The first Nor’wester of the year hit the city on Friday, two months behind schedule.

The storm with wind speed reaching 50km/hour was accompanied by the second successive afternoon downpour, which also ensured a cool start to the weekend.

The Patna Met office confirmed that the hour-long storm from 4pm — it held up nearly 20 flights at Dum Dum airport and forced six to be diverted — was a Nor’wester.

“We have had thundershowers for most of the week, topped by the Nor’wester on Friday,” said an official of the Regional Meteorological Centre in Alipore.

The first Nor’wester usually arrives in the first week of March and is followed by a few more in the next three weeks. April generally sees as many Nor’westers as March does.

Last year, however, there were only two Nor’westers the entire season.

Meteorologists and weather scientists said the late arrival of the Nor’wester this year despite conducive conditions was a mystery that needed to be explored.

“The delay is definitely a weather anomaly. It will eventually be studied in the context of climate change,” said a senior scientist at the National Centre for Medium Range Weather Forecasting in Noida.

O.P. Sharma, the chief meteorologist of a private agency, felt the “steadily changing path of western disturbances in India” could be responsible for the delay in Nor’westers hitting town.

Friday’s downpour started around 4.30pm and the rain, which slowed down traffic across the city, continued till 6.30pm.

The cloud cover over the city had kept the Celsius below normal even before the skies opened up, countering the high humidity range of 51 to 97 per cent.

According to the India Meteorological Department (IMD), a trough of low pressure stretching from sub-Himalayan Bengal to south Tamil Nadu is favouring the formation of thunderclouds over south Bengal.

“The incursion of abundant moisture under the influence of southerly and south-westerly winds is making conditions conducive to rain and thundershowers,” said an official of the IMD in Delhi.

Saturday is likely to be generally cloudy with rain or thundershowers expected in the afternoon or evening, the weatherman said.

Temperatures are likely to range between 23 and 35 degrees Celsius.