GOVERNMENT OF INDIA

INDIA METEOROLOGICAL DEPARTMENT

LONG RANGE FORECAST

FOR

2003 SOUTH-WEST MONSOON SEASON RAINFALL

IMD has developed new Long Range Forecast models for South-West Monsoon rainfall (June-September) for the country as a whole, which are being introduced this year. These are as follows:

1. A new 8-Parameter Power Regression Model requiring data only up to March, will replace the existing 16-Parameter Power Regression Model which required data up to May. This has enabled IMD to advance the date of issue of the Long Range Forecast from May 25 to April 16.

2. A new 8-Parameter Probabilistic Model requiring data only up to March, will replace the existing 16-Parameter Parametric Model which was used for giving a qualitative forecast. The new model will give the probability of the monsoon rainfall for the country as a whole in 5 different categories:

Drought (less than 90 % of Long Period Average)

Below Normal (90 to 97 %)

Near Normal (98 to 102 %)

Above Normal (103 to 110 %)

Excess (more than 110 %)

3. A new 10-Parameter Power Regression Model requiring data up to June has been developed for purposes of a long-range forecast update to be issued by July 15.

4. Considering the importance of July rainfall to agriculture, a new 8-Parameter Power Regression Model for July rainfall requiring data up to June, has been developed for issuing a forecast for July rainfall along with the forecast update.

5. The existing Power Regression models for long range forecasts of monsoon rainfall over the 3 broad homogeneous regions of the country (NW India, NE India and Peninsula) have been refined and these forecasts will be issued by July 15.

8- Parameter Power Regression and Probability Forecast Models

The Power Regression model uses 8 parameters, which need data up to March. The parameters used in the model are: Arabian Sea Surface Temperature (January + February), Eurasian Snow Cover (December of previous year), Northwest Europe Mean Temperature (January), Nino-3 SST (July + August + September of Previous year), South Indian Ocean SST Index (March), East Asia Pressure (February + March), Europe Pressure Gradient (January) and 50 hPa Wind Pattern (January + February). Data of 38 years (1958-1995) have been used for model development and data for 7 years (1996-2002) have been used for an independent verification of the model performance. The model error of the 8-Parameter model is ±5%.

The Probabilistic Model uses the statistical discriminant analysis technique applied to the same 8 parameters. The model estimates the probability of South-West Monsoon season rainfall for the country as a whole in 5 categories viz., Drought (less than 90% of LPA), Below normal (90 to 97% of LPA), Near normal (98 to 102% of LPA), Above normal (103 to 110% of LPA) and Excess rainfall (more than 110 % of LPA). In a hindcast mode, the model has proved correct in 8 out of 9 drought years.

Long Range Forecast of 2003 South-West Monsoon Season Rainfall

Based upon the newly-adopted 8-Parameter Power Regression Model, IMD’s long range forecast for the 2003 South-West Monsoon Season (June-September) is that the rainfall for the country as a whole is likely to be 96% of the Long Period Average (LPA) with a model error of ± 5%.

For the 2003 South-West Monsoon Season, for the country as a whole, the new 8-Parameter Probabilistic Model indicates:

21% probability of drought (rainfall less than 90% of LPA)

39% probability of below normal rainfall (90 to 97% of LPA)

14% probability of near normal rainfall (98 to 102% of LPA)

23% probability of above normal rainfall (103 to 110% of LPA)

03% probability of excess rainfall (more than 110% of LPA).

IMD will issue a long range forecast update in mid-July 2003.