The Hindu : South-west monsoon prediction proves correct

Online edition of India's National Newspaper
Sunday, Sep 28, 2003

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National

South-west monsoon prediction proves correct

By N. Gopal Raj

THIRUVANANTHAPURAM SEPT. 27. As the south-west monsoon draws to a close, the Indian Meteorological Department (IMD) has the satisfaction of its long-range forecast turning out correct.

The monsoon season is defined as extending from June 1 to September 30.

After last year's debacle when the IMD failed to predict the drought, the department overhauled its long-range forecasting techniques.

This year, using an eight-parameter power regression model, the IMD predicted in April that the monsoon would be 96 per cent of the long-term average (about 88 cm), with an error margin of plus or minus five per cent.

In July, after two more parameters became available, the IMD updated its forecast and said the monsoon would be 98 per cent of the long-term average, with an error margin of four per cent.

The country had received two per cent more than the long-term average as of September 17.

But, with decreasing rain as the monsoon retreats, the rainfall for the entire season was not likely to be more than one per cent above the long-term average, said one IMD expert.

Two models had forecast last year's drought. One was the global circulation model of the National Centres for Environmental Prediction (NCEP) in the United States. As early as March last year, the NCEP computer simulations began showing a drought over India.

The other was an empirical model, using sea surface temperatures, developed by scientists at the Indian Institute of Tropical Meteorology (IITM) in Pune.

This year, the NCEP model predictions issued in April and May were suggesting deficient rain during the monsoon, though nothing like last year's drought.

But from June onwards, the NCEP model began indicating normal rain.

The IITM model using sea surface temperatures, on the other hand, predicted 13 per cent excess rain during the monsoon.

This year, the IMD had also introduced a probabilistic forecast.

In its April forecast, the probability of near normal rain, which is how the current monsoon would be categorised, had been given only a 14 per cent chance.

On the other hand, below normal rain was given a 39 per cent probability and another drought a 21 per cent chance.

But in its July update, the IMD correctly predicted that near normal rain was the most probable outcome.

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