INDIA METEOROLOGICAL DEPARTMENT
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 |
