Explained | Why Cyclone Fengal dumped 'historic' levels of rain in Tamil Nadu’s interior areas

Chennai: Slow-moving Cyclone Fengal, which remained stationary over the Puducherry-Villupuram belt for over 24 hours, has wreaked havoc across coastal and northern Tamil Nadu from Chennai to Chengalpattu to far-away Krishnagiri, which received 50 cm rainfall. It was a “once in 300-year” event, in less than 24 hours.

Chennai and its neighbouring districts of Tiruvallur, Kanchipuram, and Chengalpattu were the first to face the fury of the Cyclone, which IMD scientists and independent weather bloggers found difficult to predict from the beginning, from Friday evening to Saturday afternoon. It was then the turn of Villupuram, and Cuddalore in Tamil Nadu and Puducherry to get pounded for over 24 hours, followed by Tiruvannamalai, Ranipet, Vellore, Dharmapuri, Krishnagiri, Salem, and even Mysuru in Karnataka.

It is a rare event, according to weather bloggers, that interior areas like Uthangarai in Krishnagiri receive a whooping 50 cm of rainfall. Several towns and villages in Krishnagiri, Dharmapuri, and Salem districts too registered “record” levels of rainfall on Sunday and Monday under the influence of Cyclone Fengal.

“Slow moving Cyclones are deadly… Uthangarai never got such rainfall. A slow moving Cyclone Fengal dumped everything there,” Pradeep John, a popular weather blogger, said.

K Srikanth, another weather blogger who tweets under Chennai Rains on X, told DH that Chennai and its suburbs would have faced the “fury” of the cyclone had it moved north-west instead of west, which led to the weather system dumping most of the rains in Puducherry, Villupuram, Tiruvannamalai for nearly 36 hours.

Ajith Kumar, also a weather blogger, said the wind convergence in lower to mid-levels of atmosphere and slower translational speed of Cyclone Fengal led to historic rains across north Tamil Nadu.

Usually, cyclones move around the outflow of a high pressure area but in Fengal’s case, there was a dominant ridge (Arabian) to its west, not allowing the weather system to move freely.

“Due to which, the convection of the clouds over the western part of the cyclone remained practically over the same region, recycling moisture over the same area just before the landfall dumping huge rains in Villupuram and Puducherry,” Ajith Kumar, who tweets under the handle Chennai Weatherman, told DH.

Srikanth said Cyclone Fengal practically lay stationary and spent nearly 30 hours over the Puducherry-Villupuram-Tiruvannamalai region before it moved to other interior areas. “That is why we saw extremely heavy rains pounding Puducherry, Villupuram, and Cuddalore continuously for over 24 hours,” he said.

Had Cyclone Fengal moved fast, it wouldn’t have dumped so much rainfall in interior areas like Tiruvannamalai, Dharmapuri, Krishnagiri, and Tirupattur. “Fast-moving cyclones usually retreat into the Arabian Sea in about 24 to 36 hours. But since this was a slow-moving cyclone, it took its own sweet time to cross the coast and weaken into a deep depression and later into a depression. That’s the reason for the deluge,” Ajith Kumar said.

Though the cyclone weakened, convergence of winds over the same area made clouds last longer than usual, resulting in more rains. Srikanth said several weather models did pick up the route of the cyclone with bloggers making it clear that Dharmapuri and Krishnagiri districts would receive heavy rainfall.

“But we tend to doubt the probability because we look at the models two days before the landfall. If we predict extremely heavy rainfall, we are accused of creating panic on social media. But we give warnings only to ensure that we are prepared for the worst-case scenario, not the best case scenario,” Srikanth added.