Long Period Average- IMD’s benchmark for monsoon prediction

Contact Counsellor

Last Updated16/04/2022

The country is likely to receive a normal monsoon for the fourth consecutive year according to the India Meteorological Department (IMD).
Rainfall in the four-month June-September southwest monsoon season was in the normal range in 2019, 2020, and 2021 as well.*

The IMD predicts a “normal”, “below normal”, or “above normal” monsoon in relation to a benchmark “long period average” (LPA).
It is defined as 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.
The IMD’s prediction of a normal monsoon was based on the LPA of the 1971-2020 period, during which India received 87 cm of rain for the entire country on average.
The IMD has in the past calculated the LPA at 88 cm for the 1961-2010 period, and at 89 cm for 1951-2000.
While this quantitative benchmark 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.
Therefore, along with the countrywide figure, the IMD also maintains LPAs for every meteorological region of the country — this number ranges from around 61 cm for the drier Northwest India to more than 143 cm for the wetter East and Northeast India.
Broken down monthwise for the entire country, the LPA figures for the season are 16.36 cm for June, 28.92 cm for July, 26.13 cm for August, and 17.34 cm for September.

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 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.

Southwest monsoon seasonal (June to September) rainfall over the country as a whole is most likely to be normal (96 to 104% of LPA).
Quantitatively, the monsoon seasonal (June to September) rainfall is likely to be 99% of the LPA with a model error of ± 5%.
The LPA of the season rainfall over the country as a whole for the period 1971-2020 is 87 cm.”

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

Category	Rainfall
Excess	If the rainfall is more than 110 % of this LPA.
Above Normal	If the rainfall is between 104 and 110 % of this LPA.
Normal	If the rainfall is between 96 and 104 % of this LPA.
Below Normal	If the rainfall is between 90 and 96 % of this LPA.
Deficient	If the rainfall is less than 90 % of this LPA.

Established in 1875.
National Meteorological Service of the country and the principal government agency in all matters relating to meteorology and allied subjects.
An agency of the Ministry of Earth Sciences of the Government of India.
Principal agency responsible for meteorological observations, weather forecasting and seismology.
One of the six Regional Specialised Meteorological Centres of the World Meteorological Organisation.
The Regional Specialised Meteorological Centre (RSMC) of Tropical Cyclones in New Delhi is responsible for naming the cyclones in the northern Indian ocean region.
IMD Headquarters: New Delhi
Objective:
- To take meteorological observations and to provide current and forecast meteorological information for optimum operation of weather-sensitive activities like agriculture, irrigation, shipping, aviation, offshore oil explorations, etc.
- To warn against severe weather phenomena like tropical cyclones, norwesters, duststorms, heavy rains and snow, cold and heat waves, etc., which cause destruction of life and property.
- IMD is mandated to provide meteorological statistics required for agriculture, water resource management, industries, oil exploration and other nation-building activities.
- To conduct and promote research in meteorology and allied disciplines.

Prelims Takeaway

Mains Track

Q. Discuss the nature and origin of the Indian monsoon and recent techniques of its prediction.