After a delay of two days, the Southwest Monsoon made an onset over Kerala on Thursday, marking the commencement of the four-month rainfall season in the country, the India Meteorological Department (IMD) said.

The Southwest Monsoon has made an onset over southern parts of Kerala, said IMD Director General Mrutunjay Mohapatra.

The normal onset date for Southwest Monsoon over Kerala is June 1.

Southwest Monsoon is likely to advance into remaining parts of south Arabian Sea and some parts of central Arabian Sea, remaining parts of Kerala, Lakshadweep, some parts of Tamil Nadu, Puducherry, coastal and south interior Karnataka, Rayalaseema, and south and central Bay of Bengal during the next two days, the IMD said.

This is the third time in the last six years that monsoon arrived late. In 2016 and 2019, Southwest Monsoon made an onset over Kerala on June 8.

The IMD had earlier forecast that monsoon would make an onset over Kerala around May 31.

Skymet, a private weather forecasting station, said Southwest Monsoon made an onset over Kerala on May 30. However, the IMD said conditions were not ripe for declaring the onset of monsoon.

According to IMD, three parameters need to fulfilled for declaring the onset of the Southwest Monsoon over Kerala -- If after May 10, 60 per cent of the 14 stations including Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore report rainfall of 2.5 millimetres or more for two consecutive days, the onset over Kerala can be declared on the second day, provided other two criteria are also in concurrence.

This has to be supplemented by the wind speed. The depth of westerlies should be maintained up to 600 hectopascal (hPa), in the box equator to Latitude 10-degrees north and Longitude 55 degrees to 80-degrees east. The zonal wind speed over the area bounded by Latitude 5 to 10-degrees north, Longitude 70-80-degrees east should be of the order of 15-20 knots at 925 hPa, the IMD said.

The Outgoing Longwave Radiation (OLR) should be below 200 watt per square meter (wm-2) in the box confined by Latitude 5-10 degrees north and Longitude 70-75 degrees east, it added.

The IMD said these parameters were fulfilled on Thursday, it said.

The Southwest Monsoon is likely to be normal in north and south India, above-normal in central India and below-normal in east and northeast India, the Met department had said in its second-long range forecast for Southwest Monsoon 2021 on Tuesday.

A good monsoon is critical to the Indian economy which is still largely based on agriculture and its allied activities.

Most parts of the country are expected to receive normal to above normal rainfall during the season, the IMD said.

However, there are some regions in east and northeast India like eastern parts of Bihar, some parts of West Bengal, Assam, Meghalaya, Jammu and Kashmir, Ladakh, adjoining Himachal Pradesh, southwest peninsular India, specifically some parts of Kerala, coastal Karnataka and parts of interior Maharashtra and Tamil Nadu where the probability of below normal activity is predicted, the IMD said.