2nd Stage Long Range Forecast for the 2018 Southwest Monsoon Rainfall

2nd Stage Long Range Forecast for the 2018 Southwest Monsoon Rainfall

HIGHLIGHTS Rainfall over the country as a whole for the 2018 southwest monsoon season (June to September) is most likely to be NORMAL (96% to 104% of long period average (LPA)). Quantitatively, monsoon season (June to September) rainfall for the country as a whole is likely to be 97% of the LPA with a model error of ±4%. Region wise, the season rainfall is likely to be 100% of LPA over North-West India, 99% of LPA over Central India, 95% of LPA over South Peninsula and 93% of LPA over North-East India all with a model error of ± 8 %. The monthly rainfall over the country as whole is likely to be 101% of its LPA during July and 94% of LPA during August both with a model error of ± 9 %.

1. Background

India Meteorological Department (IMD) had issued the first stage operational long range forecasts for the 2018 southwest monsoon season (June-September) rainfall over the country as a whole on 16^thApril. IMD has now prepared the 2^nd Stage Long Range forecast of the seasonal rainfall over the country as a whole, forecasts for the monthly rainfall for July & August over the country as a whole, and forecasts for the seasonal rainfall for the 4 broad geographical regions of India (Northwest India, Northeast India, Central India and South Peninsula). The 2^nd stage forecasts for the southwest monsoon season (June-September) rainfall over the country as a whole was prepared using a 6-parameter Statistical Ensemble Forecasting System (SEFS) and the operational Monsoon Mission Climate Forecast System (MMCFS).

2. Sea Surface Temperature Conditions in the Pacific & Indian Oceans

The moderate La Nina conditions developed in the equatorial Pacific in later part of the last year weakened to weak La Nina conditions early this year and currently have turned to neutral ENSO conditions. The MMCFS & other global climate models indicate conditions over the Pacific likely to continue to be Neutral during most part of the monsoon season and turn to weak El Nino conditions after the monsoon season.

At present, the warm neutral Indian Ocean Dipole (IOD) conditions are prevailing over the Indian Ocean. The MMCFS and other global climate models indicate weak negative IOD conditions are likely to develop during the middle of the monsoon season and continue to persist till the early part of the post-monsoon season.

3. Monsoon Mission Coupled Forecasting System (MMCFS)

The latest experimental forecast based on the MMCFS suggests that the monsoon season rainfall during the 2018 monsoon season (June to September) averaged over the country as a whole is likely to be 102% ± 4% of LPA.

4. The Operational Second Stage Forecasts for the 2018 Southwest Monsoon Rainfall

1. 1. Season (June-September) Rainfall over the country as a whole

Quantitatively, the season rainfall for the country as a whole is likely to be 97% 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.

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

Category	Rainfall Range (% of LPA)	Forecast Probability (%)	Climatological Probability (%)
Deficient	< 90	13	16
Below Normal	90 - 96	28	17
Normal	96 -104	43	33
Above Normal	104 -110	13	16
Excess	> 110	3	17

1. 2. Season (June-September) Rainfall over the Broad Geographical Regions

The season rainfall is likely to be 100% of LPA over North-West India, 99% of LPA over Central India, 95% of LPA over South Peninsula, and 93% of LPA over North-East India all with a model error of ± 8 %.

1. 3. Monthly (July & August) Rainfall over the country as a whole

The monthly rainfall over the country as a whole is likely to be 101% of its LPA during July and 94% of LPA during August both with a model error of ± 9 %.

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