The Long Period Average (LPA) is a meteorological concept used by the India Meteorological Department (IMD), an agency under the Ministry of Earth Sciences, as a benchmark for forecasting quantitative rainfall. It is defined as the average rainfall recorded over a particular region for a given interval, such as the four-month Southwest Monsoon season (June to September), averaged over a long period, typically 50 years.
The LPA was established to solve the problem of high year-to-year variability in rainfall, allowing for a reasonably accurate prediction by smoothing out trends like unusually high or low rainfall years. The IMD uses the LPA to issue its Long Range Forecasts (LRF).
The mechanism involves comparing the actual seasonal rainfall to the LPA, which is expressed as a percentage. The IMD maintains five rainfall distribution categories on an all-India scale: 'Normal or Near Normal' rainfall is defined as a percentage departure of +/-10% of LPA, which is between 96% and 104% of LPA. Other categories include 'Below normal' (90-96% of LPA), 'Above normal' (104-110% of LPA), 'Deficient' (less than 90% of LPA), and 'Excess' (more than 110% of LPA).
The LPA is intrinsically connected to the Southwest Monsoon, which is vital for India's agriculture and economy. Factors like the El Niño and Indian Ocean Dipole (IOD) are concepts that influence the actual rainfall compared to the LPA.
The LPA is updated periodically, typically once every decade. The base period has changed recently: the LPA for the 1961-2010 period was 88 cm, but the most recent revision, based on the 1971-2020 period, has set the new all-India Southwest Monsoon LPA at 87 cm. This revision reflects a slight reduction in the average rainfall value.