Why torrential rain caught Mumbai off guard

That the weather bureau had failed to issue alerts about the extreme weather event drew flak from residents, who were left reeling in the flooded city throughout Monday.

However, meteorologists say that almost all weather forecasting models failed to capture the extreme event which unravelled in the late hours.

What made the Monday rain record breaking?

Data furnished by the IMD showed that the Santacruz observatory received 268 mm of rain between Sunday (July 7) and Monday morning until 8.30 am, making it Mumbai’s wettest day since 2019, when the city received 375.2 mm of rain within 24 hours on July 2.

In what made it an extreme event, the maximum showers (over 200 mm rain) was received within a couple of hours. Of the 268 mm of rain, the Santacruz station recorded only 40.9 mm until 2.30 am. However, as the intensity of the rain gathered pace, the station received over 210.9 mm between 2.30 am and 4.00 am.

Meanwhile, the Brihanmumbai Municipal Corporation’s Automatic Weather System—between 1 am to 7 am—recorded 315.6 mm of rain in Vikhroli, followed by Powai, where 314.5 mm of rain was recorded.

Even as the intensity subdued thereafter, heavy rain continued to lash Mumbai throughout the day with the Colaba observatory recording 101 mm of rain between 8.30 am and 5.30 pm.

This prompted the IMD to issue a red alert for Mumbai on Monday evening. The alert remained in place until Tuesday morning.

What caused the heavy downpour?

IMD officials attributed the heavy showers to an offshore trough that suddenly strengthened around midnight.

An offshore trough refers to a shallow trough of low pressure observed (on sea-level surface chart) along the west coast of India during the monsoon season. Such systems frequently develop off the west coast of India—anywhere from north Kerala to south Gujarat—during the monsoon and are responsible for the strengthening of the rain in the coastal area.

On the phenomenon that brought heavy rain, Sushma Nair, a scientist from IMD, Mumbai, said, “Around midnight, the offshore trough along north Gujarat and the Kerala coast, which was feeble earlier, strengthened along north Konkan, leading to heavy rain. In light of this development, we had issued a nowcast warning at 12.30 am for Mumbai and its neighbouring districts.”

Raghu Murtugudde, a professor of climate studies at IIT Bombay, said the heavy showers were induced by sudden northward expansion of the southwesterly jet. “Mumbai has been at the edge of the southwesterly jet for weeks resulting in deficit rain. But yesterday, this jet expanded slightly northward and started pumping moisture directly, causing heavy rain,” he explained.

Why did the weather models fail to foresee the event?

According to Subimal Ghosh, a professor and the convener of the interdisciplinary programme in Climate Studies at IIT Bombay, all weather models—including the IMD’s—failed to register the extreme event.

Ghosh told The Indian Express, “None of the models could forecast the extreme rains. The IMD model placed no warnings. The ECMWF model forecast low rainfall. Even the NCEP-GFS model showed low rainfall. Since our model uses NCEP data to statistically downscale at local level, even our models failed to forecast the rain.”

“We are now trying to understand the factors by looking at the circulation as well as the moisture flow, where it came from, the wind directions etc,” he added.

An IMD official, on the condition of anonymity, added that the IMD models failed to capture the event as it was likely caused by an offshore vortex.

According to the IMD website, an off-shore vortex is formed when monsoon winds strike the Western Ghat mountains. On many occasions the winds do not have enough energy to climb over the Western Ghats and on such occasions they tend to be deflected round the mountains and return current forms the off-shore vortex. Such a weather system brings very heavy rain to the western coast.

“It is likely that the rain was caused by an off-shore vortex and our models could not capture an intense yet small-scale weather system that brought the very heavy rain,” the IMD official said.

Murtugudde added, “The current models have not been doing very well in capturing the edge effects of the jet, which pumped moisture and brought rain. A slight offset means you get a wrong forecast. Unfortunately, this happens as it is a tough system, especially to forecast at local levels.”

How does IMD monitor and forecast monsoon?

In a bid to monitor the monsoon, the IMD undertakes continuous monitoring of surface and upper air meteorological observations as well as real-time monitoring using remote sensing techniques like satellite and radars. It also carries out analysis of different meteorological charts with guidance from various national and international weather forecasting models at different spatio-temporal scales.

The IMD’s observation tools also derive products from satellite observations such as Cloud Top Temperatures, Cloud Motion Vector winds and other numerical weather prediction models available on the internet and world institutions like the United Kingdom Meteorological Office (UKMO), the European Centre for Medium-Range Weather Forecasts (ECMWF), the Center for Ocean-Land-Atmosphere Studies (COLA) etc.

The IMD also seeks observations from doppler weather radars.

Currently, Mumbai is home to two S-band doppler radars, which can detect, track rain and cyclones over a span of 500 km— at Veravali as well as Santacruz.