India Meteorological Department’s long
range forecast for the 2009 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 96% 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.
Following the 2007/08 La Niña event, near neutral ENSO
conditions had prevailed in the tropical Pacific from early June to November 2008.
However, La Nina conditions returned in early December 2008. From early
March, 2009 the SSTs over central and eastern Pacific have weakened and as on
now, weak La Nina conditions are prevailing. The latest forecasts from both dynamical
and statistical models indicate equal probabilities for weak La Nina and ENSO
neutral conditions till May 2009. For the
subsequent monsoon season (June to September), majority of the models predict
ENSO neutral conditions. It may be mentioned that as the lead time of the forecast
increases there is considerable spread and uncertainty in the ENSO forecasts.
IMD follows a two-stage forecast strategy for long range
forecasting 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.
During the last two years (2007 & 2008), IMD has
been using the following statistical models for forecasting 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.
The operational forecast issued by IMD for the 2008
seasonal rainfall over the country as a whole was accurate as the first and second
stage forecasts were 99% and 100% of Long Period Average (LPA) respectively and
the actual rainfall was 98% of LPA.
For preparing the first stage forecast for the 2009
South-west monsoon rainfall for the country as a whole presented here, the same
5-parameter statistical ensemble forecasting system as that used for 2008 has
Statistical Forecast System
In the IMD’s Ensemble Statistical Forecasting system
for April forecast, the following 5 predictors are used. The model error of the
April forecasting systems is ±5%.
North Atlantic Sea Surface
December + January
Equatorial South Indian Ocean
Sea Surface Temperature
February + March
East Asia Mean Sea Level Pressure
February + March
NW Europe Land Surface Air
Equatorial Pacific Warm Water
February + March
As a part of ongoing efforts to improve the long range
forecast capabilities, experimental forecast for the 2009 south-west monsoon rainfall
based on IMD’s dynamical forecast system was also generated. For this purpose,
observed sea surface temperature data of March have been used.
addition, IMD has 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, Centre for Mathematical Modelling and Computer Simulation (CMMACS),
Bangalore and 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 , the Experimental Climate Prediction Center
(ECPC), USA, and World Meteorological Organization (WMO)’s Lead Centre for Long
Range Forecasting - Multi-Model Ensemble (LRFMME).
IMD will update the above
forecast in June 2009 as a part of the second stage forecasts. Separate forecasts
for the monthly (July and August) rainfall over the country as a whole and seasonal
(June-September) rainfall over the four geographical regions of India will also
(Release ID :48526)