“Sudden re-emergence of La Niña over the equatorial Pacific was the reason”

The India Meteorological Department (IMD) has admitted that most of its long-range forecasts issued during the just-concluded South-West Monsoon season, went wrong.

Most of these forecasts had underestimated the actual rainfall situation and, therefore, were not very accurate, an end-of-the-season report released on Monday said.

The IMD has issued 12 long-range forecasts during the four-month season: one forecast for the onset of the season, two season-wise forecasts, three month-wise forecasts and individual forecasts for the four broad geographical regions in the country and a separate forecast for the second half of the season — the August-September period.

The onset forecast was right: the IMD had forecast that the monsoon would set in on May 31 with a model error of plus or minus four days. The actual onset was on May 29 and within the forecast limit.

Of the two season-wise forecasts, the first, issued on April 19, turned out to be within the limit, while the second, an update issued on June 21, proved wrong. On April 19, the forecast was that the country as a whole and for the season would receive 98 per cent of the normal rainfall with a model error of plus or minus five per cent.

This was brought down to 95 per cent with a model error of plus or minus four per cent in the June 21 update. The actual rainfall, in contrast, was 101 per cent of the normal.

Of the month-wise forecasts — for July, August and September — only the July forecast proved right.

For July, the IMD had forecast a rainfall of 93 per cent plus or minus nine per cent and the actual rainfall was 85 per cent. For August, the forecast was 94 per cent plus or minus nine per cent, but the actual was 110 per cent. For September, the forecast was 90 per cent plus or minus 15 per cent whereas the actual was 106 per cent.

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