IMD’s inaccurate Monsoon forecast
The India Meteorological Department (IMD) has erred on its forecast for monsoon rain in August. In June, it said that India would get more rain than it usually did but as of August 31, the country got 8.5 % less rain than what’s normal for the month.
In June, the agency’s updated forecast said India would receive 6% more than the 89 cm it normally gets between June and September and that August rains would be 4% more than the 26 cm that the country usually gets. An error margin of 9% is built into IMD’s forecasts of monthly rainfall but even so, an 8% deficit lies well outside the margin of error.

It suggests that the agency’s weather models are still not robust enough to capture changes in global climate that could affect the Indian monsoon.

Anticipated shift of plan 
The IMD is planning to shift next year to a forecast system that relies on a supercomputer-led dynamical weather-modelling. Moreover, unless September gets 150% more rain than normal, it is unlikely that India will meet its forecast target of 6% surplus rains for the monsoon season. So far the all-India monsoon deficit as of August 31 was 3%.

Reason behind wong prediction
Forecast models — even global ones — expected a “stronger” La Nina, the converse of an El Nino and usually associated with good rains over India, to set in around August. So far the La Nina has been extremely weak.

On the back of successive droughts in 2014 and 2015 — because of one of the strongest El Ninos on record — weather agencies had said 2016 would be a good year because El Nino had receded and was likely to give way to La Nina.

Different Models to predict monsoons:

Statistical model:
It tries to match prevailing conditions with historical records to see how the monsoon had behaved in years when similar conditions had prevailed.

Dynamical model:
This model makes continuous observation of some selected physical phenomena, and notes how the conditions for monsoon behave over a period of time. It then follows those changes to predict for the future, and comes up with a forecast.