Conditions are quite propitious for a normal onset of the South-West monsoon this year and chances are that it would arrive on time, give or take two-three days.

The normal onset date for the seasonal rains is June 1. It is also likely that the onset phase would be ‘active’ if not a ‘vigorous’ (in terms of the rainfall intensity).

The onset can happen any time after May 25, according to CK Rajan, former director, Centre for Monsoon Studies at Cochin University of Science and Technology (Cusat).

He was also Professor and Chair for Climate and Environmental Changes and Risk Reduction at the SCMS Group of Institutions.

‘Rogue’ system

The India Meteorological Department (IMD) has already come out with its long-range forecast indicating that the seasonal rainfall quantum would be normal this year (between 96 and 104 per cent of the long-period average, LPA).

Private forecaster Skymet Weather has said it would be a ‘healthy monsoon’ at 103 per cent of the LPA. Rajan, however, said that any low-pressure system forming in the South Arabian Sea (off Kerala) after the second week of May and ahead of the monsoon current reaching the Kerala coast should be watched with caution since it could disturb the whole pattern as has been the case in the last few years.

Such a ‘rogue’ system can drag the monsoon current to a totally different direction (West-North-West) in the Arabian Sea and away from Kerala and mainland India.

In the course of such movement, it could intensify and go on to become a cyclone that races away to the Oman-Yemen (Middle-East) coast.

Pre-monsoon rainfall peak

“So, even as we watch systems developing in the Bay of Bengal, we have to keep a close tab on the Arabian Sea as well to see whether a system is developing or not, and if yes, where it is headed,” Rajan told BusinessLine.

Only then can forecasts/predictions make sense to the public at large.

The ‘pre-monsoon rain peak’ can form any time between the first week of April and the second week of May.

Its timing gives prior indication of the date of monsoon onset over Kerala. For a normal monsoon onset on June 1, the raining band of cloud in the Bay of Bengal should form around April 21.

“The clouds over the East Indian Ocean and adjoining Bay of Bengal prove undeniably that the pre-monsoon rainfall peak is already on,” Rajan said. In fact, the activity was on from as early mid-April which goes to indicate that the monsoon onset over Kerala will be more or less in time this year.

Pre-monsoon in Kerala

All other available conditions, including the ongoing pre-monsoon showers in Kerala, provide further evidence. This will help the build-up of moisture in the atmosphere.

One main criterion to be fulfilled here is that upper air moisture level should be quite good up to a height of 500 millibars (5 km), he said.

“This is more or less in place. The winds are also strengthening. Onset over the Andaman & Nicobar may happen by the middle of May. This is again more or less in keeping with the normal time,” Rajan added.

The sea-surface temperatures (SSTs) over the East Indian Ocean adjoining Bay have scaled up suitably.

Migration of ITCZ

This makes the Inter tropical Convergence Zone (ITCZ) cross the Equator and move into the Northern Hemisphere in the Indian Ocean as well as the adjacent West Pacific Ocean. The ITCZ is a zone of ascending air, maximum clouds and heavy rainfall along the Equator, which merges with the monsoon circulation and sets up the trough (area of lower pressure) to drive seasonal winds and rain into India.