INDIA METEOROLOGICAL DEPARTMENT

Long Range Forecast

For 2008 South-west Monsoon Season Rainfall

1.      Background

 India Meteorological Department (IMD) follows a two-stage forecast strategy for long range forecasts of the south-west monsoon rainfall over the country as a whole. 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.

Last year, IMD introduced the following new statistical models for forecasting south-west monsoon rainfall (June – September) for the country as a whole:

a)               A 5- parameter statistical ensemble forecasting system requiring data up to March, for the first forecast in April.

b)               A 6- parameter statistical ensemble forecasting system requiring data up to May for the forecast update in June.

The same forecasting system is used for preparing the forecast for the 2008 South-west monsoon rainfall. 

 

2. Operational Statistical Forecast System

 

In the IMD’s Ensemble Statistical Forecasting system, the following 8 predictors are used, out of which the first 5 predictors are used for the April forecast.

 The model errors of the April and June forecasting systems are ±5% and ±4% respectively. 

        

3. Experimental Forecasts

 As a part of ongoing efforts to improve the long range forecast capabilities, experimental forecast for the 2008 south-west monsoon rainfall based on the IMD’s dynamical forecast system was also generated. For this purpose, observed sea surface temperature data of March have been used.

 In addition, IMD has also taken into account the experimental forecasts prepared by the national institutes like Indian Institute of Tropical Meteorology, Pune, Indian Institute of Science, Bangalore, Space Applications Centre, Ahmedabad, National Aerospace Laboratories (NAL), Bangalore and Centre for Mathematical Modelling and Computer Simulation (CMMACS), Bangalore, National Centre for Medium Range Weather Forecasting (NCMRWF), Noida and operational/experimental forecasts prepared by international institutes like the National Centers for Environmental Prediction (NCEP), USA, International Research Institute for Climate and Society (IRI), USA, Meteorological Office, UK, the European Center for Medium Range Weather Forecasts(ECMWF), UK and the Experimental Climate Prediction Center (ECPC), USA.

 

4. La Nina Conditions over the equatorial Pacific

 

During August 2007, La Nina conditions were developed over the equatorial Pacific with colder than normal Sea Surface Temperatures (SST). However, during the recent weeks, negative SST anomalies weakened across the central and east-central equatorial Pacific. During March 2008, La Nina conditions declined to moderate-strength. The recent dynamical and statistical SST forecasts indicate La Nina will become weak and persist during the next three months. Thereafter, there is considerable spread and uncertainty in the forecasts of La Nina conditions.

 

5. Forecast for the 2008 South-west monsoon rainfall

 

IMD’s long range forecast for the 2008 south-west monsoon season (June to September) is that the rainfall for the country as a whole is likely to be Near Normal.

 

 Quantitatively, monsoon season rainfall is likely to be 99% of the long period average with a model error of ± 5%. The Long period average rainfall over the country as a whole for the period 1941-1990 is 89 cm.

 

IMD will update the above forecast in June 2008 as a part of the second stage forecasts. Separate forecasts for the July rainfall over the country as a whole and seasonal (June-September) rainfall over the four geographical regions of India will also be issued.

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