NEW DELHI: The
India Meteorological Department had forecast a normal
monsoon this year, pegging the countrywide rainfall at 97% of the long period average. The season ended on Sunday at a rain deficit of 9.4%, making the national weather agency’s forecast off by more than the 4% margin of error.
While IMD’s monsoon forecasts have in recent years been getting more accurate, this year the department has not just been more than 6% off target, it’s categorisation of “normal” seasonal rainfall too proved inaccurate. The monsoon ended in the “below normal” zone, close to the deficient category (below 10%).
“It was the high monsoon shortfall in September that led to the inaccuracy. We were expecting 90-91% rainfall in September after taking into consideration an evolving El Nino. But high convection activity in northwest Pacific during the month drew moisture away from our region,” said D Sivananda Pai, IMD’s lead monsoon forecaster.
September ended with a shortfall of more than 23% and was the driest monsoon for the month since 2015.
In its first two forecasts (in April and May), IMD had said the monsoon was likely to be 97% of LPA. In an update for the second half of the season (August-September) released on August 3, IMD had forecast 95% rainfall for the two months. The actual rainfall during the period was close to 87%.
In contrast, private forecaster Skymet, which had initially predicted 100% monsoon rainfall this year, made a course correction midway into the season. In an update released on August 1, Skymet downgraded the monsoon to “below normal” at 92% of the long period average — a forecast that came close to the actual figure.
Skymet had, however, forecast poor rainfall in August (88%) and a slightly better performance in September (93%). The opposite turned out to be true, with August rainfall being slightly less than 93% of LPA.
IMD’s regional forecasts proved to be more accurate than its nationwide prediction. The department had forecast good rainfall over northwest India at 100% of LPA. The region saw its best monsoon in five years which ended at 98% of LPA. Likewise, its forecasts for central India (99% versus actual of 94%) and south peninsula (95% versus 99%) were well within the 8% error margin. East and northeast India, however, performed below expectations with just 75% rainfall during the season as against a forecast of 93%.
In the event, fairly good rainfall distribution, despite the overall shortfall, kept the rain deficiency down to 12 subdivisions of the country out of 36 (33% of total). The deficiency was higher if one looked at the districts with 38% of districts for which IMD had data (252 out of 659) showing deficient or large deficient rainfall.
