The forecast for the South-West monsoon rainfall is issued in two stages.
First stage forecast: Seasonal (June to September) rainfall over India, as a whole is issued in April.
Second stage forecast: Update of April forecast for the country as a whole is issued in June The update forecast is also more specific and gives the monsoon seasonal rainfall over four broad geographical regions of India as well as July rainfall for the entire country.
From 2007, a new statistical forecasting system is used, which is based on the ensemble technique involving 8 predictors as given in the Table For the April forecast, the first 5 predictors given in this table are used. For the updated forecast in June, 6 predictors that include 3 previous predictors (first 3 predictors in this table) are used.
In the ensemble method, all possible models based on all the combination of predictors are considered. Thus, for April (June) forecast, with 5 (6) predictors, 31 (63) different models were developed. Out of all the possible models, ensemble mean of the best few models were selected based on their skill in predicting monsoon rainfall during a common period. The weights are proportional to the multiple correlation coefficients of the models during the training period. For developing the models, two different statistical techniques namely, Multiple Regression (MR) and Projection Pursuit Regression (PPR) were considered. The model error of the April forecast system is 5% and for the June forecast system, it is 4% of the Long Period Average (LPA) rainfall. (Details of the method used in the forecasting system is given here)
For forecasting of South-West monsoon season rainfall over the four broad geographical regions of India (NW India, Central India, South Peninsula and NE India), multiple regression (MR) models based on separate set of predictors are used. All the four multiple linear regression models have model errors of 8% of LPA.
IMD has also implemented a dynamical prediction system - Seasonal Forecast Model of the Experimental Climate Prediction Center (ECPC). 20 year model climatology (1985-2004) was prepared by prescribing observed sea surface temperature (SST) data as boundary conditions. The validation of model hindcast suggests promising skill (Details here) over the Indian region. At present, this model is used for preparing experimental long range forecasts for the southwest monsoon rainfall over India.
National Climate Centre also prepares an extended range forecast for the onset of southwest monsoon rainfall over Kerala. This forecast was first issued in 2005. The forecast is issued based on indigenously developed statistical model with 6 predictors. (Details here)
In addition, IMD prepares operational long range forecasts for the Winter Precipitation (Jan to March) over Northwest India and Northeast Monsoon rainfall (October to December) over South Peninsula. For this purpose, separate statistical models have been developed.