A cyclonic circulation that developed over coastal Myanmar on Wednesday will determine the Puja weather in Calcutta.

Moving east to west, the system is expected to intensify into a depression and reach India’s east coast, over 1,000km away, by Chaturthi (Friday). How Durga Puja plays out will depend on what the system does next.

The city may receive “passing showers” on Thursday and Friday, but the intensity of the showers is likely to go up on Friday, said a Met official.

The forecast for the days ahead will be updated on Thursday, the official said.

A Wednesday bulletin predicted “light to moderate rain/thundershower” in south Bengal on Saptami and Ashtami (September 29 and 30).

Another system that caused — within a few hours early on Tuesday — the heaviest rain the city has seen in 40 years is weakening fast and will fade away by Thursday morning, the Met office said.

The proof was Calcutta’s weather on Wednesday — sunny and cloudy in phases and very little rain.

Current status

An upper-air cyclonic circulation formed over the coastal areas of central Myanmar and the adjoining east-central Bay of Bengal on Wednesday, a Met bulletin said. It is likely to move westwards, intensifying first into a low-pressure area and then a depression.

Future path

On Saturday, the depression is expected to make landfall. As of now, the touchdown is likely somewhere between southern Odisha and northern Andhra Pradesh.

“It is very likely to cross the south Odisha-North Andhra Pradesh coasts around September 27,” said the bulletin.

If the system follows the projected path, Calcutta might escape with light rain between sunny phases on the Puja days.

But any deviation to the north can spell trouble. Southern Bengal shares a border with northern Odisha. “If the system moves north, the chances of heavy rain in south Bengal will go up,” said a Met official.

Saturday worry

Thursday is likely to be sunny and cloudy in phases. Rain is not ruled out. But the intensity of the rain is likely to go up after Friday. Saturday is most likely to get heavy rain in south Bengal.

As of now, Calcutta has not made it to the list of probables. South 24-Parganas, Jhargram, East Midnapore and West Midnapore districts are most likely to get heavy rain on Saturday, according to the Met forecast.

Calcutta is almost certain — a Met official called it a 70% possibility — to get some rainfall on Saturday.

Since the system behind the rain is a depression, some clouds from the other band are likely to reach south Bengal. The sky will be gloomy with breezy conditions, said a Met official.

“Thunderstorm, lightning, and gusty wind (speed 40-50kmph) very likely to occur at one or two places over South 24-Parganas, Calcutta, Howrah, East Midnapore and West Midnapore districts of south Bengal,” said the official bulletin issued by the IMD.

The Met office recorded around 5mm of rain in Alipore on Wednesday. The sky turned bright and sunny more than once. The showers were passing drizzles.