India Meteorological Department (IMD) forecast suggests that the onset of Southwest monsoon this year over Kerala is likely to be slightly delayed. The statistical model used by IMD for predicting the onset of monsoon, indicates that the southwest monsoon is likely to set over Kerala on June 7 with a model error of ± 4 days.
The onset of southwest monsoon over Kerala signals the arrival of monsoon over the Indian subcontinent and represents beginning of rainy season over the region. The normal monsoon onset over Kerala is 1 June 1. Since 2005, India Meteorological Department (IMD) has been issuing operational forecasts for the date of monsoon onset over Kerala using an indigenously developed statistical model with a model error of ± 4 days.
The model uses the following six predictors;
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.
The forecasts of the monsoon onset issued during the past 11 years (2005-2015) except 2015 were proved to be correct.
The southwest monsoon advances over the Andaman Sea normally around May 20 with a standard deviation of about one week. The strong convective activity has been observed over the equatorial Indian Ocean for the last several days. A well marked low pressure area has formed over the Indian Ocean which will concentrate into a depression and move north-westwards. As a result, conditions are becoming favourable for the onset of southwest monsoon over Nicobar Islands, south Andaman Sea and parts of south Bay of Bengal around May 17 and advancement over the entire Andaman Sea close to its normal date. Past data suggest that there is no association of the date of monsoon advance over the Andaman Sea either with the date of monsoon onset over Kerala or with the seasonal monsoon rainfall over the country.