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The India Met Department (IMD) has confirmed that a low-pressure area is likely to form over the South-East Arabian Sea off the Kerala-Karnataka coast by Sunday, which will power the onset phase of the monsoon.
But the IMD has deferred by two days the onset from Thursday (June 6) to around Saturday (June 8), though falling with the larger window (+/-4 days) originally set by it.
Wind direction
The Climate Prediction Centre (CPC) of the US National Weather Service attributes the delay to anomalous easterly winds (due to pre-monsoon activity) in the lower levels. These winds need to be south-westerly to precipitate the onset of monsoon.
According to the US agency, model guidance suggests a weakening of the anomalous easterlies and the return to a normal monsoonal flow pattern towards the end of the week from June 12-18.
The CPC joined the IMD in the outlook for an intensified ‘low’ developing during the ongoing week in association with the enhanced phase of the Madden-Julian Oscillation (MJO) wave over the Indian Ocean.
The MJO represents a global band of lower pressure travelling periodically along the tropics and underwriting the formation of low’s, depressions, cyclones and even monsoon onsets.
The eastward-bound MJO could rustle up the waters of the Bay of Bengal next, and likely trigger the formation of a ‘low’ between June 12-18.
Ecosystem developing
On Thursday, the IMD said an East-to-West wind-shear zone of monsoon turbulence — the ecosystem required to generate active monsoon conditions — lay extended across the Lakshadweep-Maldives-Comorin area.
This would shift northwards and intensify gradually even as an offshore trough, could materialise around Saturday off the Maharashtra-Kerala coast.
With these favourable conditions, the monsoon could set in over Kerala in the subsequent two days. The expected low and the offshore trough could in fact ensure spectacular onset conditions.
Conditions are also expected to become favourable for advance of the Bay of Bengal arm of the monsoon into some parts of the North-Eastern States during the next 2-3 days, the IMD said.
Overall, the CPC noted that below-normal rainfall would be the dominant theme over a larger part of the Indian subcontinent until June 18, except in the South, where the monsoon would make known its presence.
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