moderate El Nino conditions, unless the Indian Ocean Dipole also known as Indian Nino, is able to negate the former.

El Nino was responsible for the droughts in the year 2014 and in 2015.

In its monsoon forecast for 2019, the private agency said it expects the upcoming monsoon to be below normal to the tune of 93 per cent of the long period average (LPA) of 887 mm for the four-month period from June to September. It had said there is an error margin of 5 per cent in the forecast.

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