It has been said by Indian Meteorological Department (IMD) that this year there will be normal monsoon but they also commented that it is still premature to forecast, which can be done only towards the end of May or early June. IMD’s forecasts for the last few years have been way off the mark.

Since 1886, the IMD has been forecasting how well or bad the summer monsoon was likely to be over the country as a whole. The primary purposes of the IMD forecasts were therefore, to forewarn the government so that it could be prepared in case of a bad monsoon. Despite all the advances in science, the advent of powerful computers and availability of satellite-based sensors, the mechanisms which cause the year-to-year variations in the monsoon are still poorly understood. Understanding the physics of the vagaries of the Indian monsoon is one of the most challenging problems in atmospheric science today.

In the absence of such knowledge, the IMD has depended on statistical methods to predict the all-India rainfall during the summer monsoon (June to September). Its early forecasts are dependent on just one predictor, the Himalayan-snow cover. More snow in the Himalayas was bad for the monsoon while less snow presaged a good monsoon. But this soon proved unsatisfactory.  In 1907 IMD scientifically established the correlation of various meteorological parameters with the rainfall. The subjectivity of the earlier method was replaced with mathematical multiple regression models to compute the rainfall from these predictors. Since 1988, the IMD has relied on its power regression model for forecasting the monsoon for the country as a whole.

IMD    had    to    periodically update its multiple regression models. The IMD’s power regression model   has performed badly in the last few years. The obvious solution is to predict rainfall for smaller and ' more   homogenous   regions. Since 1999, the IMD has reintroduced   separate   monsoon forecasts for north-west India separately due to failure of appropriate forecast in these regions.  Consequently, the IMD gives its forecasts a greater error margin of plus or minus eight per cent. Moreover, north-west and peninsular India are still quite large and disparate regions, the first forms about 30 per cent of the Indian land area and the latter close to half. So even if the monsoon forecasts are tolerably accurate, the issue could still arise as to whether the aggregated rainfall is truly representative of the region.

Ideally, rainfall recorded in all stations in a region should be strongly correlated with one another.  The outcome of the monsoon issuing predictions only for some regions and not others, however sounds the scientific justification, is not an option open to a national body such as the IMD. Even if such factors limit how much in advance the monsoon can be predicted, shorter range forecasts too can be valuable. Ultimately, improving prediction is going to depend on understanding the complex processes which determine the progress of the monsoon. There are still major gaps in the availability of data needed for this purpose which needs attention.