New Delhi, 2 August 2012
Government of India
Ministry of Earth Sciences
India Meteorological Department
Long Range Forecast Outlook for the Rainfall
During the Second Half (August –September) of
2012 Southwest Monsoon
India Meteorological Department (IMD) issues operational long range forecasts for the southwest
monsoon Season (June to September) rainfall over the country as a whole in two stages; in April and in
June. Along with update forecast for the season rainfall over the country as a whole issued in June, forecast
for monthly rainfall (for July and August) over the country as a whole and season (June to September)
rainfall over the four geographical regions (Northwest India, Central India, Northeast India and South
Peninsula) are also issued. From 2009, IMD started to issue forecast outlook for the rainfall during second
half (August-September) of the monsoon season. This forecast is issued in the end of July. IMD has now
prepared a forecast outlook for the second half of the 2012 monsoon season. For this, a new Principal
Component Regression (PCR) Model based on 5 predictors is used
2. First & Second Stage Forecasts issued respectively in April and June, 2012
This year, the first stage forecast was issued on 26th April and Second stage forecast was issued on
22nd June. Summary of IMD’s long range forecasts for the 2012 south-west monsoon season (June to
September) issued in two stages are given in the table below. In the last column, the actual rainfall received
upto 31 July is also given.
Forecast Actual Rainfall
(% of LPA) Received so far
(1 June- 31 July)
1st Stage 2nd Stage
% of LPA
All India June to September 99 ± 5 96 ± 4 81
Northwest India June to September - 93 ± 8 64
Central India June to September - 96 ± 8 85
Northeast India June to September - 99 ± 8 90
South Peninsula June to September - 95 ± 8 77
All India July - 98 ± 9 87
All India August - 96 ± 9 -
3. Details of the 5- Parameter Operational Model used for Preparing the Forecast
During the second half of the monsoon season (August to September) the long period average
(LPA) of the rainfall over the country as a whole is 43.5cm (49% of the average season rainfall) based on the
1951-2000 climatology. The coefficient of variation (C.V) of the rainfall time series is 15%. For the forecast of
August - September rainfall over the country as a whole, a new Principal Component Regression (PCR)
Model based on 5 predictors has been used. The model error of 5-parameter PCR model is 8% of LPA.
The 5-parameter PCR model was also used to prepare probability forecasts for the pre-defined 3
(tercile) categories of rainfall during the second half of the monsoon season. These are below normal (<94%
of LPA), near normal (94-106% of LPA), above normal (>106% of LPA). The tercile categories have equal
climatological probabilities (33.33% each). The forecasted probabilities for the rainfall during the second half
of 2012 southwest monsoon season over the country as a whole in percentage for the above tercile
categories are 62%, 33%, and 5% respectively.
4. SST conditions in Pacific and Indian Oceans
The weak to moderate La Nina conditions prevailed over the equatorial east Pacific during later part
of 2011 continued till first half of February 2012, after which it started weakening and dissipated in the early
April 2012. As of now, ENSO neutral conditions are prevailing. But El Nino conditions are building up in the
equatorial Pacific with sea surface temperature (SST) anomalies of ≥ 0.5oC observed over much of the
equatorial east Pacific during the recent two weeks. The latest forecasts from a majority of the dynamical
and statistical models indicate strong possibility (with a probability of about 65%) for weak to moderate El
Nino conditions to emerge during the next two months. Therefore, the El Nino conditions are likely to have
adverse impact on the rainfall over the country during the second half of the monsoon season.
Over Indian Ocean, the latest forecasts from some climate models indicate possibility for the
development of a weak negative Indian Ocean Dipole (IOD) event during the next few months. Therefore,
IOD may not have much influence on the monsoon.
5. Summary of the Forecast outlook for the Rainfall During the Second Half of the 2012
Southwest Monsoon Rainfall
(a) The rainfall during August is likely to be normal (96 ± 9% of LPA) as was forecasted in June.
(b) Rainfall over the country as a whole for the second half (August to September) of the 2012
southwest monsoon season is likely to be below normal (<94% of long period average (LPA)).
(c) Quantitatively, rainfall for the country as a whole during the period August to September, 2012 is
likely to be 91% of LPA with a model error of ±8%.
(d) Based on the rainfall distribution over the country till date and outlook for the second half of the
season, the seasonal rainfall of the entire southwest monsoon season (June to September) is
likely to be deficient (< 90% of LPA).