The Telegraph
Since 1st March, 1999
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Mumbai caught in middle of rain plot

New Delhi, July 4: A low pressure zone rolling westward over central India and a cyclonic wind pattern hanging over the northeast Arabian Sea worked in tandem today to batter Mumbai with rain, weather scientists said.

The combined effect of the two weather systems is likely to produce extremely heavy rainfall ' more than 25 cm over the next 24 hours ' over coastal Konkan, Goa, Mumbai and southern Gujarat, they said.

“These conditions are very favourable for some intense rainfall over coastal Maharashtra and southern Gujarat,” said S. Krishnan Subramanian, the deputy director-general at the Indian Meteorological Department in New Delhi.

The incessant rain and waterlogging have kindled concerns over whether the city might be headed for another freak event of the kind that struck on July 26 last year when northern Mumbai received 94 cm of rainfall within a few hours.

But scientists said there is no sign yet of any such freak event.

“There is a basic difference between last year and now,” said Lakshman Singh Rathore, a senior scientist at the National Centre for Medium Range Weather Forecasting. “Last year, the rainfall was confined to a small area, compressed into just a few hours. In the present scenario, the rainfall is likely to be spread over large areas and over a longer time.”

“A long stretch of the west coast ' roughly from Ratnagiri to Baroda ' typically receives heavy rainfall after the formation of the upper air cyclonic wind pattern in the northeast Arabian Sea,” Subramanian said.

This cyclonic circulation, hovering about three to five kilometres above the surface of the sea, was close to hugging the north Maharashtra coast on Tuesday, drawing moisture from the sea to boost rainfall on the coast.

The IMD today issued a “severe weather” advisory, cautioning that some parts of Konkan, Goa, Mumbai and southern Gujarat could receive “extremely heavy rainfall”.

It is not unusual for depressions or low pressure zones to form over the Bay of Bengal and move west-northwestward across the mainland. The depression over Chhattisgarh had slipped from the Bay into the mainland on Sunday night.

On its own, such a typical monsoon season depression would cause copious rainfall in Karnataka and Goa. Sometimes, as has happened now, another system forms in the northern Arabian Sea.

Scientists said the two systems work together to produce downpours. “We call the system over the Arabian Sea a sympathetic circulation,” said Subramanian.

The IMD defines anything more than 12 cm as heavy rainfall, and a value higher than 25 cm as extremely heavy rainfall. But scientists said they have no model yet to predict how the 25 cm rainfall would be distributed over time or space.

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