Explained: What is the ‘Long Period Average’, IMD’s benchmark for monsoon prediction?

What is the News?

India Meteorological Department(IMD) has released its first Long Range Forecast(LRF) for 2022.

What is the Long Range Forecast(LRF) for 2022?

According to IMD, India is likely to receive a normal monsoon for the fourth consecutive year. This forecast is based on the Long Period Average(LPA) of the 1971-2020 period.

What is the Long Period Average(LPA)?

The IMD predicts a “normal”, “below normal”, or “above normal” monsoon in relation to a benchmark Long Period Average(LPA). 

According to the IMD, the LPA of rainfall is the rainfall recorded over a particular region for a given interval (like month or season) averaged over a long period like 30 years, 50-years etc. 

The IMD maintains LPAs for every meteorological region of the country.

Hence, LPAs act as a benchmark while forecasting the quantitative rainfall for that region for a specific month or season. 

What is the range of a Normal Monsoon?

The IMD maintains five rainfall distribution categories on an all-India scale.These are:

Normal or Near Normal: When the percentage departure of actual rainfall is +/-10% of LPA, that is, between 96-104% of LPA

Below Normal: When the departure of actual rainfall is less than 10% of LPA, that is 90-96% of LPA

Above normal: When actual rainfall is 104-110% of LPA

Deficient: When the departure of actual rainfall is less than 90% of LPA

Excess: When the departure of actual rainfall is more than 110% of LPA

What is the significance of LPA?

Smooth Out Trends of Rainfall: An LPA is needed to smooth out trends so that a reasonably accurate prediction can be made because the IMD records rainfall data at more than 2,400 locations and 3,500 rain-gauge stations.

Covers Large Variations in Either Direction: A 50-year LPA covers large variations in either direction caused by freak years of unusually high or low rainfall (as a result of events such as El Nino or La Nina), as well as for the periodic drought years and the increasingly common extreme weather events caused by climate change.

Source: This post is based on the article “Explained: What is the ‘Long Period Average’, IMD’s benchmark for monsoon prediction?” published in Indian Express on 15th April 2022.