IMD throws open monsoon onset window in the Bay

Article dated 16-May-10
India Meteorological Department (IMD) has officially thrown open the monsoon onset window in the fast-churning Bay of Bengal, which is now home to some of the warmest tropical waters around the globe.

Sea surface temperatures (SSTs) in the south-east Bay are in the 31-32 deg Celsius-range, which is most ideal for weather systems (low-pressure areas and depressions) to prosper.

CONDITIONS FAVOURABLE

The IMD said on Sunday that conditions are favourable for onset of south-west monsoon over south Andaman Sea, Nicobar Islands and adjoining southeast Bay of Bengal during the next two days.

An upper air cyclonic circulation has already formed over southeast Bay of Bengal.

A warning valid for this period said that isolated heavy rain or thundershowers would occur over Andaman and Nicobar Islands.

The Thailand Meteorological Department (TMD) has said that an ‘active low-pressure cell' covered the Andaman Sea on Sunday.

This region is forecast to stay very cloudy with scattered thundershowers and isolated heavy rain.

Southwesterly winds are clocking variable speeds of 15 km/hr to 30 km/hr.

The Canadian Meteorological Centre (CMC) and the Global Forecasting System (GFS) model of the US National Centres for Environmental Prediction (NCEP) have hinted that a ‘low' or depression may be generated from the Bay system and travel to the east coast.

EAST COAST SYTEM

While the CMC sees the system to be active until Thursday making a landfall over the north Andhra Pradesh coast, the NCEP has suggested a slightly different trajectory positing the system into the warmer waters adjoining coastal Tamil Nadu to cross the Chennai coast by Saturday.

From here, the system might take a north-northwest course and travel all the way into northern Maharashtra and adjoining west Madhya Pradesh, the NCEP said.

Meanwhile, the IMD has said that scattered rain or thundershowers would start unfolding over Tamil Nadu, south interior Karnataka, Kerala and Lakshadweep during the first three days of this week.

In its guidance for the rest of the week, it expected to see an increase in rainfall over south peninsular India and Lakshadweep Friday onwards in clear indication that the Arabian Sea would soon get into the act to facilitate onset of rains over the Kerala coast.

Satellite imagery showed convective clouds (rain or thundershower-causing) over parts of the Northeastern States, south peninsular India, Bay Islands, south Arabian Sea, the Comorin region and east, central and south Bay of Bengal and the Andaman Sea.


The TMD sees the monsoon making an onset over the southwest coast (Kerala) as early as Sunday (May 23). This was based on the active phase of an ongoing Madden-Julian Oscillation (MJO) wave in the equatorial Indian Ocean and adjoining peninsular seas.

Meanwhile, the US Joint Typhoon Warning Centre (JTWC) has signalled building activity with separate systems in the Arabian Sea and the Bay of Bengal. The Arabian Sea system, though, was brewing far away to the west of the Indian coast, but had a ‘fair' chance of developing into a tropical storm. Exact coordinates and direction of movement would be closely monitored for implications for monsoon flows.

As for the Bay system, the JTWC pointed to persisting area of convection, more than 1,000 km south of Kolkata. Satellite pictures revealed a broad unorganised region of convergence, deep convection and cyclonic turning.

Maximum sustained surface winds are estimated up to 28 km/hr. But JTWC assessed as ‘poor' the potential for the development of a significant tropical cyclone within the next 24 hours.

The US National Centres for Environmental Prediction (NCEP) maintained its outlook for a wave of enhanced rainfall for the India's southwest coast (Kerala) during the week ending May 23.

The entire Arabian Sea from the far west to the south-east (off Kerala) is shown to be ‘lit up' in the immediately following week (May 24 to June 1). Monsoon onset may happen mid-way through this period, which is earlier than usual. During this period, a ‘rain head' (likely storm) is shown to dig a track north-northeast from central Bay of Bengal and smash into the Thailand-Myanmar region.