The India Meteorological Department (IMD) and private weather forecaster Skymet are yet again differing on the date of arrival of monsoon in Kerala.

Skymet on Sunday declared that the South-West monsoon has arrived over mainland India along the Kerala coast in what it described as a ‘mild onset’, just two days before the due date of June 1. This is in line in with its earlier prediction that the onset will happen on May 30 (Sunday) with a margin of +/-2 days.

On the other hand, the IMD chose to push back the onset by four days to June 3 after south-westerly winds strengthen likely from June 1 resulting in enhancement of rainfall over Kerala. Hence, the onset may happen around June 3 against the predicted May 31 with a model error of +/-4 days.

 IMD Director General M Mohapatra said there is cyclonic circulation along the Karnataka coast which is hindering the progress of the southwest monsoon. 

“The southwesterly winds could strengthen further gradually from June 1, resulting in likely enhancement in rainfall activity over Kerala. Hence the monsoon onset over Kerala is likely to take place around June 3,” the IMD tweeted.

 Earlier this month, the IMD had predicted the arrival of monsoon over Kerala by May 31 with an error margin of plus or minus four days. On Sunday morning, the IMD, in its daily bulletin, said the onset of the monsoon over Kerala was expected to be around May 31. However, by afternoon it said the onset is expected to be by June 3.

According to the IMD, the onset of the southwest monsoon over Kerala depends on three parameters. If after May 10, 60 per cent of the 14 stations -- 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 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 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.

Mohapatra said the depth of westerlies is not as much as it is expected. Plus, the rainfall parameters remain unfulfilled to declare onset of monsoon over Kerala.

On the other hand, Skymet said the monsoon kept date with Kerala two days before the normal date of June 1 despite two pre-monsoon cyclones, Tautkae and Yaas, to either side of the peninsula having robbed much of its kinetic energy.

Prior to its arrival, most parts of the state have been receiving strong pre-monsoon showers. Both the

IMD and Skymet have differed on monsoon arrival dates earlier too.

 “A set of environmental conditions need to be fulfilled as a criterion for the arrival of monsoon. By and large, all parameters have satisfied the required threshold, essentially the rainfall and depth and speed of westerly winds over the Southeast Arabian Sea and adjoining Equatorial Indian Ocean,” the Skymet said.