Monster thunderstorm twice Mt. Everest's height dumps over 250mm rain in Mumbai

The Saturday thunderstorm in Mumbai deposited 235mm of rain at the India Meteorological Department's (IMD) monitoring station in Santacruz. While early Friday's extreme rainfall totalling 253mm was confined to the western suburbs, on Sunday, more than 204.5mm rain fell in 24 hours, including 180mm to 270mm received across the city. Weather experts were amazed by the monster thunderstorm which, as per the images from the weather department's doppler radar, had a cloud top height of nearly 18kms, or 60,000 feet.

According to the IMD, most locations in Mumbai received well over 150mm of rain in just a few hours ending at 3:30 am Sunday, leading to severe water logging, interruption in water supply and at least 18 deaths. The storm, which was described by expert meteorologists and forecasters as "massive" and "monstrous", formed initially over Raigad district late Saturday, then slowly moved north along the west coast till it reached south Gujarat.

The intensity of rains began to subside between 3:30 and 4am for most parts of the city, as the cloud formation moved on to Palghar district.

Sridhar Balasubramanian, an expert tracking weather at the Indian Institute of Technology, Bombay (IIT-B), attributed the intense spell to an offshore vortex, adding that IIT-B had recorded over 250mm of rain in just three hours (between midnight and 3am), touching 305mm by 7am. "Luckily, this freak event happened during the wee hours when traffic and movement of people were minimal," he Tweeted.

The incident marked the fifth thunderstorm for Mumbai this month alone, pointed out Akshay Deoras, independent meteorologist and PhD researcher at the University Of Reading, England. "Such thunderstorms are definitely more frequent in June (6.1 days) than in July (2.9 days)," he said.

Deoras pointed out that easterly winds in the mid-atmospheric levels are bringing relatively dry air over the warm and moist air conditions in lower levels, which are brought by westerly winds, making the situation ideal for convective instability, which is further facilitated by day time heat. "Further, the vertical wind shear is significant, which is prolonging the lifespan of thunderstorms," Deoras said.

Similar observation was made by another independent forecaster Athreya Shetty. "Upper level winds seemed to be almost absent, hence there was no movement of the thunderstorm system. The winds that were there seemed to be due to a mini temporary circulation over Mumbai Metropolitan Region, hence the net wind movement was negligible," he Tweeted.

The absence of upper level winds further exacerbated the intensity because of the sheer height of the cloud structure which had formed over Mumbai. Images from the IMD's doppler radar showed that the thunderstorm had a cloud top height of nearly 18kms, or 60,000 feet. "To put in other words, the vertical extent of this monster thunderstorm is approximately twice that of Mount Everest!" Deoras said.

As per IMD's impact based forecast issued at 3am this morning, Mumbai has been placed under a red alert, "indicating heavy to very heavy rainfall with extremely heavy rainfall at isolated locations" for the rest of the day.