Long Range Forecast Update for the 2016 Southwest Monsoon Rainfall

The India Meteorological Department (IMD), Ministry of Earth Sciences, issues the operational long range forecasts for the southwest monsoon season (June-September) rainfall over the country as a whole in two stages i.e. in April and in June. The update for the forecast for the seasonal rainfall over the country as a whole, issued in April, was released today. The Forecast predicts rainfall over the country as a whole, for the 2016 southwest monsoon season (June to September), is most likely to be Above Normal (>104% to 110% of long period average (LPA)).

The forecasts for the monthly rainfall for July & August over the country as a whole and for the season rainfall for the 4 broad geographical regions of India (NW India, NE India, Central India and South Peninsula) have been issued using a 6-parameter Ensemble Forecasting System.

The 6 predictors used are: North East Pacific to North West Atlantic SST Anomaly Gradient (December + January), Southeast Equatorial Indian Ocean Sea Surface Temperature (February), East Asia Mean Sea Level Pressure (February + March), Central Pacific (Nino 3.4) Sea Surface Temperature (March to May + tendency between March to May & December to February), North Atlantic Mean Sea Level Pressure (May) and North-Central Pacific 850 zonal wind gradient (May).

Recent changes in the atmospheric conditions over the Pacific reflect the weakening El Ni conditions. Latest forecast from IMD-IITM (Indian Institute of Tropical Meteorology) coupled model indicate ENSO (El Ni - Southern Oscillations) neutral conditions are likely to continue and turn to weak La Nina conditions in the latter part of the monsoon season. The rapidly declining El Nino conditions became moderate in early April 2016, weak in early May and now have turned to neutral ENSO conditions.

Over Indian Ocean, the sea surface temperatures are warmer than normal over most parts except along the coast off central and south Africa. The latest forecast from IMD-IITM coupled model indicates positive Indian Ocean Dipole (IOD) conditions are most likely during early part of the monsoon season and same to turn to negative IOD during the latter part of the monsoon season.

The Monsoon Mission Experimental forecast, based on the ESSO-IMD-IITM coupled dynamical model, suggests that the monsoon rainfall during the 2016 monsoon season (June to September) averaged over the country as a whole is likely to be 112% 5% of long period model average (LPMA). The experimental five category probability forecasts for the 2016 monsoon season rainfall over the country as a whole using the experimental dynamical prediction system are 0% (deficient), 0% (below normal), 18% (normal), 18% (above normal) and 64% (excess).

The details of the Second Stage Forecasts for 2016 Southwest Monsoon Rainfall are as follows:

Quantitatively, the season rainfall for the country as a whole is likely to be 106% of the long period average (LPA) with a model error of 4%. The LPA rainfall over the country as a whole for the period 1951-2000 is 89 cm. Thus there is no change in the updated quantitative forecast from the first stage operational forecast issued on 12th April, 2016.

The 5 category probability forecasts for the Season (June to September) rainfall over the country as a whole is given below:

Category Rainfall Range
Forecast Probability (%) Climatological
Deficient < 90 0 16Below Normal 90 - 96 4 17Normal 96 -104 33 33Above Normal 104 -110 40 16Excess > 110 23 17

The season rainfall over Broad Geographical Regions is likely to be 108% of LPA over North-West India, 113% of LPA over Central India, 113% of LPA over South Peninsula, and 94% of LPA over North-East India all with a model error of 8 %.

The rainfall over the country as a whole is likely to be 107% of its LPA during July and 104% of LPA during August both with a model error of 9 %.

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