Will it rain or not?

YOGINDER K ALAGH | Updated on January 20, 2018 Published on April 17, 2016


The IMD hedges its predictions this time

The Indian Meteorological Department and Skymet have come out with optimistic monsoon forecasts. Yet, after two droughts policymakers cannot be too careful. In fact, irresponsible talk can have serious consequences in a largely private sector-dominated, open trade regime, globalised at the margin.

Skymet would be discounted in trading decisions, as since it was wrong only last year.

Quite reliable

Not so, the IMD. I admire Indian statisticians and meteorologists. They are competent, have a good reputation globally, and do difficult jobs with politicians and senior bureaucrats always sniping at them. When I was their minister I considered it a privilege to defend them in Parliament.

On April 13, the IMD issued a sober forecast talking of another poor year. I was to meet a channel representative at 4 pm, so I was tracking the releases on a minute-by-minute basis. The only silver lining I was thinking of commenting on, was the forecast that the worst region hit would be the North-West, with kharif rains less than 90 per cent of the long-term average.

Since the region is largely irrigated, apart from pockets like south Punjab and parts of Haryana, it could cope with a small yield loss, I felt. But this release was withdrawn by the IMD.

The interesting fact is that the final IMD forecast for the first time includes a probability number to the numerical forecast. For example the kharif forecasts in 2015 do not have such a number.

Forecasting system

The model used is the Experimental Coupled Dynamical Model Forecasting System. This year the main forecast is that kharif rainfall will be 106 per cent of the long period average; instead of 89 cm it will be 94 cm.

This number has a range of error of plus/minus 5 per cent. Suppose you have to forecast a number from a model. Depending on the regularities and variation seen in the past there will be a probability associated with each forecast. This is the accuracy estimate for your forecast. Now kharif rainfall will be either 94 cm, or it will not be so. The new probability number will give you an idea of the accuracy you can expect on your decision.

The director-general of IMD, LS Rathore, said this probability number is 94 per cent. In other words, there is a 94 per cent probability that the kharif rainfall will be 94 cm. Normally on such numbers much higher accuracy can be attempted. Since such a figure was not there in earlier IMD forecasts for kharif, it is extremely likely that much higher precision was aimed at. This is important.

The probability of the forecast at 99 per cent would mean that certainty is much greater. Numbers with adverse consequences will become ‘acceptable’ as the probability of their being correct declines. The 94 per cent probability to the decision is a new concept. The least the IMD could have done is tell us what this number was in their earlier predictions.

Anyway, I remain an admirer of professional statisticians and meteorologists.

The writer is the chancellor of Central University of Gujarat and was a Union minister

Published on April 17, 2016
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