Thereafter, it covers the entire country in next 45 days.
The onset marks the start of the rainy season over the region and as the monsoon progresses northward, it provides relief from scorching summer temperatures experienced over the areas.
However, a timely onset of monsoon over Kerala does not guarantee its normal progress over other parts of the country and also overall performance of the rains, the IMD clarified.
“The southwest monsoon advances over the Andaman Sea normally around May 20 with a standard deviation of about one week. Conditions are likely to become favourable for the advance of southwest monsoon into some parts of the Andaman Sea and the southeast Bay of Bengal around May 23,” the IMD said.
The met department in its first forecast of monsoon for 2018 released last month predicted that monsoon is expected to be ‘normal’, brightening chances of recovery in farm sector, which has seen fluctuating growth rates in the first four years.
IMD said that rainfall in June to September period is expected to be at 97 per cent of the Long Period Average (LPA) with model error of plus and minus 5 per cent.
LPA is the average rainfall across the country from 1951-2000 estimated to be 89 centimeters.
IMD said there is 42 per cent chance of rainfall this year to be normal while 30 per cent probability of it being below normal.
Earlier, private weather forecasting agency Skymet also said that monsoon 2018 is expected to be ‘normal’.
Skymet said that rains this year could be 100 per cent of the Long Period Average (LPA) with a model error of plus and minus 5 per cent.
Skymet also said that East and North-East India along with parts of Southern India might get slightly ‘below normal’ rains in 2018, while the agriculturally crucial regions of North and Central India are expected to get ‘normal’ to ‘excess’ rains in 2018.
IMD would come out with region-wise forecast in June along with its second update.