On Tuesday, meteorological department officials said a low pressure system was forming in the Bay of Bengal that could turn into a cyclonic storm and hit the country’s east coast by December 4. Tamil Nadu is bracing for the storm to hit its northern coast, though it is unclear as of now if it will do so or move further up to southern Andhra Pradesh.

“So far, we expect a low pressure system to form in the Andaman sea in the next few days,” said SB Thampi, head of the Regional Meteorological Centre in Chennai. He said a trough – a region of low atmospheric pressure – had already formed over south-western Bay of Bengal and the adjoining areas of southern Sri Lanka. “In the next few days it will move towards the Bay of Bengal, and is likely to come closer to Chennai and take a northerly turn,” he said. “Whether it will reach cyclone intensity or not, it cannot be said at this point in time. We will have to wait and see.”

According to private weather services company Skymet, this would be the fourth low pressure area to be formed in the Bay of Bengal during the North-East Monsoon that could affect the weather over Tamil Nadu.

The southern state is currently in the middle of the North-East Monsoon cycle, which sets in at the end of October and accounts for around 48% of its annual rainfall. Rainfall distribution across the state has been uneven so far and the cyclone, if it lands, could widen the imbalance by drenching the rain-fed northern coastal districts.

A storm alert for Tamil Nadu around this time of the year is not unusual. Two cyclones had struck the state’s coastal areas in December last year. Cyclone Nada had landed near Puducherry, weakening into a depression as it neared the coast. The more powerful of the two cyclones, Vardah, had made landfall in North Chennai and was accompanied by powerful winds that brought down trees and power lines. Many parts of the city had no power, phone or internet services for around a week.

Normal rainfall

This year’s North-East Monsoon has seen a rainfall deficit of around 17% in Tamil Nadu and Puducherry. However, meteorological department officials said they still expected a normal monsoon. “A deficit up to 18% is considered normal,” Thampi explained. “We hope that in the coming days, the departure from normal rainfall amount will come down.”

This graph represents normal rainfall received by the southern peninsula in the past two months.

Credit: India Meteorological Department

The graph below shows that there have been spikes of heavy rainfall in the past two months. But weather blogger Pradeep John says this is a normal feature of the North-East Monsoon. “Unlike the South-West Monsoon, the North-East Monsoon is erratic,” he said. “You see good rainfall one day, and then a break for a while.”

Credit: India Meteorological Department

Unequal distribution

The above normal rainfall experienced at the end of October and in the first week of November, as shown in the graph above, was caused by two low pressure systems. They brought heavy showers to the northern coastal belt of Tamil Nadu. The districts of Thiruvallur, Chennai, Kancheepuram and Nagapattinam recorded excess rainfall.

Meanwhile, pre-monsoon rain in early October brought showers to the interior districts of Krishnagiri and Vellore, which usually do not receive much rainfall.

But for most of the monsoon season, the central and southern districts have not received much rainfall. Ramanathapuram district, in particular, has had very deficient rain.

The southern districts are finally seeing some showers as a result of a third low pressure system in the Bay of Bengal. “As the low in the south-west Bay moves into Comorin sea in a day or two, most of the action will be in South Tamil Nadu and the delta belt,” John wrote on his Facebook page. “Let them get the much needed rains.”

The fourth low pressure system that is still forming is predicted to bring rain to northern Tamil Nadu, although it is not yet clear how close it will get to the coast. “Whether it brings rain over the land area or the ocean, we are yet to see,” said Thampi.

Satellite image of weather conditions on November 28. (Credit: India Meteorological Department)