A normal monsoon, really?

The India Meteorological Department (IMD), the government’s principal agency that does weather forecasting and rainfall monitoring, expects the country to receive normal rainfall during the coming south-west monsoon season.

India receives nearly 75 per cent of its rainfall every year during the south-west monsoon season, which spans from June to September. Kharif crops including rice, jowar, bajra, ragi, groundnut and cotton are dependent on the south-west monsoon.

Skymet Weather Services, a private weather forecaster, has also predicted a normal monsoon, but expects the rainfall to be 100 per cent of the Long Period Average (LPA).

The IMD expects the country to receive rainfall of 97 per cent of the LPA with a modal error of +/-5 per cent.



What is normal rainfall?

LPA is the average rainfall received by the country as a whole between 1951 and 2000, during which the average rainfall was 89 cm.

It is normal rainfall if it falls at 96-104 per cent of the LPA. It is said to be deficient if the rain received is less than 90 per cent of the LPA and excess if greater than 110 per cent. If the rainfall is 104-110 per cent of the LPA, it is considered to be above normal. For this year’s south-west monsoon, the IMD has predicted 40 per cent probability of normal rainfall and 14 per cent probability of deficient rainfall.

Sea surface temperature is key to India’s rainfall conditions. According to the IMD, weakened La Nina conditions (cooling of water in the equatorial pacific which is associated with widespread weather changes) and neutral Indian Ocean Dipole, also known as Indian Nino (irregular sea-surface temperature that influences weather patterns), should keep the monsoon normal.

IMD’s forecast

In the past five years, though IMD has forecasted normal rainfall for the country, its prediction missed the mark in all the five years. For instance, in 2016, the expected rainfall was to be 100 per cent of the LPA, but the actual rainfall received was 97 per cent of the LPA. Similarly, in 2013, when the IMD predicted rainfall to be 98 per cent of the LPA, which is normal, the country received above-normal rainfall at 106 per cent of the LPA, due to neutral El Nino/La Nina conditions.

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