Wednesday, October 06, 2010

Big brother Bay "LOW 95 B" drives rains across peninsula

The upper air cyclonic circulation over South-East Arabian Sea and the low-pressure area over West-Central Bay of Bengal have together set up an active trough across peninsular India.

The Bay “low” is approaching the Chennai-South Coastal Andhra Pradesh coast and is expected to strengthen along the way but weather models continued to suggest that it would be taken away North-North-East along the coast.
This would happen under the overarching influence of an upper air trough embedded in a passing western disturbance travelling east from North-West India and dipping low over Central India.
The system is expected to weaken after its western flanks are compromised due to proximity to land.
The western disturbance will ensure that it gets staggered along for an eventful landfall either over Orissa and Gangetic West Bengal coast, according to various tracking models.
In any case, the interaction between the two weather systems would bring moderate to heavy rainfall to most of the South-East Coast and parts of East India during its pendency.

The European Centre for Medium-Range Weather Forecasts (ECMWF) has maintained the outlook for a follow-up “low” over SouthWest Bay around October 13.
This was seemingly corroborated on Tuesday by the Climate Prediction Centre (CPC) of the US National Weather Services and separately by the US National Centres for Environmental Prediction.
Interestingly, the CPC has put the Bay of Bengal under a cyclone watch during October 12 to 18, though the confidence level about the outcome was admittedly low on Tuesday.

It based its outlook on La Nina conditions (reverse of El Nino) in the East Equatorial Pacific and an enhanced phase of the Madden-Julian Oscillation (MJO) wave, the periodical upper level disturbance that passes over the region with implications for ground-level weather.
Leading MJO trackers indicated that the Arabian Sea, Central Equatorial Indian Ocean and the Bay of Bengal are all under the grip of a strong MJO phase concurrently.
The Earth System Research Laboratory of the US National Oceanic and Atmospheric Administration expected the MJO phase to linger in varied strength over the region until October 24.
Two other MJO trackers did not believe it would last this long and might peter out in a week's time. The Empirical Wave Propagation technique indicates that the next active phase of MJO would not be in until November 8.
On Tuesday, over upstream South China Sea, a seasonal monsoon trough packing monsoon winds on retreat (North-East monsoon) continued to hold itself strong overnight on Tuesday.
This trough lay across the Upper South and East Thailand through the country's North-East linking with active “low” over upper South China Sea, according to a Thailand Met Department (TMD) update.
On Tuesday, the trough looked to sneak into extreme South-East Bay of Bengal, the next gateway for North-East monsoon. But here, the winds were southwesterly to the south of the existing “low.”
Winds have to be easterly to northeasterly along the Tamil Nadu coast before the onset of North-East monsoon can be declared.
It does not get declared before October 10 in any case, even if the wind pattern and other parameters fall into place ahead of this cut-off date.
According to the TMD, torrential rain and isolated heavy falls over central, eastern and southern parts of the country would continue for three more days during when flash floods have been warned of.
The Andaman Sea would continue to be very cloudy with fairly widespread thundershowers and isolated heavy rain. As mentioned, the winds are going to be southwesterly with speeds ranging between 15 to 35 km/hr.
Meanwhile, an India Meteorological Department (IMD) update said that the 24 hours ending Tuesday that fairly widespread rainfall occurred over Maharashtra, Tamil Nadu, Kerala and Andaman and Nicobar Islands. It was scattered over Lakshadweep, Jharkhand and Gangetic West Bengal.
While the marauding winds were southwesterly over Kerala, the rest of the peninsula was buffeted mostly by easterlies to northeasterlies as dictated by the Bay “low.”
The upper cyclonic circulation over South-East Arabian Sea was reduced to merely playing second fiddle.
IMD satellite imagery on Tuesday showed the presence of convective clouds over parts of Tamil Nadu, Coastal Andhra Pradesh, Orissa, Gangetic West Bengal, the Bay of Bengal, the Andaman Sea and South Arabian Sea.
A warning issued by the IMD said that isolated heavy to very heavy rainfall would occur over Andaman and Nicobar Islands, Lakshadweep, Kerala, Coastal Karnataka, Coastal Andhra Pradesh, Coastal Orissa and Tamil Nadu during the next two days.
Isolated heavy to very heavy rainfall is also likely over Coastal West Bengal and the North-Eastern States from Wednesday onwards.

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