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End of monsoon, not of showers

Monsoon has finally withdrawn from Calcutta but not rain.

On Monday, monsoon winds bade goodbye to the city after an unusually long innings, but there was no reprieve for the city in terms of rain. Localised thunderstorms powered by a low-pressure area over the Bay of Bengal near Tamil Nadu triggered the wet spell that had Calcuttans crying “No, not again!”

The thunderstorms that started late on Sunday could persist for another 24 hours at least, the weather office said.

The conditions brought scattered rainfall in the city through Sunday night, Behala being the wettest spot with 30mm. Elsewhere in the city, rainfall ranged between 4mm and 23mm. The Alipore weather office recorded 14mm.

More rainfall is likely as the low-pressure area off the Tamil Nadu coast is penetrating the coastal regions with rain-bearing clouds.

“The low-pressure area is a well-marked one. It is gaining strength and is expected to turn into a depression. Because of the moisture incursion taking place due to this system, Calcutta and large stretches of south Bengal will experience light to moderate rainfall till Tuesday. The rainfall may continue on Wednesday, depending on how strong the system becomes,” said G.C. Debnath, director of the India Meteorological Office, Calcutta.

The rainfall triggered by the low-pressure area off Tamil Nadu marks the commencement of the northeasterly monsoon in that state. Unlike Bengal, the southern state receives two monsoons.

Once Calcutta comes out of the influence of the low-pressure area, which should happen by Wednesday evening, the seemingly ceaseless rainfall that has bugged the city this festive season could stop. But don’t rejoice yet. The threat of more low-pressure areas and cyclones over the Bay will be there throughout November.

The normal date for the withdrawal of the southwesterly monsoon from the city is October 10, but recent years have witnessed a late retreat.

In 2010, monsoon withdrew on October 26 but that year the monthly rainfall had been much less. This year, with 10 days still left for October to end, aggregate rainfall has already reached 323.1mm for a surplus of 202.3 per cent. Only in 2005 was the surplus percentage anywhere close — 191.7 per cent with the month’s aggregate at 460.4mm.

Recent years have seen aggregate rainfall close to or lesser than the monthly average of 157.8mm, which is based on data collected over 30 years, starting 1971.

In 2010, when monsoon retreated very late, the total rainfall was just 105mm, a deficit of 33.5 per cent.