How IMD got its previous monsoon forecasts wrong

Data from previous 16 years shows that IMD’s first monsoon forecast was mostly at a large variance from actual rainfall values


Last year, all IMD forecasts, except the one for July for all of India, were overestimates when compared to actual rainfall values. Photo: Mint
Last year, all IMD forecasts, except the one for July for all of India, were overestimates when compared to actual rainfall values. Photo: Mint

New Delhi: The India Meteorological Department (IMD), which released on Tuesday its first long-range forecast saying India will receive normal rains or 96% of its long-period average during the southwest monsoon, has rarely got its forecasts right.

Data from previous 16 years shows that IMD’s first monsoon forecast was mostly at a large variance from actual rainfall values. The southwest monsoon is considered normal when the rainfall is 96-104% of its long period average (LPA).

Out of 16 years, only in 2008, the IMD’s first long-period average forecast was closest to its actual value, with the difference between the two values being very less.

However, over the years, the variance between the first long-range forecast and actual rainfall values has reduced, especially after 2010, IMD data suggests.

“There is improvement in the operational forecast system in the recent 10-year period (2007-2016) compared to the later 10-year period (1997-2006) as the absolute error between the forecast and actual values was only 6.24% of the LPA during recent 10-year period compared to 8.68% during the later 10-year period,” said D.S Pai, director of long-range forecast at IMD, Pune.

Last year, all IMD forecasts, except the one for July for all of India, were overestimates when compared to actual rainfall values. The forecasts and the actual rainfall for July were exactly the same. The seasonal rainfall over central India and northeast India were within forecast limits. However, all other seasonal rainfall forecasts, including for northwest India and south Peninsula, for August and the second half of the monsoon season, were incorrect, with actual rainfall values being less than even the lower limits of the respective forecasts.

A State Bank of India’s report released on Tuesday said, “An analysis of monsoon forecast (1st and 2nd stage) since 2008 indicates that IMD has only once increased its 2nd stage forecast (2014), else the forecast remained same or slightly less than the 1st. With a 38% probability of rainfall in the normal category, we believe rainfall is most likely to be higher than 96% of LPA.”

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