Chennai received rains in excess of 20 cm on the night of November 6, the heaviest spell since the 2015 floods. However, a broken, outdated and ill-maintained weather prediction system used by the Regional Meteorological Department failed to predict that the city would witness such heavy rainfall in a short span of 24 hours. An official from the Regional Meteorological Department, Chennai, confirmed that two weather radars — one in Chennai and another in Karaikal — had been under maintenance on November 6, totally failing to predict the record rainfall on November 7. But this is just one part of the problem.
Two out of three radars were out of order just before the floods of November 6. And a look at weather bulletins show that IMD and Regional Met department made conflicting predictions.
While old and failed radars are an issue, there are other specific problems too, including different predictions by the same department and the overdependence on single systems. To understand Chennai’s problems, it is important to know the two kinds of weather predictions — nowcasting and forecasting.
Nowcasting is a technique used to forecast weather conditions in the short range, i.e. within a period of zero to six hours. Nowcasting is done by taking estimates of the wind speed and direction of wind movement to forecast weather in the short range. Forecasting, on the other hand, predicts weather changes for longer time periods.
While doppler weather radar systems are useful for nowcasting, satellite images help in both nowcasting and forecasting for longer periods of time, especially while tracking events like Cyclones etc.
‘Conflicting’ reports
On November 6, Saturday, Chief Minister MK Stalin called for a preparatory meeting as IMD had forecasted heavy rains after the onset of the North East monsoon. An alert on November 6 afternoon predicted heavy to very heavy rainfall in north coastal Tamil Nadu. North Coastal Tamil Nadu includes around 14 districts, and there were no other specifics in this bulletin.
But in the same bulletin, IMD gave an orange alert — which is a direction to be prepared for floods — for the entire state. Ideally, the IMD should have given a red alert which is to ask the state to take action.
However, it’s the Regional Met department’s job to specify which areas in the state will be affected — and while the daily bulletin from the Regional Meteorological Centre at 1 pm on November 6 spoke about a cyclonic formation, and predicted rainfall in many areas, it did not predict heavy rainfalls for Chennai on November 6. The regional office predicted heavy rains only for Cuddalore, Mayiladuthurai, Nagapattinam, Tiruvarur, Thanjavur and Pudukottai districts of Tamilnadu and Karaikal area on November 6.
For November 7, the same bulletin predicted moderate rainfall for the city, with some heavy spells in some areas.
The Alert by the Regional Meteorological Centre which did not mention Chennai
That’s because on November 6, the Regional Met Department had radar imagery only from the S-band Doppler Weather Radar (DWR) in Sriharikota, which has a range of 500 km. Though there are three S-band radars, only one worked, and a weather blogger TNM spoke to said that this radar (used for nowcasting) would have picked up the cloud patterns, but the Regional Met Department failed to make the right prediction for later on — that is, their nowcast failed.
A recurring issue with radars
This is not the first time that one or more weather radars around the city have stopped working ahead of a major spate of rains. In November 2020, in the week preceding cyclone Nivar, the IMD website showed that the Karaikal doppler weather radar was under maintenance. So in November 2020, the Meteorological department had to rely on the Sriharikota radar.
In November 2021, it also had readings from an X-band Doppler Weather Radar situated inside the National Institute of Ocean Technology (NIOT) in Chennai. This X-band radar however is less powerful than the main radar located inside the Chennai Port Trust, with a maximum range of 150 to 200 km.
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