The Long Period Average (LPA) of the south-west monsoon season rainfall over the country as a whole for the period 1951-2000 is 89cm. The first long range forecast for the south-west monsoon season (June-September) rainfall is issued in April and the forecast update is issued in June.

From 2007 onwards, IMD has been using the following statistical models for preparing quantitative and probabilistic forecasts of the south-west monsoon rainfall (June – September) for the country as a whole:

• A 5- parameter statistical ensemble forecasting system requiring data up to March, for the first forecast in April
• A 6- parameter statistical ensemble forecasting system requiring data up to May for the forecast update in June. Three of these 6-parameters are same as that used for April forecast.

For preparing the first stage forecast for the 2011 south-west monsoon rainfall for the country as a whole presented here, the 5-parameter statistical ensemble forecasting system has been used.  

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