IMD downgrades monsoon forecast to ‘below normal’

• Monsoon rainfall over the country is likely to be below normal at 90 per cent of long period average (LPA) with a model error of ±4 per cent, the India Meteorological Department (IMD) said in its second stage forecasts on May 29.

• The LPA of the seasonal rainfall over the country, based on data from 1971 to 2020, is 87 cm.

• LPA of rainfall is the rainfall recorded over a particular region for a given interval (like month or season) average over a long period like 30 years, 50 years, etc. It acts as a benchmark while forecasting the quantitative rainfall for that region for a specific month or season.

• In its first forecast for the monsoon season on April 13, the IMD had said that India might witness 92 per cent of LPA rainfall this year.

• According to the latest release issued by IMD, there is a 60 per cent probability of a “deficient” monsoon (below 90 per cent of LPA).

• While the northeast is likely to witness normal rainfall this monsoon season, the remaining parts of the country may see below-normal rainfall.

• If the monsoon season sees less than 90 per cent of LPA rainfall, the IMD classifies it as ‘deficient’.

• While the IMD had earlier said that the monsoon’s onset over Kerala was likely to happen on May 26, the department has now stated that this may take place in the next seven days.

• Typically, Kerala witnesses the onset of southwest monsoon around June 1, which marks the beginning of the southwest monsoon season in the country.

How does the IMD forecast monsoon?

• The India Meteorological Department (IMD) issues operational forecast for the Southwest Monsoon season (June to September) rainfall for the country as a whole in two stages. 

• The first stage forecast is issued in April and the second stage forecast is issued by the end of May.

• Since 2021 IMD has implemented 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 uses both dynamical and statistical forecasting system. Multi-Model Ensemble (MME) forecasting system based on coupled global climate models (CGCMs) from different global climate prediction centres, including IMD’s Monsoon Mission Climate Forecast System (MMCFS) is used in dynamical forecast system. 

• The MME forecast for the 2026 Southwest Monsoon seasonal rainfall was prepared based on the April initial conditions and using a group of coupled climate models that have highest prediction skill over the Indian monsoon region.

There are 5 categories of monsoon rainfall: 

i) Deficient (less than 90 per cent of LPA)

ii) Below normal (90 to 95 per cent of LPA)

iii) Normal (96 to 104 per cent of LPA)

iii) Above normal (105 to 110 per cent of LPA)

iv) Excess (more than 110 per cent of LPA).