IMD Forecast For Monsoon rainfall (June-September 1998)
(Issued on the 25 May 1998)

Around this time, the India Meteorological Department(IMD) issues long range seasonal forecast for the ensuing southwest monsoon rainfall spanning the four month period (June to September) over the country as a whole.IMD has developed models like Parametric, Power regression, Multiple regression, and Dynamic Stochastic Transfer models for issuing long-range seasonal forecasts.

1. SUMMARY OF LONG RANGE FORECAST FOR 1998 MONSOON

The long range forecast for the 1998 southwest Monsoon formulated on the basis of Parametric and Power Regression Model is given below:

a) In 1998, the rainfall over the country as a whole for the entire southwest monsoon season(June to September) is likely to be normal, thus making the year 1998 the eleventh normal monsoon year in succession. The normal being defined as rainfall with + or - 10% of its long period average.

b) Quantitatively, the rainfall for the country as a whole for the entire southwest monsoon season (june to september) is likely to be 99% of its long period average within an estimated error of + or - 4%.

2. DETAILS OF FORECAST MODELS

2.1 Parametric Model

This qualitative forecast model utilizes the signals from 16 antecedent global and regional land-ocean-atmospheric parameters.Analyses of signals from these parameters indicates that this year 56% of them are favourable for a normal monsoon.The seasonal rainfall for the country as a whole is thus expected to be normal ( defined as + or - 10% of the long period average value).

2.2 Power Regression Model

This quantitative forecast model is based on the physical relationship of monsoon rainfall with 16 different individual parameters. According to this model the total amount of rainfall over the country as a whole during the monsoon season from June to September 1998 is likely to be 99% of its long period average within an estimated model error of + or - 4%.

2.3 Multiple Regression Model

This quantitative model is based on statistical correlations between monsoon rainfall and certain global and regional meteorological parameters. The model utilizes seven predictors and the forecast based on this model indicates that the monsoon rainfall during June to September 1998 is likely to be 106% of its long period value.

2.4 Dynamic Stochastic Transfer Model

This model considers the atmosphere as a dynamic system. It utilizes regional parameters as inputs and yields monsoon rainfall over the country as a whole as output. Computations from the model indicate that monsoon rainfall over the country as a whole during June to september 1998 is likely to be 93% of its long period average.

2.5 Experimental Models

Recently IMD has developed a couple of new long-range forecast models for monsoon rainfall over India as a whole. The performance of these models indicates that three of the models have reasonable accuracy. Details are given below:

a) Power transfer model:
The model has been developed based on Dynamic Stochastic Transfer and Power Regression techniques. The model uses 12 regional and global parameters as inputs and indicates that the monsoon rainfall during 1998 over the country as a whole is likely to be 97% of its long period average.

b) Principal Component Regression Model
The model has been based on the Empirical Orthogonal Function (EOF) analysis technique and makes use of 8 regional and global parameters. The forecast from this model indicates that the monsoon rainfall during 1998 over the country as a whole is likely to be 103% of its long period average.

c) Hybrid Artificial Neural Network Model
The model has been based on the Empirical Orthogonal Function (EOF) analysis technique and makes use of 8 regional and global parameters. The forecast from this model indicates that the monsoon rainfall during 1998 over the country as a whole is likely to be 105% of its long period average.

3. PERFORMANCE OF THE FORECAST MODELS

When tested with past data, these long-range forecast models have shown varying accuracy.The parametric and power regression models have proved to be most accurate over the past ten years and thus these alone are being used since 1988 for issuing the official forecast of IMD.

This information was produced by the Indian Meteorological Department and more details can be found by contacting directly:

The Director General
India Meteorological Department
Mausam Bhavan
Lodi Road
New Delhi 110 003
India