The Long Period Average (LPA) is a meteorological concept used by the India Meteorological Department (IMD) to establish a benchmark for the country's monsoon 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 from June to September, averaged over a long period, typically 50 years. The LPA was created to smooth out the large year-to-year variations in rainfall caused by phenomena like El Niño or La Niña, allowing for a reasonably accurate prediction of the upcoming monsoon season.
The mechanism works by calculating the mean rainfall from thousands of rain-gauge stations across the country over the specified long period. The IMD uses this LPA value as a quantitative benchmark to categorize the monsoon's performance each year. For instance, a monsoon is considered Normal or Near Normal if the actual rainfall is between 96% and 104% of the LPA. Rainfall below 90% of the LPA is classified as Deficient, while rainfall above 110% of the LPA is categorized as Excess.
The LPA connects directly to the IMD's Long Range Forecast (LRF), which is crucial for India's economy, as the monsoon delivers nearly 70% of the country's annual rainfall and is vital for the agricultural sector. The LPA is updated periodically, once every decade, to incorporate the latest data and reflect the changing climate. The all-India southwest monsoon LPA was recently updated from 88 cm (based on the 1961-2010 period) to 87 cm (based on the 1971-2020 period), reflecting a slight reduction in the average rainfall. This revision means the benchmark for a "normal" monsoon has been lowered, while the core mechanism of using a 50-year average to define rainfall categories remains the same.