The norm is that officials assess the data recorded at 14 weather stations in Lakshadweep, Kerala and Mangalore from May 10.
The southwest monsoon normally hits Kerala around June 1. It advances northwards, usually in surges, and covers the entire country by around July 15.
Last year, while the IMD forecast the date for the onset of the southwest monsoon as May 30, the actual onset took place on June 5.