The Hindu Business Line : Brewing Bay `low' to decide monsoon play-out over plains

The Arabian Sea arm of the monsoon continued to whiplash the northern flank of the west coast on Thursday (June 1) on what would have been its normal date of onset over the southwest coast in Kerala.

But its counterpart, the Bay of Bengal current, kept the weatherman guessing on how and where it proposed to move next. Easterlies in the upper levels, a vital component in normal monsoon ensemble, had not accelerated to the desired extent. This is holding back the `east-west shear zone' of monsoon turbulence from evolving fully.

As Mr Jim Andrews of AccuWeather notes: "The all-important easterly jet is not fully developed yet; but it will build northward sooner. In a classic southwest monsoonal setting, westerly or southwesterly winds in the lower atmosphere give way upwards to easterlies. This results in a distinctive smeared-out look to satellite imagery of cloudiness amidst heavy convective rains."

Dr Akhilesh Gupta of the National Centre for Medium Range Weather Forecasting (NCMRWF) told Business Line that the easterlies would establish in strength with the formation of the westward-oriented seasonal trough. The trough will be held into place by a signature `monsoon low' to its eastward end (over the Head Bay).

"Onset phase not backed fully by easterlies has been recorded on earlier occasions too, especially when the system is progressing pronouncedly northward. It is left to the monsoon low to take the Bay of Bengal arm westward into Central and Northwest India with the help of easterlies. The east-west shear zone would also come into place by then," Dr Gupta said.

Model outputs continue to assess as `fair' the chances of formation of a low-pressure area in the North Bay of Bengal around Sunday. Although there is no consensus on the direction of its likely progression, at least two models seem to indicate that it is likely to move west to take a monsoon-friendly track.

Both the US Navy and the European Centre for Medium-Range Weather Forecasts (NCMRWF) agree that the system is likely to move west. The Fleet Numerical Meteorology and Oceanography Center (FNMOC) of the US Navy even said the seasonal monsoon trough would settle into place by Monday/Tuesday.

Optimistic as these forecasts are, Dr Gupta said that before taking a call, he would track the models for at least another day.

Monsoon advances

Meanwhile, the southwest monsoon advanced into entire Konkan on Thursday. The northern limit of monsoon passed through Dahanu, Pune, Sholapur, Kurnool, Ongole and Gangtok.

An NCMRWF forecast said widespread rains with heavy to very heavy rains are likely at a few places over coastal Karnataka, Konkan, Goa, the South Gujarat region, West Bengal, Sikkim, the North-Eastern States and at one or two places over North Gujarat, Madhya Maharashtra, Kerala, Andaman and Nicobar Islands.

The monsoonal flow is expected to maintain its strength over Arabian Sea and Bay of Bengal during the next two days resulting in continuance of good monsoon rainfall activity over the west coast and the Northeastern States during this period.

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