Chennai: Did extreme rain catch IMD off guard?

Commuters make their way through a street during a heavy rain shower in Chennai on December 30. Pic/PTI

Chennai and its surrounding districts were pounded by more than 100 and 200 cms of rainfall between Thursday morning till early Friday, throwing the life of residents out of gear and leaving three people dead.

The state government declared a holiday on December 31 owing to further prediction of heavy rainfall in Chennai, Kancheepuram, Tiruvallur, and Chengalpattu districts due to heavy rainfall as a precautionary measure.

But could the three lives have been saved? Could the damage to property, waterlogging leading to traffic jams on Chennai roads, people being stuck in many offices and, in general, the troubles for the common citizen been avoided?

The India Meteorological Department (IMD) had in its 'All India Impact Based Weather Warning Bulletin' on Thursday afternoon predicted, "Thunderstorm accompanied with lightning at isolated places very likely over Tamil Nadu, Puducherry and Karaikal" and for Friday, predicted: "Thunderstorm accompanied with lightning at isolated places very likely over coastal Andhra Pradesh and Yanam, Tamil Nadu, Puducherry and Karaikal."

Even the Qualitative Precipitation Forecast (QPF) in IMD's Hydromet Bulletin from Chennai did not indicate anything beyond 25 mm rains with a 'NIL warning' for heavy rainfall.

It was, however, the nowcast - special forecast for metro cities that warns of changing weather conditions two-three hours in advance - that predicted the extreme rainfall in Chennai and surrounding districts.

The IMD's nowcast has a limited reach and most people suffered due to sudden torrential downpour as they had no knowledge of it.

"IMD Chennai had informed the disaster management and other relevant authorities with appropriate colour coding mention," an IMD official said. Colour coding refers to yellow, orange, red alerts commensurate to the quantum of precipitation.

Stating that they were expecting the rainfall to increase from Friday, "but it occurred one day in advance," IMD's Director-General Dr Mrutyunjay Mohapatra said, "There was a trough in westerly winds and the easterly winds from north-east monsoon was approaching... both interacted. Large scale wind flow patterns, which interacted with each other led to this type of activity."

K Srikanth, who runs a widely followed blog about Chennai weather, 'Chennaiyil Oru Mazhaikkallam' but popular as Chennai Rains, concurred that "nobody predicted yesterday's event".

He uses global model output and IMD's satellite and radar images for issuing warnings, forecasts on Chennai Rains' social media handles and writing his blogs. "The models did not show this event at all," he said.

"While the focus was on the lower-level easterlies bringing in moisture from Bay of Bengal, the mid-tropospheric winds created perfect conditions for the available moisture to develop into intense thunderstorms around Chennai and surrounding, Srikanth said, adding: "The sudden change in direction of winds at 5.5 kms above sea level (ASL) off the coast of Chennai enhanced the lower-level moisture brought in from the Bay."

Also Read: After heavy rains, govt offices, schools, colleges shut in Chennai, adjoining districts

The IMD has predicted further rains on Friday and Saturday, which would be heavy but widespread, unlike Thursday's rainfall.

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