India Meteorological Department (IMD) issues various monthly and seasonal operational forecasts for rainfall during the Southwest Monsoon season. Operational forecasts for the Southwest Monsoon season (June – September) rainfall are issued in two stages. The first stage forecast is issued in April and the second stage forecast is issued in June.
1. The forecast for 2017 southwest monsoon rainfall over the country as a whole based on both the SEFS and the Monsoon
Mission CFS (MMCFS) are presented here. IMD’s SEFS model for the April forecast uses the following 5 predictors that require data monitored upto March.
2. Sea Surface Temperature (SST) Conditions in the equatorial Pacific & Indian Ocean
The weak La Nina conditions developed in the later part of the last monsoon season peaked in December 2016 and started weakening thereafter. Currently, neutral conditions are prevailing over the equatorial Pacific. The atmospheric conditions over the Pacific also reflect neutral El Nino conditions. The latest forecast from MMCFS indicates weak El Nino conditions to develop during the latter part of the monsoon season. However, there is no one to one relationship between El Nino and Indian Monsoon. For example, during 34% of El Nino years,monsoon season rainfall was normal or above normal.
At present, neutral Indian Ocean Dipole (IOD) conditions are prevailing over the Indian Ocean. The latest forecast from the MMCFS indicates weak positive IOD conditions are likely to develop during the middle of the monsoon season and to persist for some more months subsequently. Positive IOD conditions are likely to be favorable for a normal/above normal monsoon.
As the extreme sea surface temperature conditions over the Pacific particularly El Nino conditions over the Pacific (El Nino or La Nina) and positive IOD development over the equatorial Indian Ocean are known to have strong influence on the Indian summer monsoon, IMD is carefully monitoring the sea surface conditions over the Pacific and Indian oceans.
3. Forecast For the 2017 Southwest monsoon Season (June – Season) rainfall over the Country as a whole
3a. Monsoon Mission Coupled Forecasting System (MMCFS) based Assessment
For generating the forecast for the 2017 southwest Monsoon season rainfall, atmospheric and oceanic initial conditions during March 2017 were used. The forecast was computed as the average of the 44 ensemble members. The forecast based on the MMCFS suggests that the monsoon rainfall during the 2017 monsoon season (June to September) averaged over the country as a whole is likely to be 96% ± 5%of the Long Period Average (LPA).
3b. Forecast Based on the Operational Statistical Ensemble Forecasting System (SEFS)
Quantitatively, the monsoon seasonal rainfall is likely to be 96% of the Long Period Average (LPA) with a model error of ± 5%. Further, forecasts for the seasonal rainfall for the country suggest 38% probability for near normal rainfall scenario.
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4. Summary of the Forecast for the 2017 Southwest Monsoon Rainfall
1. Quantitatively, the monsoon seasonal rainfall is likely to be 96% of the Long Period Average (LPA) with an error of ± 5%.
2. Forecast assessment suggests 38% of probability for near normal monsoon rainfall.
IMD will issue the update forecasts in early June, 2017as a part of the second stage long range forecast of monsoon rainfall. Along with the update forecast, separate forecasts for the monthly (July and August) rainfall over the country as a whole and seasonal (June-September) rainfall over various geographical regions of India will also be issued. By that time, more information on the evolution of El Nino and IOD will be available.
According to Skymet Weather, Monsoon 2017 is likely to be below normal at 95% (with an error margin of +/-5%) of the long period average (LPA) of 887 mm for the four-month period from June to September.
