Meteorologists said forecast models had “failed” to predict the extreme weather event that caught Delhi residents by surprise early Friday, bringing a record rainfall of 228.1 mm.

This amount is more than three times the June average of 74.1 mm and the highest for this month since 1936.

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An IMD official said the monsoon winds interacted with the remnants of a western disturbance, causing heavy rains in Delhi, Himachal Pradesh and Jammu and Kashmir.

Independent scientists suspect that a thunderstorm over north Delhi may have triggered the torrential rains.

On June 26, the IMD had forecast only light to moderate rain and thunderstorms with gusty winds for Friday (June 28).

On Thursday afternoon, the weather department noted that a trough zone of a cyclonic circulation in the lower tropospheric layers extended over central Gujarat to western Bihar.

In the weekly weather report uploaded on the IMD's YouTube page, scientist Soma Sen Roy said that this low pressure area is pumping moisture into north and central India.

“The east-west low is likely to strengthen during the week and rainfall over northern India will increase,” she explained.

The IMD's extended weather forecast released on Thursday evening predicted “fairly widespread to widespread light to moderate rainfall accompanied by thunderstorms and lightning over northwest and eastern India during the next five days”.

There were also forecasts of “very heavy rains” over Delhi on June 29 and 30, but no one expected the torrential rain on Friday morning.

At 4:58 am on Friday, the IMD issued an alert saying, “Light to moderate rain, with heavy rain at some places and wind speed of 20 to 40 kmph, will occur in entire Delhi and NCR, Gannaur, Sonepat, Kharkhoda, Jhajjar, Sohna, Palwal, Baraut, Baghpat, Khekra, Pilakhua and Sikandrabad and adjoining areas during the next two hours.”

It was later reported that the Safdarjung Observatory had recorded 148.5 mm of rainfall between 2:30 am and 5:30 am, indicating that much of the rain had already fallen before the IMD warning.

It also declared the arrival of monsoon in Delhi by Friday noon, less than 15 hours after its forecast that the primary rain system would reach Delhi within two to three days.

“There was not much rainfall due to the eastern arm of the monsoon in West Bengal, Bihar and up to Uttar Pradesh. The eastern arm of the monsoon was moving slowly. But suddenly a surge arrived from Madhya Pradesh. Nobody expected this huge amount of moisture,” said an IMD official, who wished to remain anonymous.

“The models could not capture it. In addition, convective clouds began to form in the evening and intensified over time. Predicting thunderstorms in advance is not easy,” the official added.

Akshay Deoras, scientist at the National Centre for Atmospheric Science and the Institute of Meteorology, University of Reading, said, “A thunderstorm developed north of New Delhi at around 2:45 am IST on June 28. The presence of a large storm over Uttar Pradesh may have played a role in triggering it.”

“The mega storm in Delhi intensified into a severe storm by 4:15 am IST. The storm from Uttar Pradesh was also approaching the city. The outflows of the storm from Uttar Pradesh might have played a role in the intensification of the storm in Delhi,” he posted on X.

The IMD issued an orange alert on Friday and predicted heavy to very heavy rain in the city on Saturday and Sunday.