More rain in offing for Mumbai but repeat of July 26 deluge unlikely

For citizens of Mumbai, there is going to be heavy rain till the morning of August 3. But this time, meteorological experts are sure that it will just be heavy rain and not another offshore vortex bringing the city to its knees. The last time, forecasting was not possible without dedicated doppler radars which the Met department is yet to procure for this kind of localised area and short time-frame.

After the deluge, the National Centre for Medium Range Weather Forecasting (NCMRWF) sent out mails to other weather stations to see if they were able to forecast what Mumbai had in store.

The only Met station that came close was the UK Meteorological Department: When they ran the model again, it came up with 80 cm of rain for the city.

Met experts say this phenomenon was unusual for Mumbai but not for India. It was the intensity that took everyone by surprise. The cloud column that’s normally supposed to be 6-7 km during the monsoon was as high as 15 km that day. Essentially, it was this column that drowned the city.

‘‘The reason why these rains are not going to be as deadly as July 26 rain is because right now there is only an offshore trough and not a vortex,’’ said Akhilesh Gupta, Director, NCMRWF.

This dangerous offshore vortex was formed when moisture-laden strong winds came in from the Arabian Sea but ran into an obstacle in the form of the Western Ghats.

Unable to cross over directly, it began flowing in a channel from the side, slightly northwards of the Western Ghats. The moisture-laden winds took a turn forming a sort of a wedge.

This wind trough became a vortex when the winds took a full 360-degree turn, lifting the air upwards in an anti-clockwise direction.

What made it deadlier was the low pressure area over central Madhya Pradesh. Even though the monsoon winds are still flowing from the Arabian Sea, the low pressure area has moved from central Madhya Pradesh to Gujarat and is gradually going to weaken in three days.

Though the amount of rain is similar to a cloudburst, it is different as it is constantly being fed with moisture from the Arabian Sea making it more lethal. In a cloudburst, it is the collected moisture that results in heavy rain.

This vortex is not an unknown phenomenon: it is just that the intensity and the location created havoc. It was so localised that even in Mumbai, it was North Mumbai and its suburbs that bore the brunt. Essentially, it is like a cyclone but spread over a small area compared to 400-500 km in a cyclone.

On May 4, 2004, a station in Minicoy had 116.5 cm of rain. On the same day, Virar lake had 104.5 cm of rain.

Though, it is going to rain for another 48 hours or so, the fact that it is spread over a large area, starting north of Ratnagiri and covering almost the entire state, it means that July 26 is not likely to be repeated, at least this monsoon.