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Updated: April 20, 2011 02:11 IST

Southwest monsoon rainfall "most likely" to be normal

P. Sunderarajan
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The India Meteorological Department (IMD) on Tuesday forecast that rainfall in the coming southwest monsoon season is “most likely” to be normal, at about 98 per cent of the Long Period Average (LPA), with a model error of plus or minus 5 per cent.

According to an IMD note, there was a 53 per cent probability of the rainfall to be between 96 per cent and 104 per cent of the LPA [which is considered normal], 30 per cent for it to be between 90 per cent and 96 per cent of the LPA [below normal], 10 per cent for it to be between 104 per cent and 110 per cent of the LPA [above normal], 6 per cent for it to be below 90 per cent of the LPA [deficient] and 1 per cent for it to be above 110 per cent of the LPA [excess].

The forecast is, however, not final. The IMD will update it in June after taking into account parameters, for which data would be available only by then. This is an annual routine.

Among other things, a majority of international models monitoring El Nino-La Nina have indicated that sea surface temperatures over equatorial Pacific are warming up. Consequently La Nina conditions, which could, otherwise, have been beneficial for monsoon, are weakening and are expected to reach the El-Nino Southern Oscillation condition by June. But long term forecasts for El Nino-La Nina are fraught with large uncertainties.

The El Nino-La Nina phenomena may unfold in a different way from the present forecast.

IMD Director general Ajit Tyagi said the department would keep a close watch on the phenomena to ensure that the country was well prepared in case of any major changes.

The IMD would also closely monitor the developments relating to the Indian Ocean Dipole (IOD), which relates to anomalies in the sea surface temperatures between eastern and western parts of the Indian Ocean.

At present, international models are forecasting that a negative IOD could develop in the last part of the monsoon. Consequently, as of now, there does not seem to be any possibility of a major impact.

It, however, remains to be seen how it will actually develop.

If it were to develop earlier, there could be a negative impact on the quantum of rainfall.

Dr. Tyagi declined to comment on when the monsoon was expected to set in, saying it was too early to forecast that.

He, however, ruled out a suggestion that the onset may be delayed as the weather over North and Central India continues to be cool.

“It may be a popular concept. But, studies have not shown any correlation between the onset, or for that matter the monsoon as a whole and temperatures in North and Central in April.”

Asked about the report of a recent meeting of the South Asian Climate Outlook Forum, which has indicated below normal monsoon over the northwestern and northeastern parts of the region, he said it was possible some pockets in the northeast and the western most parts of Punjab, Haryana and Rajasthan adjoining Pakistan experience deficiency. But the impact would not be much on the overall rainfall in the country.

Keywords: monsoon rains

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