Yesterday's well-marked low-pressure over South-West Arabian Sea intensified into a depression around the same coordinates by the evening, India Met Department (IMD) said. It expects the system to further intensify into a deep depression (numbered tropical cyclone) and further into a named cyclone by later tonight or early tomorrow.

Conditions right

Prevailing sea and atmospheric conditions are just right for its intensification, according to an assessment of the US Joint Typhoon Warning Centre. Sea-surface temperatures are above 31 deg Celsius, which provides sufficient fuel through convection and cloud-building at the ground level.

The vertical wind shear values, or the speed at which the winds suddenly change direction and speed within the system, are currently low or within manageable limits. High wind shear values, as are typical of a full-fledged monsoon, don't allow the critical storm tower to grow into the desired height that decides its strength and intensity.

This is why cyclones form mostly during the run-up to the monsoon or just after it has withdrawn, and not midway through.

Benign MJO watch

Models project the brewing cyclone to move North-North-West and head towards the East Yemen-South Oman coast during the next four to five days. This morning, the depression is located close to 600 km to the West-South-West Lakshadweep Islands off India and 1,060 km South-South-East of Salalah, Oman.

Activity in the Arabian Sea has been peaking under the benign watch of a Madden-Julian Oscillation (MJO) wave that moves from West to East periodically over Indian Ocean. Model forecasts indicate that the MJO wave would be active until June 4, progressively triggering activity in the Bay of Bengal to keep the progress of monsoon on an even keel.

The MJO wave in the heights of the atmosphere packs convection, clouds and heavy rain and underwrites the onset of monsoons and initiates depressions/cyclones on ground. According to MJO trackers, the current 'wet phase' may last into the first week of June covering the onset phase of the monsoon as projected by India Met Department (IMD).

Bay watch on

This is precisely why forecasters are looking at the behaviour of circulations over North Tamil Nadu and off Sri Lanka for signs of further development. An IMD assessment indicates that they would merge and set up a depression, if not a cyclone, tracking along the East Coast towards Kolkata/Bangladesh.

This outlook has found consensus with other models, which, if proven true, would make up for the early loss of traction that the monsoon has suffered thanks to the Arabian Sea cyclones. The IMD has already forecast for heavy rain for parts of Tamil Nadu and Karnataka over the next three to four days in the context of the buzz developing in the Bay.

Monsoon flows this morning are seen directed into the Bay through two different corridors to the North and South of the Maldives and across Sri Lanka. These are away from the sphere of disruptive influence of the brewing Arabian Sea cyclone, and they should manage to mostly hold out as the Bay gets into the monsoon stride.

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