The southwest monsoon is expected to set in over Kerala on 29 May 2018, with a model error of ± 4 days, according to the latest forecast by the India Meteorological Department (IMD).
The rainy season normally sets in over Kerala on 1 June with a standard deviation of about 7 days, marking the start of the rainy season over the region as the monsoon progresses northward in the days following.
The IMD has been using an indigenously developed state-of-the-art statistical model to issue operational forecasts for the date of monsoon onset over Kerala from 2005 onwards. It has been forecasting monsoon date with a model error of ± 4 days.
The six predictors used in the models are: minimum temperatures over North-west India, Pre-monsoon rainfall peak over south Peninsula, Outgoing Long wave Radiation (OLR) over south China Sea, Lower tropospheric zonal wind over southeast Indian ocean, Upper tropospheric zonal wind over the east equatorial Indian Ocean, and Outgoing Long wave Radiation (OLR) over the south-west Pacific region.
IMD’s operational forecasts of the date of monsoon onset during the past 13 years (2005- 2017) were proved to be correct except in 2015.
The southwest monsoon advances over the Andaman Sea normally around 20 May with a standard deviation of about one week. Conditions are likely to become favorable for the advance of southwest monsoon into some parts of Andaman Sea and southeast Bay of Bengal around 23 May 2018.
However, 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.