The weather office has predicted a wet weekend with tall, dark clouds and squalls.
The sky over Calcutta and adjoining districts is expected to turn overcast on Saturday. Rain is likely towards evening. The cloud cover and rain are expected to continue through Sunday.
“The sky is likely to clear up from Monday,” said an official of the India Meteorological Department, Calcutta.
Meteorologists expect two or three spells of rain over the weekend, accompanied by gusts of wind of speed up to 50- 60kmph. If that happens, the storm would be the season’s first kalbaisakhi.
For a squall to qualify as a kalbaisakhi, it must blow at a speed of at least 45kmph for a minute and there has to be a sudden jump in the wind speed.
“Two cyclonic circulations formed on Thursday — one to the north of Calcutta, over north Bengal-Assam-Bangladesh, and the other to the south, over Odisha and Chhattisgarh. These two systems are connected by a trough of low pressure. The three together would make for a wet weekend with squalls and thunder,” said Gokul Chandra Debnath, director, India Meteorological Department, Calcutta.
He said the systems were pulling air towards themselves as their atmospheric pressure was lower than that of the surroundings. “They were weak on Friday but were gaining strength,” said an official of the weather department.
As the systems intensify, they would pull moisture from the Bay of Bengal, leading to the formation of tall, dark cumulonimbus clouds that bring rain with storm and thunder.
As the circulation to the north of the city — spread over Jalpaiguri, Cooch Behar, Bangladesh and Assam — was stronger, thunderstorm is expected from that direction.
“Dry wind is likely to blow in the upper levels of the atmosphere, while the lower levels would have warm and moist air from the Bay of Bengal. The intermingling of these two flows would lead to the formation of thunder clouds,” said a weather scientist.
The Met department does not expect a significant fall in temperature because of the rain. Friday’s maximum temperature was 34.8 degrees Celsius, normal for this time of the year. It’s expected to be around 34 degrees on Saturday and Sunday.
The discomfort level might rise because of the increase in relative humidity.
The maximum relative humidity came down from 98 per cent on Wednesday to 77 on Friday, which is why it felt less sultry and the perspiration evaporated fast.
The muggy summer that Calcutta is used to might be back from Monday.