IMD’s Monsoon forecasts: Rarely spot on!

Updated: May 21, 2013, 13:35 PM IST

Team ZRG

When it comes to predicting Monsoon, the India Meteorolgical Department (IMD) rarely gets it right even after factoring in the error-margin.

This year the IMD has predicted that the Monsoon is likely to hit Kerala coast on June 3. But can the forecast be taken on its face value? No, if the preliminary forecast performance of IMD over the years is any indication!

About a decade long study of IMD preliminary forecast data versus actual rainfall shows the trust deficit. The data shows huge deviation between projected (preliminary forecast in April) and actual rainfall received during the year of prediction. The IMD forecast during the decade was cent percent correct in 2010. Earlier, in 2005 and 2008, the prediction was quite close to the actual rainfall.

The IMD offers a preliminary forecast around end April every year and follows up with a revised version sometime in June. The initial prediction is generally seen as the performance benchmark.

In 2012 the Monsoon hit Kerala four days after the June 1 deadline. In its preliminary forecast for the upcoming June–September rainy season of 2013, the Met department has predicted a normal Monsoon by stating that rainfall would be 98 per cent of Long Period Average (LPA). In Monsoon terminology, any rainfall figure between 96 per cent and 104 per cent of the average seasonal rainfall is categorised as a normal Monsoon.

In its first-stage forecast for the June- September rainfall season of 2012, IMD predicted a normal Monsoon by stating that rainfall would be 99 per cent of LPA. Unfortunately, 2012 turned out to be rainfall deficient year as the actual rainfall recorded for the season was 92 per cent of LPA.

The pattern was similar in earlier years` forecast as well (2002, 2004, and 2009). While IMD predicted normal Monsoon for these years, the country actually reeled under severe drought. In 2009, IMD predicted a normal Monsoon by stating that rainfall would be 96 per cent of LPA.

However by the end of 2009, India witnessed its worst drought as actual rainfall recorded for the season was 77 per cent of LPA. The deviation is as huge as 19 per cent. Moreover, the deficiency gap in 2002 and 2004 was around 20 and 13 per cent respectively. Isn’t it a big question mark over the IMD’s ability to forecast?

The IMD preliminary forecast in 2007 actually turned out to be completely out of sync with the actual rainfall. It then predicted that rainfall would be 95 per cent of LPA. The reverse happened. The actual rainfall turned out to be better than normal at 105 per cent of LPA.

The IMD management, however, defends its forecast performance. SB Gaonkar, Scientist at IMD Pune argues, “There is always a variation between the actual and predicted amount of rainfall. Rainfall varies from place to place, time to time. Cross-equatorial flows, Synoptic systems, drastic lull in rainfall activity are some reasons which lead to variation in rainfall figure.”

As regards Monsoon this season, the detailed and more confirmatory forecast including seasonal and spatial pattern of rainfall would be available only in June 2013. This would be followed by forecast outlook for the rainfall during second half (August-September) of the Monsoon season to be issued in July and that for the September rainfall to be issued in August.