A fresh weather system is likely to form over the north Bay of Bengal on October 10, the date when the monsoon usually retreats from south Bengal, indicating a delayed withdrawal this year, Met officials said.
"An upper air cyclonic circulation is likely to form over north Bay of Bengal around October 10," stated a national bulletin from the India Meteorological Department (IMD) on Tuesday.
"The system is unlikely to have much impact in Bengal but a delayed exit of monsoon is almost certain this year," a Met official said.
The withdrawal of the southwest monsoon typically begins from Rajasthan around September 17. For Calcutta, the usual withdrawal window is between October 10 and 12.
This year, the process began a few days ahead of schedule but was stalled by a previous system over the Bay, which had intensified into a deep depression and entered land in Odisha on Dashami. Now, the withdrawal has resumed.
The IMD bulletin said the monsoon withdrawal line now passes through Veraval and Bharuch in Gujarat, Ujjain and Jhansi in Madhya Pradesh, and Shahjahanpur in Uttar Pradesh. The line of withdrawal moves west to east.
"Conditions are favourable for further withdrawal from remaining parts of Gujarat; some more parts of Madhya Pradesh and Uttar Pradesh and some parts of Maharashtra during the next three to four days," the bulletin added.
Weather scientists use three key conditions to confirm the withdrawal of monsoon: three to four consecutive rain-free days, a dip in atmospheric moisture, and a reversal of wind direction — from moist Bay winds to dry winds from north India.
In Bengal, however, none of these conditions are currently being met.
Multiple weather systems are injecting moisture into the state. North Bengal was just lashed by torrential rain, while parts of south Bengal, including Calcutta, have seen isolated thunderstorms.
On Tuesday, Calcutta witnessed a bright morning, but the sky turned overcast by afternoon. A sharp spell of rain hit parts of south Calcutta and nearby areas around 1.30pm, accompanied by lightning and thunder. The Met office issued thunderstorm alerts for several districts, including Calcutta, South 24-Parganas, Howrah, and East and West Midnapore.
"There are two upper-air cyclonic circulations currently affecting Bengal — one over northeast Jharkhand and the other over south Bangladesh," said a Met official in Calcutta.
Light to moderate rainfall, mainly in the form of thunderstorms, is expected across Bengal over the next three to four days. There is no warning for heavy rain in either north or south Bengal.
This year’s southwest monsoon has brought surplus rainfall to Calcutta (see chart). Met officials attributed the surplus to a higher number of Bay systems than usual.
“There have been 14 weather systems that have impacted India’s east coast this monsoon. The usual number is around 10,” said a Met official. “Monsoon rain in south Bengal is usually dependent on Bay systems.”