CHENNAI: Chennai’s northeast monsoon debut has delivered an extraordinary burst of rainfall, with North Chennai recording more than 300 mm in 24 hours at individual rain gauges, raising questions about the limits of weather forecasting and the ability of disaster management agencies to prepare for hyperlocal extreme events.
The heaviest rainfall was concentrated around Parrys, Kathivakkam, Sathangadu, Ennore and Kargil Nagar on Saturday evening and night. The Greater Chennai Corporation’s rainfall dashboard recorded 361.9 mm at its Zone 5 Parrys gauge.
The event was particularly northworthy because the downpour was not uniformly distributed across the city. Some North Chennai locations received several times the rainfall recorded at official IMD observatories elsewhere in the metropolitan area.
The IMD had forecast heavy to very heavy rainfall and issued an orange alert for Chennai and neighbouring districts on Saturday. However, the concentrated burst in North Chennai exceeded what civic authorities had anticipated. The episode has renewed the debate over whether such intense, localised rainfall should be described as a cloudburst.
Why the rainfall was difficult to predict
VR Durai, head of Regional Weather Forecasting Centre at the Regional Meteorological Centre (RMC), Chennai, told TNIE that the event was associated with an upper-air cyclonic circulation that helped generate intense convective activity.
The circulation initially lay over North Tamil Nadu and adjoining south coastal Andhra Pradesh. According to the IMD’s assessment, it enhanced moisture transport and wind convergence over north coastal Tamil Nadu, supporting the development of deep convective clouds capable of producing intense rainfall.
Durai explained that the movement and persistence of the circulation, combined with daytime heating, helped intensify the thunderstorms. “The clouds developed well, and because of that, convective rainfall and thunderstorm activity occurred, resulting in this much rain,” he said, explaining the meteorological conditions behind the spell.
The key challenge was not identifying the possibility of heavy rain, but anticipating precisely where the most intense convection would develop and how much rain it would produce within a short period.
Weather models provide an indication of the likely intensity and geographical spread of rainfall. Forecasters must then interpret the model output alongside observations and atmospheric conditions to arrive at a practical forecast. A thunderstorm can intensify over a small area, producing extraordinary rainfall while nearby locations receive considerably less.
“We cannot (predict it at that level), but we can give a range,” Durai told TNIE, explaining the limitations of forecasting such localised events.
The difficulty is consistent with the IMD’s own explanation of cloudbursts. Rainfall exceeding 100 mm in an hour at a station is classified as a cloudburst, but predicting one is difficult because of its extremely small spatial and temporal scale.
The contrast between the official observatories and the denser network of automatic weather stations (AWS), automatic rain gauges (ARG) and other monitoring stations illustrates how sharply rainfall varied across the region.
The official IMD observatories recorded 157.8 mm at Nungambakkam and 146.7 mm at Meenambakkam, while the AWS at Ennore Port recorded 222.5 mm. The much higher figures from north Chennai’s municipal gauges explains why citywide forecasts cannot always convey the full intensity of a neighbourhood-scale event.
Durai also said the rainfall observations from nearby instruments were being examined and calibrated. The IMD’s initial meteorological assessment, meanwhile, attributed the event to organised convective activity associated with the cyclonic circulation rather than an isolated, highly localised cloudburst.
What the forecast says now
For Chennai, the forecast indicates that the immediate risk has not passed. Durai said the upper air circulation, which brought in copious amounts of rain on Saturday, continues to persist in the same location.
"We have issued a red alert for Chennai for Sunday. The consequences of further rain would depend on the ground conditions. After such an extreme spell, already saturated soil and accumulated water can make even moderate rainfall consequential. The subsequent alert was therefore intended as a precaution, not necessarily a prediction of another spell matching Saturday night’s intensity," the official said.
Durai said the movement of the upper-air circulation would determine how the next spell develops. If it moves away, the impact should ease; if it remains stationary and continues feeding successive bands of clouds, rainfall could persist.