Making the announcement in its mid-Sunday press release, the IMD pointed out that rain/thundershowers have occurred at many places over West Bengal, Sikkim, coastal Andhra Pradesh and at a few places in Chhattisgarh, Assam, Meghalaya and Kerala.
It has been raining in Kerala for the past few days and the weather officials are analysing the data recorded at the 14 weather stations located at the Lakshadweep, Kerala and in Mangalore in Karnataka.
The norm is that if after May 10, 60 per cent of these 14 stations report rainfall of 2.5 mm or more for two consecutive days, the onset of monsoon over Kerala is declared on the second day, provided the wind and cloud formations are also favourable.
Southwest monsoon normally sets in over Kerala around June 1. It advances northwards, usually in surges, and covers the entire country around July 15.
Last year while the IMD forecast the onset of the southwest monsoon by May 30, the actual onset took place on June 5.
Since 2005, the IMD has been issuing operational forecasts for the date of monsoon onset over Kerala using an indigenously developed statistical model with a model error margin of four days.