NEW DELHI: The southwest (summer) monsoon is likely to set in over Kerala on May 26 with a model error of ± 4 day, IMD said on Friday while making a forecast of early arrival of rainy season in India. Monsoon normally sets in over Kerala on June 1 with a standard deviation of about seven days.The advance of the southwest monsoon over the Indian mainland is marked by monsoon onset over Kerala and is an important indicator characterising the transition from the hot and dry season to the rainy season.Last year, monsoon arrived over Kerala eight days in advance on May 24. Early or late arrival of monsoon has, however, nothing to do with the overall quantitative or spatial rainfall during the four-month rainy season.The Met department has already predicted ‘below normal’ monsoon rainfall this year amid the growing risk of El Niño – a climatic pattern associated with warming of the ocean surface in the central and eastern tropical Pacific Ocean. El Niño is invariably linked with depressed monsoon rainfall in South Asia.IMD has been issuing operational forecasts for the date of Monsoon onset over Kerala from 2005 onwards. An indigenously developed state of the art statistical model with a model error of ± 4 days is used for the purpose.The six predictors used in the models for its onset forecast are: minimum temperatures over north-west India, pre-Monsoon rainfall peak over south Peninsula, 0utgoing Long wave Radiation (OLR) over south China Sea, lower tropospheric zonal wind over equatorial southeast Indian Ocean, outgoing Long wave Radiation (OLR) over southwest Pacific Ocean and upper tropospheric zonal wind over equatorial northeast Indian Ocean.The Met department claimed that its operational forecasts of the date of Monsoon onset over Kerala during the past 21 years (2005-2025) were proved to be correct except in 2015.