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Understanding Monsoon Forecast 2017 by IMD

(Before reading this report do read about meanings of terms used in LRF report for better understanding.)

IMD’s Long Range Forecast (LRF) for April 2017 in brief:

India Meteorological Department the country’s national weather forecaster predicted normal monsoon rainfall for this year on 18th April. Further IMD said that monsoon rainfall likely to be 96 % of LPA (Long Period Average). As per IMD normal means  between 96 % and 104%. So this year monsoon rainfall will be 96 % means normal at lower side. At the same time department told that there is 38 % chances of becoming 96 % of LPA.

So let’s understand what is the meaning of the above LRF:

1) Rainfall amount suggested in LRF 2017:

The LPA is 889 mms which is average of all sub divisions in India. So 96% of 889 mms means 852 mm as a whole.

2) What does 38% of 96% of LPA means?

Surprisingly like last year, IMD this time haven’t mentioned the table of probability for five categories. Due to which LRF doesn’t provide clarity about what IMD has to say.

However it can be interpreted as give below.

IMD gives probability forecasts in five category, such as:

1) Deficient, 2) Below Normal 3) Normal 4) Above Normal & 5) Excess

IMD mentions that 38% chances of becoming 96 % of LPA. It means that as per model analysis 38% is highest probability assigned to normal conditions out of the five categories mentioned above.


About Further Updates on Monsoon:

The distribution of rainfall is also likely to be good said IMD chief K.J.Ramesh. According to IMD the probability of El Nino developing around July – August 2017 has come down to 40 % which was earlier 55 % in IMD bulletin.

IMD is carefully monitoring the development in two ocean, namely – Pacific and Indian ocean. It expects more information on El Nino and Indian Ocean Dipole (IOD) will be available in time for forecast update in June 2017. IMD will release its prediction of monsoon onset over Kerala in 3rd week of May. The normal date of onset of monsoon over Kerala is 1st June.

Following data to judge the prediction of IMD.


IMD predicted in % Actual r/f in % El Nino/La Nina IOD


100 98 Mod La Nina



96 77 Mod El Nino



98 102 Mod La Nina



95 101 Weak La Nina



99 93 neutral



98 106 neutral



96 88 neutral



93 86 V strong El Nino

Strong negative

2016 106 97 Weak La Nina

Weak positive

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