Stage set for monsoon onset in Bay of Bengal

Conditions are becoming favourable for advance of southwest monsoon into south Andaman Sea and adjoining southeast Bay of Bengal during the next two to three days.

Rain-bearing southwesterly winds are forecast to head increasingly into the region powered by a compliment of relatively stronger southerlies from the southern hemisphere.

ONSET LIKELY

This is expected to precipitate the onset in the Andamans where prevailing southwesterly winds were weak on Friday. On the other side of the peninsula, India Meteorological Department (IMD) expects an increase in rainfall over peninsular India from Monday.

According to the European Centre for Medium-Range Weather Forecasts, weather conditions would take a decisive turn after May 20 when wind speeds and moisture carry are forecast to scale up dramatically.

'LOW' LIKELY

This phase could also throw up the expected low-pressure area in the southeast Arabian Sea and the adjoining equatorial Indian Ocean-southwest Bay of Bengal. The Head Bay may also be readying to host a separate circulation.

Three-hour forecasts being made available by an international model suggests that the flows would become southwesterly over Sri Lanka by Monday and the monsoon get established over the island nation.

But Thiruvananthapuram in Kerala, the monsoon gateway for India, would still feature predominantly west-northwesterlies around the same time. A series of thundershowers is forecast to unravel over the region as a prelude to monsoon onset.

THUNDERSHOWERS

The US National Centres for Environmental Prediction (NCEP) has said that the week ending May 22 would see Kerala awash with rain or thundershowers, which may spill into neighbouring Tamil Nadu and Karnataka.

The rains would have simultaneously become well established over southwest and central Bay of Bengal, east and northeast India where a westerly trough is shown to combine with the monsoon current to trigger widespread rainfall through the following week (May 23 to 31).

HEAT LOW

On Friday, the 'heat low' over west Rajasthan and adjoining east Pakistan held relatively strong aided by prevailing heat wave conditions. The 'heat low' must sustain to be able to provide the ideal pressure differential for monsoon to drive up from the relative 'high' over the south peninsula.

Meanwhile, India Meteorological Department (IMD) has warned of the possibility of thunder squall and hail flaring up over the seasonally 'vulnerable' West Bengal, Sikkim, the Northeastern states, Jharkhand, Orissa and coastal Andhra Pradesh during the next two days.

TROUGH WEAKENS

In a major development, the weather-maker trough/wind discontinuity from Orissa to south Tamil Nadu has become less marked on Friday. A counterpart trough in westerlies from Assam and Meghalaya to northeast Bay of Bengal too has become less marked.

This is a major enabling condition for the southwest monsoon to set in over the Bay of Bengal. The unsettled weather triggered by the Nor'westers (winds associated with a prevailing western disturbance) has to shut out for the monsoon to set in.