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All you wanted to know about...Long Period Average

Bavadharini KS | Updated on April 24, 2018 Published on April 23, 2018

India Meteorological Department (IMD), the government’s principal agency for weather forecasting and rainfall monitoring, brought smiles and cheers to farmers, government officials and consumers last week. It forecast that the country would receive normal rainfall during the coming south-west monsoon season (June to September 2018), with a low probability of deficient rainfall. IMD brands the monsoon as ‘normal’ or ‘deficient’ based on how it fares against its benchmark Long Period Average (LPA).

What is it?

LPA is the average rainfall received by the country as a whole during the south-west monsoon, for a 50-year period. The current LPA is 89 cm, based on the average rainfall over years 1951 and 2000. This acts as a benchmark against which the rainfall in any monsoon season is measured. The country is said to have received deficient rainfall if the actual rainfall falls below 90 per cent of LPA. Similarly, the country is said to have received excess rainfall if the rainfall is greater than 110 per cent of LPA. It is deemed ‘normal’ when the actual rainfall received falls between 96 and 104 per cent of LPA.

In 2018, the IMD expects the rainfall to be at 97 per cent of LPA. The IMD also budgets for a ‘model error’ of plus or minus 5 per cent from its forecasts.

IMD’s rainfall data is based on actual rainfall recorded in 2,412 locations, across its 3,500 rain-gauge stations. Based on daily rainfall data received in these stations, monsoon statistics are prepared for the administrative zones such as districts, States and for the whole country. The statistics is compiled for the 36 meteorological subdivisions and for the four broad regions — South peninsula, North West India, Central India and North and North-East India — before being aggregated for the whole country.

Why is it important?

The LPA uses a 50-year average because annual rainfall can be highly variable, thanks to the whimsical rain gods. A 50-year average is expected to smooth out the day-to-day, month-to-month variations, while also accounting for freak weather events like the El Nino and La Nina.

Once in every three or four years, Indian monsoons have witnessed aberrations such as severe drought, flooding and storms owing to El Nino — the abnormal warming of waters in the Pacific Ocean. You can also know if IMD has got its prediction right with the help of LPA. For instance, IMD forecast normal rainfall for the year 2013 but the actual rainfall received was 106 per cent of LPA, which is above normal rainfall.

But it is important to understand that a ‘normal’ monsoon doesn’t automatically guarantee farm prosperity or rural spending. The actual distribution of those 89 cm of rainfall over India’s key growing regions and over the critical sowing months (called spatial and temporal distribution), plays a key role too.

Why should I care?

Whether you’re a city dweller grappling with a drinking water problem or a farmer deciding what and when to sow, the south-west monsoon plays quite a big role in your life. Understanding what’s a ‘normal’ monsoon by IMD’s norms and what’s a ‘deficient’ one, can help you prepare for unexpected contingencies.

It is IMD’s forecast that prompts the Central and State governments to make their pre-monsoon preparation to deal with flooded drains, pot-holed roads or drought-afflicted farms. Similarly, the Centre’s flood alerts and storm warnings are based on expected rainfall in relation to the LPA. Understanding the annual deviations from LPA helps shed light on the extent to which climate change and seasonal aberrations such as El Nino are impacting India’s rainfall patterns.

The bottomline

There are many grey areas to a normal monsoon forecast.

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Published on April 23, 2018
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