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Why IMD failed to forecast the Delhi storm?

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The India Meteorological Department (IMD) may have been caught off-guard on Monday, as squally winds of up to 100 km/hr, accompanied by thundershowers and even hail, struck the Capital at around 4pm, leading to widespread damage across the city.

On Sunday, IMD only forecast “thundery development” for the Capital on Monday and had a ‘green’ alert in place for Delhi; it upgraded this to a ‘yellow’ alert at around 3pm Monday; and then, finally, it sounded an ‘orange’ alert at around 4.30pm, by which time it was already too late for people and agencies to take precautionary measures.

IMD issues green alerts for days when no weather phenomenon is expected, with the thundery development forecast generally signifying overcast skies, without any rain. It issues a yellow alert, in case a weather activity is expected, while the orange alert is issued, asking for people to take action.

IMD officials say capturing such sudden changes in the weather, largely occurring in the pre-monsoon months of May and June, are hard to capture on medium-term models that rely on a combination of software, satellite data and doppler weather radars which cover a radius of 400km around Delhi. Instead, they rely on short-term models, issued in the form of ‘nowcasts’, which track weather around Delhi every 2-3 hours.

IMD scientist RK Jenamani says forecast reliability is generally around 95-100% in most months, but drops below 80% in May and June, due to unstable atmospheric conditions.

“It is extremely difficult to forecast such spells of rain and thunderstorm activity during this time of the year, as both the maximum temperature and the relative humidity is high. The more the moisture, the greater are the chances of it suddenly leading to the development of thunderclouds that form within a matter of hours and bring a short but intense spell of rain,” he says, stating both these conditions were met on Monday, with an active western disturbance (WD) acting as the trigger for the formation of thunderclouds.

“The WD over northwest India and Pakistan, along with an induced cyclonic circulation over Rajasthan, both added more moisture to the air, causing a quick formation of thunderclouds. (An alert on) this sudden formation was promptly issued through our nowcast feature,” he added.

At 1.02pm, IMD issued a nowcast , stating thunderstorm with light to moderate intensity rain, and wind speeds of up to 40km/hr would occur over parts of UP and Rajasthan in the following two hours. At 3.35 pm, it issued an alert stating light to moderate intensity rain and gusty winds of up to 50km/hr would hit parts of west, north-west, south and south-west Delhi, along with parts of Haryana and UP in the following two hours. At 4.15 pm, it issued a third nowcast , stating wind speeds would touch up to 60km/hr and the thunderstorm would cover all of Delhi and NCR over the following two hours.

Mahesh Palawat, vice-president of meteorology at the private weather forecaster Skymet, says that generally for such weather events, only a two-hour window is available to predict and issue alerts.

“This is a phenomenon generally occurring during the pre-monsoon months and Delhi-NCR has sufficient moisture now after the spell of rain on May 23… The air, which is high on moisture, rises to a sufficient height and starts to cool down, leading to condensation and formation of cumulonimbus or thunderclouds. This formation occurred at a height of around 12-14km on Monday and the higher the height, the more intense is the spell,” said Palawat,

IMD’s own forecasting accuracy data shows while its accuracy is lowest in June, with only 73% accuracy last June and 80% in June 2020. Similarly, for May, it was 84% in 2021 and 2020. It is also lowest in the pre-monsoon season as compared to other seasons, averaging at around 85% over the last three years.

Met officials say they cannot rule out another such sudden thunderstorm over the next month, stating that high humidity and high temperatures could once again cause it.

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