A Nor’wester with wind speed touching 84kmph struck Kolkata on Monday evening, uprooting trees, tearing off hoardings, choking traffic and disrupting trains.
The storm, which was followed by a spell of rain, was the strongest so far this season, the Met office said. More such storms are likely till May 20.
Monday’s storm began with an ominous, howling wind and by the time it was over, signs across the city bore testament to its ferocity.
Largely bereft of thunderstorms this season and reeling under successive heatwaves, the city was cooled by Monday's storm and rain. But coinciding with the evening rush hour, it left many people returning home from work stranded on the road in cars, buses and cabs.
A trough of low pressure in the upper atmosphere from Bihar to Odisha could trigger more such thunderstorms in the next few days, according to the Met forecast.
The Met office had on Sunday predicted thunderstorms in south Bengal, including Kolkata, from May 17 to 22. The forecast was based on the assumption that Cyclone Mocha would dissipate gradually, following which moisture from the Bay of Bengal would enter south Bengal.
“But the cyclone died earlier than expected. While it was on land, moisture from the Bay moved towards the cyclone. But as the system dissipated, the same moisture started entering south Bengal. There was sufficient heating, which turned the moisture into clouds. The trough of low pressure provided the perfect lifting mechanism for the clouds,” said G.K. Das, director, India Meteorological Department, Kolkata.
Monday's storm qualifies as a Nor'wester because it originated over the Chhotanagpur Plateau. The timing also ticked the box.
A Nor’wester originates over the heated Chhotanagpur Plateau in the late afternoon and sweeps through parts of eastern India over the next three to five hours at a wind speed of over 45kmph. It is usually followed by a brief spell of rain.
The clouds reached the city via Paschim Medinipur, Jhargram, Birbhum, Bardhaman, Hooghly and Howrah, unleashing ferocious winds in the districts. By the time the clouds reached Kolkata, they were 10-12km above the earth’s surface, said a Met official.
Nadia, Murshidabad and North and South 24-Parganas also felt the brunt of the storm.
In Kolkata, the day was reasonably hot. The sky started becoming cloudy in the late afternoon. The gusts of wind started around 5.15pm.
Around 5.30pm, the sky was pitch dark as the winds kept gaining in strength.
The Met office recorded a “squall” that clocked the maximum wind speed of 84kmph at 5.41pm in Alipore. It lasted three minutes, the Met office said.
In Dum Dum, the maximum wind speed was 62kmph at 6pm, lasting a minute.
The formation of the trough had prompted the Met office to issue an “enhanced thunderstorm” bulletin hours before the storm struck.
“In the presence of an upper air trough from Bihar to Odisha and strong moisture incursion from Bay of Bengal, enhanced thunderstorm with lightning activity along with gusty wind is very likely over the districts of West Bengal from May 15 to 20, with peak activity during May 18 to 20,” said the bulletin.
Before Monday, the city was struck by a powerful thunderstorm on April 27. The maximum wind speed was 79kmph. But the rain that followed was sharper than it was on Monday.
On April 27, the city received around 40mm of rain. In comparison, the Met office recorded around 15mm on Monday.