Context: Recently, the India Meteorological Department (IMD) released its first Long Range Forecast (LFR) stating that the monsoon would be normal this year.
About Long Period Average (LPA)

• The IMD predicts a “normal”, “below normal”, or “above normal” monsoon in relation to a benchmark “long period average” (LPA).
• The 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.
• LPA refers to the average rainfall recorded from June to September for the entire country, the amount of rain that falls every year varies from region to region and from month to month.
• The IMD’s prediction of a normal monsoon is based on the LPA of the 1971-2020 period, during which India received 87 cm of rain for the entire country on average.
• It has in the past calculated the LPA at 88 cm for the 1961-2010 period, and at 89 cm for the period 1951-2000.

Why LPA is needed?

• The IMD records rainfall data at more than 2,400 locations and 3,500 rain-gauge stations.
• Because annual rainfall can vary greatly not just from region to region and from month to month, but also from year to year within a particular region or month.
• An LPA is needed to smooth out trends so that a reasonably accurate prediction can be made.
• A 50-year LPA covers for large variations in either direction caused by freak years of unusually high or low rainfall, as well as for the periodic drought years.
• It also takes into account the increasingly common extreme weather events caused by climate change.

Range of normal rainfall
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 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 departure of actual rainfall is less than 90% of LPA; and
• Excess, when the departure of actual rainfall is more than 110% of LPA.

Reasons for variation in Rainfall

• The decrease in the seasonal rainfall is due to the natural multi-decadal epochal variability of wet and dry epochs of India’s rainfall.
• The monsoon season rainfall shows an epochal behaviour, wherein the monsoon can shift between dry and wet epochs (30-to-50-year periods) in certain decades.
• The other factor is the Indian Ocean Dipole (IOD), which is likely to remain in its neutral phase till June and later turn negative, which will not be good for the monsoon.
• It is defined by the difference in sea surface temperature between two areas (or poles, hence a dipole) a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia.
• The prevailing La Nina conditions will continue till at least the first half of the monsoon season.
• La Nina is the abnormal cooling of the sea surface temperatures observed over the equatorial Pacific region.
• It is known to favour the Indian monsoon.
• The land heating this year has been good and the existing La Nina conditions will favour a good monsoon.

Significance of Rainfall for India 

• With India’s agriculture being primarily rain-fed, the southwest monsoon is vital for kharif crops.
• About 78 percent of the country’s gross cropped area is supported by monsoon rainfall.
• Besides agriculture, the key driver of the country’s economy, India’s reservoirs and sectors like transport, aviation and power are also monsoon-dependent.

About India Meteorological Department (IMD)

• It is an agency of the Ministry of Earth Sciences.
• It is the principal agency responsible for meteorological observations, weather forecasting and seismology.
• It is also one of the six Regional Specialised Meteorological Centres of the World Meteorological Organisation.
• IMD releases the long range forecast in two stages in April and June.