Rains: IMD proposes; will weather gods dispose? Here’s a data check
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?
Since 2003, the IMD has been issuing its forecasts for the southwest monsoon rainfall average over the country in two stages: in April and end of May. Rainfall between 104% and 110% of the LPA is considered above normal, while 96-104% is normal. The LPA is the average rainfall received during the season over an extended period: currently, 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 don’t have a 100% strike rate, and the IMD’s forecast has an error range 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 improved over the past two decades, though there were some hiccups recently,
The average absolute error in the last five years (2019 to 2023) was 6.8% of LPA, slightly higher than the previous five-year period (20142018), when the average error was 5.9% of LPA.
However, when you take the past two decades into account, the past five years represent a sharp improvement: 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%. 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, the 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 normalmonsoon years since 2001, the IMD got it right five times, under-projected twice and over-projected just once.