New Delhi | Forecasting models "failed" to predict the extreme weather event that took Delhi residents by surprise early Friday, resulting in a record 228.1 mm of rainfall, meteorologists said.
This amount is more than three times the June average of 74.1 mm and the highest for the month since 1936.
An IMD official explained that the monsoon winds interacted with the remnants of a western disturbance, causing heavy rains in Delhi, Himachal Pradesh, and Jammu and Kashmir.
Independent scientists suggested that a thunderstorm over North Delhi could have triggered the torrential rains.
On June 26, the IMD had predicted only light to moderate rain and thunderstorms with gusty winds for Friday (June 28).
On Thursday afternoon, the Met office noted that a trough extended from a cyclonic circulation over central Gujarat to west Bihar in the lower tropospheric levels.
In the weekly weather briefing uploaded on the IMD's YouTube page, scientist Soma Sen Roy said this trough was pumping moisture into north and central India.
"The east-west trough is likely to strengthen during the week and rainfall will increase over north India," she explained.
The IMD's extended range forecast issued on Thursday night predicted "fairly widespread to widespread light to moderate rainfall accompanied by thunderstorms and lightning over northwest and east India during the next five days".
It also suggested "very heavy rainfall" over Delhi on June 29 and June 30, but no one expected the torrential rain on Friday morning.
At 4:58 am on Friday, the IMD issued an alert, stating "light to moderate intensity rain with heavy intensity rain over a few places and winds of 20-40 kmph would occur over and adjoining areas of entire Delhi and NCR, Gannaur, Sonepat, Kharkhoda, Jhajjar, Sohna, Palwal, Baraut, Baghpat, Khekra, Pilakhua, and Sikandrabad during the next two hours".
Later, it reported that the Safdarjung Observatory recorded 148.5 mm of rainfall between 2:30 am and 5:30 am, indicating that much of the rain had already occurred before the IMD issued the alert.
Also, it declared the arrival of the monsoon in Delhi on Friday noon, less than 15 hours after it had projected that the primary rain-bearing system would reach Delhi within two to three days.
"Not much rainfall was happening due to the eastern arm of the monsoon in West Bengal, Bihar, and up to Uttar Pradesh. The eastern arm monsoon moved slowly. But one pulse suddenly arrived from the Madhya Pradesh side. Nobody expected this massive amount of moisture," an IMD official said, requesting anonymity.
"The models could not capture it. Also, convective clouds started developing in the evening and intensified over time. Predicting thunderstorms in advance is not easy," the official added.
Akshay Deoras, a research scientist at the National Centre for Atmospheric Science and the Department of Meteorology at the University of Reading, said: "At around 2.45 am IST on June 28, a thunderstorm popped up to the north of New Delhi. The presence of a big storm over Uttar Pradesh might have played a role in triggering it."
"The Delhi megastorm strengthened to a severe storm by 4.15 IST. The storm from Uttar Pradesh also approached the city. Outflow from the Uttar Pradesh storm might have played some role in intensifying the Delhi storm," he posted on X.
The IMD on Friday issued an orange-colour warning, predicting heavy to very heavy rain in the city on Saturday and Sunday.