The forecast, made public on May 15, is in line with a forecast by private forecaster Skymet, which on May 14 said that the monsoon would hit Kerala on June 4.

The India Meteorological Department (IMD) on Wednesday said the southwest monsoon would be “slightly delayed” over Kerala and arrive on June 6. The normal onset date is June 1.

The forecast is in line with a forecast made by private forecaster Skymet, which on Tuesday said the monsoon would hit the State on June 4.

To forecast the monsoon arrival, the IMD uses a customised weather model which, it stated, had been wrong only once - in 2015 - since 2014.

This model crunches 6 meteorological parameters: the minimum temperatures over northwest India; the pre-monsoon rainfall peak over south Peninsula; the outgoing long-wave radiation (OLR) over the South China Sea; the lower tropospheric zonal wind over southeast Indian Ocean; the upper tropospheric zonal wind over the east equatorial Indian Ocean; and the outgoing long-wave radiation (OLR) over the southwest Pacific region.

The model has a built-in error margin of four days - a June 6 onset can mean any day from June 2-10.

“Conditions are becoming favourable for the advance of the southwest monsoon over the southern part of the Andaman Sea, Nicobar Islands and adjoining southeast Bay of Bengal during 18-19 May,” says the IMD.

Generally, the monsoon reaches Kerala within 10 days of reaching the Andamans. However, meteorologists had indicated that the monsoon — impeded by high temperatures in the seas surrounding India, and an El Nino — will advance sluggishly after reaching Kerala.

The monsoon’s arrival time, the IMD said, had no bearing to the quantum of rainfall in June-September period.

The IMD said it expected a “normal monsoon” but pointed to a “significant probability” of below normal rains.