New Delhi, 24th June 2009
Long Range Forecast
Update for the
2009 Southwest Monsoon
Rainfall
1.
Background
India Meteorological Department (IMD) issues operational
long range forecasts for the southwest monsoon rainfall in two stages. First
stage forecast is issued in April and the second stage forecast is issued in
June. This year, the first stage forecast for the southwest monsoon rainfall
over the country was issued on 17th April, 2009. IMD has now
prepared the second stage forecasts.
2. First Stage Forecast issued on 17th
April, 2009
“IMD’s long range forecast for the 2009 southwest 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.”
3. Second Stage Forecasts
The following forecasts are being released
now:
a) Forecast update for the southwest
monsoon season (June-September) rainfall over the country as a whole using a
6-parameter ensemble statistical model with a model error of ± 4%.
b) Forecast
for the monthly rainfall over the country as a whole for the months of July
& August using separate principle component regression models with a model
error of ± 9%.
c) Forecasts
for the southwest monsoon season (June-September) rainfall for the
following four broad geographical regions of India using separate multiple
linear regression models with a model error of ± 8%:
Northwest India – Jammu and Kashmir, Himachal Pradesh, Punjab, Rajasthan,
Haryana, Chandigarh, Delhi, Uttaranchal and Uttar Pradesh.
Northeast India – Arunachal Pradesh, Meghalaya, Assam, Nagaland, Manipur,
Mizoram, Tripura, Sikkim, West Bengal, Bihar and Jharkhand.
Central India – Gujarat State, Madhya Pradesh, Chattisgarh, Maharashtra,
Goa and Orissa.
South Peninsula – Andhra Pradesh, Karnataka, Tamil Nadu, Kerala, Lakshadweep and
Andaman and Nicobar Islands.
4. 6-Parameter Ensemble Forecasting System
The 6 predictors used in the ensemble forecasting for the
update forecast for the southwest monsoon season (June-September) rainfall over
the country as a whole are: North Atlantic Sea Surface Temperature (December +
January), Equatorial SE Indian Ocean Sea Surface Temperature (February +
March), East Asia Mean Sea Level Pressure (February + March), Central Pacific
(Nino 3.4) Sea Surface Temperature Tendency (MAM-DJF), North Atlantic Mean Sea
Level Pressure (May) and North Central Pacific Zonal Wind at 850hPa (May).
The model error of the 6-parameter ensemble forecasting system is ±4%.
5.
Experimental Forecasts
IMD has also generated experimental forecast for
the 2009 southwest monsoon rainfall based on the IMD’s dynamical forecast
system (Seasonal Forecast model of the Experimental Climate Prediction Centre
(ECPC), USA). The forecast was generated using observed global sea surface
temperature data of May.
In addition, IMD has
taken into account the experimental forecasts prepared by the national
institutes like Indian Institute of Tropical Meteorology (IITM), Pune, Indian
Institute of Science (IISc), Bangalore, Space Applications Centre (SAC),
Ahmedabad, National Aerospace Laboratories (NAL), Bangalore, Centre for
Mathematical Modelling and Computer Simulation (CMMACS), Bangalore, National Centre for Medium Range Weather
Forecasting (NCMRWF), Noida and Centre for Development of Advanced
Computing (C-DAC), Pune and operational/experimental
forecasts prepared by international institutes like World Meteorological
Organization (WMO)’s Lead Centre for Long Range Forecasting - Multi-Model
Ensemble (LRFMME), 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 Asian-Pacific Economic Cooperation
(APEC) Climate Centre, Korea.
In general, the
experimental forecasts based on both statistical and dynamical models suggest
below normal to normal monsoon season rainfall over the country as a whole.
6.
Conditions over the
equatorial Pacific and Indian Oceans
The recent La
Nina event that started in early December 2008 has ended. Since the middle of April,
2009, ENSO neutral conditions are prevailing with positive SST anomalies
observed over the equatorial Pacific from the beginning of May. The latest
observations and forecasts from both dynamical and statistical models suggest
high probability (about 60%) for El Nino conditions to appear during the
monsoon season. The probability for ENSO neutral conditions is about 40% and that
for La Nina is negligible.
It is
important to note that other factors such as the Indian Ocean Sea surface
temperatures also influence the monsoon rainfall over India in addition to El
Niño and La Niña events. Forecasts from few climate models suggest possibility
of the development of a weak positive Indian Ocean Dipole event during the 2009
monsoon season, which may not have much impact on the Indian monsoon. However, IMD is carefully monitoring the possible evolution of El
Nino conditions over Pacific and the Indian Ocean Dipole.
7.
Summary of the Update Forecasts
for 2009 Southwest Monsoon Rainfall
i) South-West Monsoon
Season Rainfall
IMD’s long range forecast update
for the 2009 south-west monsoon season (June to September) is that the rainfall
is likely to be below normal. Quantitatively, monsoon season rainfall for the
country as a whole is likely to be 93% of the long period average with a model
error of ±4%. The Long period average rainfall over the country as a whole for
the period 1941-1990 is 89 cm.
ii) Monthly (July
& August) Rainfall
Rainfall
over the country as a whole in the month of July 2009 is likely to be 93% of
its LPA and that in the month of August is likely to be 101% of LPA both with a
model error of ± 9 %.
iii) Rainfall over Broad
Geographical Regions
Over the four broad geographical regions of the
country, rainfall for the 2009 South-West Monsoon Season is likely to be 81% of
its LPA over North-West India, 92% of its LPA over North-East India, 99% of its
LPA over Central India and 93% of its LPA over South Peninsula, all with a
model error of ± 8 %.
The long period average and coefficient of
variation of rainfall based on the 1941-1990 data for all India and 4 broad
geographical regions are given below along with the forecasts:
|
Area |
Long period Average (mm) |
Coefficient of variation (%) |
Forecast (% of LPA) |
|
All India (June to September) |
890 |
10 |
93 |
|
All India (July) |
293 |
13 |
93 |
|
All India (August) |
262 |
14 |
101 |
|
NW India |
612 |
19 |
81 |
|
Central India |
994 |
14 |
99 |
|
NE India |
1429 |
8 |
92 |
|
South Peninsula |
725 |
15 |
93 |