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Given the vagaries of the weather, forecasting the advent of the South West monsoon is always a tricky business, but the India Meterological Department (IMD) got the date wrong this year even with the error range of plus or minus four days in its model.
The IMD had forecast May 30 as the date for the onset of the monsoon over Kerala; but that happened only on June 5.
Some weeks ago the IMD said it had been able to get the date right within the plus or minus 4 day range for the the years between 2005-2014.
If the error range cushion is taken out of the reckoning, what we see is that it came very close to getting the date exactly right in two years - 2014 and 2010, when the forecast was within a day's range. And there have been three years in which it fell within a two day range. Otherwise, it has been in the three or four day range.
The IMD had said in a release that past data does not suggest any 'one to one association' between the date of the advance of the monsoon over the Andaman Sea with the date of monsoon onset over Kerala or the seasonal monsoon rainfall.
The advance of the South West monsoon over the Andaman Sea usually occurs around May 20, with a standard deviation of about one week. Conditions had been favourable for the onset of monsoon over Kerala, it had said in mid-May.
The 'indigenously developed' statistical model that the IMD uses for the forecast is based on six predictors: minimum temperature over north-west India, pre-monsoon rainfall peak over the south peninsula, outgoing long wave radiation over south China Sea, lower tropospheric zonal wind over south east Indian ocean, upper tropospheric zonal wind over east equatorial Indian Ocean and outgoing long wave radiation over South West Pacific region.
Finally on June 5, the IMD declared that the monsoon had set over Kerala and was advancing further. If that is so, the monsoon seems to have made a low-key, barely noticeable entry to the country sans the usual show of thunder and lightning associated with its coming.
