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Expert meteorologists and researchers may be still trying to figure out what befell them on Thursday when the skies literally crashed down on Chennai, but it is likely that one may not need to go farther beyond the dynamics of the current winter weather system over North India for a simple and proximate trigger.
It may all have to do with a resetting of the seasonal anticyclone over Rajasthan and its neighbourhood bringing its southern flanks to to bear on the South Peninsula, says Air Vice Marshal (retd) GP Sharma, President-Meteorology and Climate Change at leading private forecaster Skymet Weather.
Anticyclone, the culprit?
The anticyclone with its clockwise movement of air establishes over North India to preside over the winter.
The clockwise motion forces the air to sink to the ground level, and along with it, the chill that north-westerly winds from across the international border with Pakistan fetch.
The recent spurt in western disturbance activity had forced this anticyclone to move south of its normal position - down to the latitude of Maharashtra, Telangana and North Karnataka. Its eastern flanks bore down heavy on the Bay of Bengal, suddenly altering the wind flows therein.
Adds sting
The clockwise winds of the anticyclone worked up on the easterly to north-easterly flows of the North-East monsoon across the Bay. Late December also represents the fag-end of the North-East monsoon, but the overwhelming influence of the anticyclone on easterlies suddenly added a sting in its tail.
“Skymet had hinted about some moderate rain for Tamil Nadu but then we did not expect the very heavy rain that ultimately materialised. There was simply no system - some circulation, low-pressure area or depression - that could have given us a concrete lead,” he said.
“I will not be surprised to see one or two heavy spells towards the evening on Friday around Chennai, Tambaram and even to the south till Puducherry. The timing of the event surprised, since during this phase of the North-East monsoon, most activity moves South to concentrate around Sri Lanka,” he said.
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