IMD proposes, will weather gods dispose? Here’s a data check.

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For only the second time in the 21st century, India’s weather office has predicted an above-normal monsoon in 2024. The total rainfall between June and September is estimated at 106% of the long-period average (LPA), the India Meteorological Department (IMD) said on Monday. The last time it predicted above-normal monsoon, in 2016, the rainfall ended up being much lower (97.4%). Is the IMD being over-optimistic again this time?

Since 2003, IMD has been issuing its forecasts for the southwest monsoon rainfall averaged over the country in two stages: once in April and an updated one by the end of May. Rainfall between 104% and 110% of the LPA is considered above normal, and it is said to be normal between 96% and 104% of LPA. The LPA is the average rainfall received during the season over an extended period: currently, it refers to the 1971–2020 average.

Over the past 23 years, the IMD has over-predicted rainfall 11 times and under-predicted 12 times, a historical analysis shows. However, statistical forecasts are not expected to achieve a 100% strike rate, and the IMD’s forecast has an error tolerance of 5% on either side. Allowing for that, the IMD has over-predicted nine times, under-predicted seven times, and got it right seven times.

The error rates in India’s official monsoon predictions have sharply improved since the beginning of this century, but inched up in the last few years, IMD data shows. The average absolute error in the last five years (2019 to 2023) has been 6.8% of LPA, slightly higher than the previous five-year period (2014-2018), when the average error was 5.9% of LPA. However, there has been a sharp improvement in the past decade, since the average errors were 7.8% in the five-year period of 2009–2013, and 7.5% during 2004–2008.

The analysis is based on absolute errors, which means it treats an error of both 4% and (-)4% equally.

Ironically, the IMD’s biggest under-prediction of rainfall (outside of the acceptable range) also came in the last five years. This happened thrice between 2019 and 2023, with the biggest miss in 2019, when the actual rainfall was 110.4% of LPA, against a prediction of 96% of LPA. Meanwhile, in 2022 and 2023, the gap between the forecast and the actual rainfall narrowed to (-)7.5% and 1.6% of LPA, respectively.

Of late, the country has been witnessing growing instances of extreme weather conditions.

The forecaster has been fairly successful in predicting normal monsoons but the real challenge lies in predicting droughts and extreme rainfall. Since 2001, the country has had five deficient-rainfall years (2002, 2004, 2009, 2014 and 2015), out of which four ended with droughts, an earlier Mint analysis showed.

In the first three of these years, IMD had predicted a normal rainfall, over-projecting with an average error of nearly 18% of LPA. In 2014 and 2015, the error stood around 7%. Out of the eight normal-monsoon years since 2001, IMD got it right five times, under-projected twice and over-projected just once.

IMD’s expectations of above-normal rainfall in 2024 is expected to cheer farmers across the country. It bodes well for the country’s agricultural output, with 65% of the gross cropped area being rain-fed, which influences both summer and winter crops.

In the past two decades, the GVA (gross value added, or the value of goods and services) of agriculture and allied sectors has seen a healthy expansion in nearly all years of normal or above-normal rains, at an average of 5.2%, shows a Mint analysis.

With below-normal rainfall, the farm output grew meekly below 1% last fiscal while it rose 4.7% when the country witnessed above-normal showers in 2022-23. In all years of deficient rainfall, GVA in these sectors contracted, except on two occasions (2004 and 2015) when it remained flat.

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