It is going to be a late start. Soon after private weather forecaster Skymet set the date of onset of monsoon over mainland India (Kerala) as June 4, India's official forecaster, the Indian Meteorological Department, has now announced that the southwest monsoon is likely to set over Kerala on June 6, with a model error of four days either ways. The IMD has termed the date of advent as "slightly delayed", which clearly means that even with the given standard deviation, the country is expecting a delay in the onset.
The southwest monsoon usually hits the coast of Kerala on June 1, with a standard deviation of seven days. The IMD has added that "conditions are becoming favourable for advance of sourthwest monsoon over the southern part of the Andaman Sea, Nicobar Islands and adjoining southeast Bay of Bengal during May 18 to 19". As per Skymet, the monsoon will hit the far flung archipelago of India on May 22.
The IMD uses an indigenously developed state-of-the-art statistical model with a model error of four days to predict the onset date. The six predictors it uses include minimum temperatures over north west India, pre-monsoon rainfall peak over south peninsula, outgoing long wave radiation over south China sea, lower tropospheric zonal wind over southeast India ocean, upper tropospheric zonal wind over the east equatorial Indian Ocean and outgoing long wave radiation over the south west Pacific region.
A late advent does not necessarily mean a weak monsoon. The monsoon is a dynamic phenomenon with many aspects to it—advent, progress, distribution and strength—each of which is decided by a whole host of factors. Last year, for instance, while there was a deluge in Kerala, several other states, specially in the northeast, had rain deficits.
However, the bad news is that Skymet also announced that monsoon this season would be below normal across the country, with northeast, east and central regions of India likely to be the worst affected. However, IMD has not made any fresh updates. It had, during its annual announcement of the monsoon in April, said the monsoon this year will be "near normal" which means between 96 to 104 per cent of the Long Period Average. It had also said the monsoon would be evenly distributed across the country.
Last year, both IMD and Skymet had initially announced normal monsoon, but Skymet downgraded the prediction midway through the season to below normal. While IMD did not revise its monsoon prediction, the overall monsoon did end up 9.4 per cent deficit, thanks to a 23 per cent deficiency in September. This was not just off the model error of the IMD, but also was clearly in the "below normal" category.
While the forecast may not have been as accurate the regarding monsoon performance, the IMD prides itself in being correct about the onset date, ever since it began making these announcements in 2005. Only in 2015 did it go way off the mark, predicting the arrival on May 30, though the monsoon finally came on June 5.
The pre-monsoon activity so far, too, has been rather disappointing, with an overall seasonal deficit in the country at 21 per cent. The northwest with 37 per cent deficit and the peninsula with a deficit of 39 per cent, have fared the worst, according to IMD.