Rainfall over the country as a whole during the second half (August to September period) South-West monsoon season is most likely to be normal with a tendency to be on the positive side of the normal.

Monthly rainfall for August is most likely to be normal — 94 to 106 percent of Long Period Average (LPA), Indian Meteorological Department (IMD) Director General Mrutyunjay Mahapatra said.

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He was addressing a virtual press conference to declare southwest monsoon rainfall forecast for the second half of the season and for August.

The LPA of the August to September period rainfall over the country as a whole for 1961-2010 is 428.3 mm. The IMD said this year, it expects the rainfall to be 95 to 105 percent of the LPA for August to September.

The spatial distribution suggests that below normal to normal rainfall is likely over many parts of the northwest, east and northeast parts of the country and normal to above normal rainfall is most likely to be experienced over most parts of peninsular India and adjacent central India, he said.

Explaining the details about the probabilistic forecast for August, the LPA of the August rainfall over the country as a whole for the period 1961-2010 is 258.1 mm. The spatial distribution of probabilistic forecasts for tercile categories (above normal, normal, and below normal) for the August rainfall suggests that below normal to normal rainfall is likely over many areas of central India and some areas over northwest India. Normal to above-normal rainfall is most likely over most parts of peninsular India and the Northeast.

July month witnessed very heavy rainfall (115.6 to 204.5 mm) events at 567 stations of IMD across India while 112 stations witnessed extremely heavy rainfall events (more than 204.5 mm). In July 2020, 90 stations witnessed extremely heavy rainfall while 447 witnessed very heavy rainfall, IMD data showed.

The latest global model forecasts indicate that the prevailing neutral ENSO conditions (El Nino Southern Oscillations, global ocean-atmospheric phenomena that control the periodicity of monsoon) are likely to continue over the equatorial Pacific Ocean. However, sea surface temperatures over central and east equatorial the Pacific Ocean are showing cooling tendency and there is an increased possibility of re-emergence of the La Nina condition at the end of the monsoon season or thereafter. Prevailing negative IOD conditions over the Indian Ocean are likely to continue during the remaining part of the monsoon season.

“As the changes in the sea surface temperature (SST) conditions over the Pacific and the Indian Oceans are known to influence the Indian monsoon, IMD is carefully monitoring the evolution of sea surface conditions over these Ocean basins,” Mahapatra said.

This year, the IMD has adopted a new strategy for issuing monthly and seasonal operational forecasts for the southwest monsoon rainfall over the country by modifying the existing two-stage forecasting strategy.

The new strategy is based on the existing statistical forecasting system and the newly developed Multi-Model Ensemble (MME) based forecasting system. The MME approach uses the coupled global climate models (CGCMs) from different global climate prediction and research centers, including IMD’s Monsoon Mission Climate Forecast System (MMCFS) model.

Accordingly, IMD had issued the first stage forecast for the 2021 southwest monsoon seasonal (June to September) rainfall over the country on April 16 and the first update for the forecast on June 1. IMD had also issued the monthly forecast outlook for June rainfall over the country along with the updated forecast on June 1 and that for the July rainfall on July 1.

IMD will issue the forecast for September month rainfall towards the end of August or the beginning of September 2021.