New Delhi, 30
Long Range Forecast Update
For 2007 South-West Monsoon
Since 2003, India Meteorological Department (IMD) has been following
a two-stage forecast strategy for the southwest monsoon rainfall over
the country as a whole, in which the first forecast is issued in April
with the data up to March and the forecast update is issued by end of
June with the data up to May. This year, IMD introduced the following
new statistical forecast models for operational long range forecast of
southwest monsoon rainfall over the country as a whole.
a)A new 5- parameter statistical forecasting system requiring data up
to March, for the first forecast in April.
b)A new 6- parameter statistical forecasting system requiring data up
to May for the forecast update in June.
The most important aspect of the new forecast system is the introduction
of the concept of ensemble forecasts. For developing the models, two
different statistical techniques namely, Multiple Regression (MR) and
Projection Pursuit Regression (PPR) were considered.
Using the new 5-parameter statistical model, on19 April, IMD issued the
following forecast for the 2007 southwest monsoon rainfall over the country
as a whole.
"IMD's long range forecast for the 2007 south-west monsoon season (June to
September) rainfall is that the rainfall over the country as a whole is
likely to be 95% of the long period average with a model error of ± 5%."
2.Second Stage Forecasts
Now, with the availability of data up to May, IMD has prepared the second
stage forecasts, which are being released now:
a)Forecast update for the 2007 southwest monsoon rainfall over the country
as a whole using the new 6-parameter ensemble statistical forecast system.
The forecasting system has an error of ± 4%. The Long period average of
southwest monsoon rainfall averaged over the country as a whole is 89 cm
(based on the 1941-1990 data) with a coefficient of variation of 10%.
b)Forecast for the rainfall over the country as a whole in the month of July
based on a 6-parameter model, which has a model error of ± 9%. The Long
period average of monsoon rainfall in the month of July is 293 mm (based on
the 1941-1990 data) with a coefficient of variation of 13%.
c)Forecasts for the South-west Monsoon Season(June-September) rainfall for
the following four broad geographical regions of India 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.
The long period average rainfall and coefficient of variation of rainfall of
the 4 broad geographical regions of India are given below:
||Long period average (mm)
||Coefficient of variation (%)
As a part of ongoing efforts to improve the long range forecast capabilities,
experimental forecasts for the 2007 south-west monsoon rainfall based on the
IMD's dynamical forecast system (Seasonal Forecast model of the Experimental
Climate Prediction Centre (ECPC), USA) were also generated. Forecasts have
been generated using observed sea surface temperature data of May and using
the persistence method.
In addition, IMD has also taken into account the experimental forecasts
prepared by national institutes like Indian Institute of Tropical
Meteorology, Pune, National Centre for Medium Range Weather Forecast
(NCMRWF), Noida, Indian Institute of Science, Bangalore, Space Applications
Centre, Ahmedabad, National Aerospace Laboratory, Bangalore and Centre for
Mathematical Modelling and Computer Simulation (CMMACS) Bangalore 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 Tokyo Climate Centre and the Experimental
Climate Prediction Center (ECPC), USA.
4.Onset and Advance of Monsoon 2007
Southwest monsoon advanced over south Andaman Sea on 11 May, almost
10 days earlier than the normal date. IMD predicted that the monsoon
onset over Kerala this year would take place on 24th May with a model
error of ±3 days. Monsoon advanced over Kerala on 28th May, 4 days earlier
than the normal date of 1st June. By 29 May, monsoon covered the entire
coastal Karnataka and some parts of south interior Karnataka and south Tamil
Nadu. After this, there was hiatus in the advancement of monsoon for about
one week. The hiatus was caused by the formation of the Super Cyclone
"Gonu" over Arabian Sea, and its northwestward movement towards the
On 8 June, monsoon advanced over some more parts of south interior
Karnataka, Tamil Nadu, the entire Nagaland-Manipur-Mizoram-Tripura,
some parts of Assam & Meghalaya and Arunachal Pradhesh. By 10 June,
it covered most parts of Northeast India and advanced over some parts
of Bihar. Between 13 to 18 June, monsoon covered Konkan and Goa,
Andhra Pradesh, Orissa, West Bengal, most parts of Orissa and some parts
of Maharashtra, Orissa, Chattisgargh and Jharkhand. On 26 June, southwest
monsoon has further advanced into some parts of east Uttar Pradesh, remaining
parts of Uttarakhand, entire Himachal Pradesh, Jammu and Kashmir and
northern parts of west Uttar Pradesh, Haryana and Punjab. On 28 June,
Southwest monsoon has further advanced into the remaining parts of Madhya
Pradesh & Uttar Pradesh, most parts of East Rajasthan, some parts of west
Rajasthan and some more parts of Haryana (including Delhi) and Punjab.
As on 28 June, the northern limit of monsoon passes through Barmer,
Jodhpur, Ajmer, Jaipur, Narnaul, Rohtak, Ludhiana and Amritsar.
The accumulated seasonal rainfall over the country as a whole during the
period 1-27 June was 107 % of long period average.
5. La Nina Conditions over the equatorial Pacific
The warm sea surface temperature (SST) anomalies over the east equatorial
Pacific associated with the moderate El Nino of 2006 have disappeared during
February 2007. By the end of February 2007, SSTs were near average in the
vicinity of the date line, and below average over the eastern equatorial
Pacific. ENSO-neutral conditions continued in the tropical Pacific during
May and June 2007. Equatorial SSTs during the last four weeks were more than
1ºC below average in areas between 120ºW and the South American coast, and
more than 0.5oC above average in portions of the western Pacific. For the
week ending 13 June, the SST anomalies were +0.2ºC and +0.4ºC in the Niño
3.4 and Niño 4 regions respectively.
Most statistical models show ENSO-neutral conditions persisting through
August 2007, while most dynamical models indicate La Niña will develop within
the next three months. However, for the past few months the dynamical models
have been predicting a stronger and more rapid cooling than has actually
6. Summary of the forecasts
i) South-West Monsoon Season Rainfall
IMD's Long Range Forecast update for the 2007 South-West Monsoon Season
(June-September) is that for the country as a whole the seasonal rainfall
is likely to be 93% of the Long Period Average (LPA) with a model error
of ± 4%.
ii) July rainfall
Rainfall over the country as a whole in the month of July 2007
is likely to be 95% of its LPA with a model error of ± 9 %.
iii) Rainfall over broad geographical regions
Over the four broad geographical regions of the country,
rainfall for the 2007 South-West Monsoon Season is likely to be
90% of its LPA over North-West India, 98% of its LPA over North-East
India, 96% of its LPA over Central India and 94% of its LPA over
South Peninsula, all with a model error of ± 8 %.