Myriad pieces of the monsoon onset puzzle have started falling into place, according to an India Meteorological Department update on Tuesday.

An offshore trough, an elongated area of lower pressure and a receptacle for the westerly to south-westerly flows, is in the process of stretching out into place along the Kerala coast.

The intensity of the monsoon will depend on the strength of the offshore trough. As on Tuesday evening, it was a feeble formation and in need to deepen further to precipitate the onset. This should take place over the next couple of days, according to the Met Department outlook.

Also in place was a crucial “east-west shear zone of turbulence” over the south peninsula, another monsoon enabler.

This “shear zone” features opposing winds in an area rising from three km and six km into the atmosphere.

This is the height-level in the atmosphere where the monsoon is most active. The “shear zone” also sets up the platform for rain-bearing systems (low-pressure area et al) to play around in and drive the monsoon upcountry.

The Met Department assessed conditions as favourable for the onset of monsoon over Kerala, and its further advance into some parts of Tamil Nadu and adjoining Bay of Bengal over the next two days.

Conditions are also favourable for the advance of the monsoon into the north-eastern States over the next three days.

The European Centre for Medium-Range Weather Forecasts said cross-equatorial flows strengthening from Friday and peaking through the next week.

This will translate into heavy to very rain variously for peninsular India, the east and west coasts.

The US National Centres for Environmental Prediction suggests that a rain-head racing away along the east coast during the week starting on Tuesday.

India Met Department also indicated the possibility of deep trough building along the east coast, thanks to the intense heating of East India in the recent past.

Heavy rain

Heavy rain is expected to wallop the west coast from January 11 onwards, according to this forecast. A cyclone tracker featured by the US Climate Prediction Centre continued to put the south-east Arabian Sea (off Kerala) for formation of a weather system (likely low-pressure area) during this week.

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