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Explained: First monsoon forecast this season

The Indian Meteorological Department will announce on Wednesday its first forecast for the rainfall the country is likely to receive from June to September.

Written by Amitabh Sinha |
April 22, 2015 1:15:12 am
Indian Meteorological Department, Met Department, first monsoon forecast, monsoon forecast, monsoon season India gets about 70 to 75 per cent of its annual rain during the four-month long monsoon.

The Indian Meteorological Department will announce on Wednesday its first forecast for the rainfall the country is likely to receive in the four-month monsoon from June to September. AMITABH SINHA explains its significance.

Why is the forecast important?

India gets about 70 to 75 per cent of its annual rain during the four-month long monsoon. With less than half of India’s cultivable land having irrigation facilities, these rains are critical for agricultural productivity, which in turn impacts overall economic growth. This forecast, which comes about a month and a half before the onset of monsoon, only gives a prediction for the overall rainfall the country as a whole is likely to receive in the four months. It does not give regional or monthly distribution of rainfall. Those will be given in later forecasts, in June and then in an update in July. Still, this first forecast is considered very important for the purposes of planning and making preparations for contingency.

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Why is the regional and monthly forecast not made in April?

Because it is too early to give the forecasts with a reasonable degree of accuracy.

How accurate are these forecasts?

They have been fairly on target in the past few years, within the error margin of a few percentage points, except in 2009, when the rainfall was 23 per cent below normal. Forecasts made in June and July tend to be more accurate than the one in April.

How is the forecast made?

There are several factors known to influence Indian monsoon. The April forecast is made using five predictors – Sea Surface Temperature (SST) between North Atlantic and North Pacific oceans, SST in equatorial South Indian Ocean, mean sea-level pressure around East Asia, land and surface air temperature in northwest Europe, and equatorial Pacific warm water volume. The last predictor, in case the warming takes place, is known as El Nino effect.

There are reports of an El Nino developing in the equatorial Pacific ocean this year. Does that mean this will be a bad monsoon?

As mentioned above, there are several factors that affect the monsoon. El Nino is just one of them. The US weather agency, NOAA, has predicted that there is a 70 per cent chance of El Nino developing this year. There is a strong correlation between an El Nino and a bad monsoon, but it is not one-on-one correlation. There have been instances of good monsoon in an El Nino year, an example being 1997. Similarly, 2014 was a drought year but there was no El Nino.

So what will be announced on Wednesday?

The IMD will say what percentage of ‘normal’ rainfall the country is likely to receive in the four-month period. The normal is taken to be the long-period average (LPA) of annual rainfall between 1951 and 2000. This average comes to 89 cm of rainfall. If the rainfall is more than 110 per cent of this LPA, it is categorised as excess. Between 104 and 110 per cent of rainfall is called above normal. Rainfall between 96 and 104 per cent is considered to be normal. 90 to 96 per cent is below normal, and less than that is categorised as deficient. The IMD prediction on Wednesday will say what percentage of the LPA the monsoon rainfall is likely to be. It will also release the respective probabilities of the rainfall falling in each of these brackets, i.e the probability of rainfall being deficient or excess, or normal.

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