CHENNAI: Twice in two consecutive months, Chennai city received over 200 mm of rainfall and on both the occasions, the meteorological department failed to forecast the unfolding extreme weather event. While the city received 215 mm rainfall on November 7, it recorded 201 mm in the last 24 hours ending 8.30 am on December 31.
Officials TNIE spoke to cited the lack of adequate upper air data — which tells the air circulation’s location and direction — as the reason for this failure. Every two days, a telemetry instrument called radiosonde is flown on a weather balloon from Chennai and Karaikal to generate upper air data. For several months, however, this exercise was not carried.
A senior official at the Regional Meteorological Centre here said they received the instruments for the purpose only a few days back, and started generating data from Thursday. “During this entire north-east monsoon, we gave forecasts without having upper air data. We have now started to release weather balloons from Chennai, and are yet to do it from Karaikal,” the official said.
‘Predicting localised rain event difficult’
Addressing a press conference on Friday, N Puviarasan, director of Area Cyclone Warning Centre, said there were certain practical difficulties in predicting a localised weather event. "A local convergence coupled with upper air trough caused the extreme rainfall over Chennai.
Based on satellite data and wind speed, we thought upper air circulation was over sea and would move over land on Friday bringing heavy rains over coastal areas on Friday, but things didn't go as we thought." Experts opined the officials would have seen what was coming if they had the upper air data. Chief Minister MK Stalin was also critical about the met department being unable to predict the extreme rains.
59% above normal rains
Statistically, as the north-east monsoon ends on Friday, Tamil Nadu has recorded rainfall of 711.6 mm against the normal 448 mm, which is 59% above normal. Puducherry received 1491.4 mm against 882 mm, which is 69% above normal. The highest seasonal average district rainfall of 1360.4 mm was recorded in Chennai.
Rains to continue till Sunday
The met department said the rainfall activity would continue till January 2. On Friday, Nungambakkam station in Chennai received 32 mm of rainfall, Ennore received 40 mm and Virunchipuram in Vellore recorded 63 mm till 6.15 pm. Heavy rainfall warning has been issued to delta districts, Villupuram, Cuddalore, Chengalpattu, Kancheepuram, Chennai and Tiruvallur.