New Delhi, 20 April 2005




Long Range Forecast for 2005

South-West Monsoon Season (June-September) Rainfall



1.          Background


The India Meteorological Department (IMD) has been issuing long range forecasts of the south-west monsoon rainfall for a number of years using models based on statistical correlations between monsoon rainfall and antecedent atmospheric and oceanic parameters. IMD’s operational long range forecasting system has undergone changes in the approach and scope from time to time. From 1988 to 2002, IMD used the 16 parameter power regression and parametric models. In 2003, IMD adopted a two stage long range forecast strategy and introduced 8 and 10 parameter models.  As per the present forecast strategy, the first forecast for the south-west monsoon season (June-September) rainfall is issued in April using the 8 Parameter Power regression and Probabilistic models. The forecast update is issued by end of June using the 10 parameter Power regression and probabilistic models.


 Forecasts for the 2003 south-west monsoon season were accurate. However, in 2004, the realized seasonal rainfall was 87% of the Long Period Average (LPA) as against the forecast of 100% of LPA. The deficient seasonal rainfall in 2004 was due to suppressed rainfall activity in July 2004, which was caused by the unexpected development of warming in the equatorial central Pacific. No Long-range prediction group in India or abroad could correctly predict the suppressed monsoon activity in July 2004.


2.              New initiatives


IMD continued its consistent efforts to further improve the Long range forecast capabilities. Several new statistical models were developed indigenously based on objective methods like ensemble linear regression, artificial neural network and projection pursuit regression.


In addition, under a collaborative research programme with the Indian Institute of Science, Bangalore, a dynamical prediction system was also installed in 2004 at the National Climate Centre, Pune. For this purpose, the seasonal forecast model of National Centers for Environmental Prediction (NCEP), USA was adopted. These statistical and dynamical models however need more testing and review before they are put in for operational use.


In addition, IMD also consulted the experimental forecasts for the 2005 south-west monsoon rainfall prepared and supplied by Indian Institute of Tropical Meteorology, Pune, Indian Institute of Science, Bangalore and Space Applications Centre, Ahmedabad.


3.              El Nino


Near neutral conditions prevailed over the equatorial Pacific region during the first half of 2004. However in July 2004, a sharp rise in positive SST anomalies was observed over the equatorial central Pacific region. During the remaining part of the year, SST anomalies over this region remained large positive. However, from early December 2004, substantial cooling has been observed over this region suggesting a transition to ENSO-neutral conditions.


          A majority of the statistical and coupled model forecasts indicate that a transition from weak warm-episode (El Nino) conditions to ENSO-neutral conditions will continue during the next three months, and that ENSO-neutral conditions are likely to prevail during the monsoon season.


4.              Operational Forecasts for 2005 South-West Monsoon Rainfall


IMD’s operational Long Range Forecast for the 2005 South-west Monsoon season (June-September) is that the rainfall for the country as a whole is likely to be 98% of the Long Period Average (LPA) with a model error of ± 5 %.


The probabilistic model suggests a very high (75%) probability for the 2005 south-west monsoon season rainfall over the country as a whole to be near normal and above. 


IMD will issue a Long Range Forecast update by end of June 2005 using the 10 parameter models. Forecasts for July rainfall over the country as a whole and forecasts for four broad homogenous regions of India also will be issued.