A day after private forecaster Skymet Weather announced the onset date for the South-West Monsoon over Kerala as June 4, the India Meteorological Department (IMD) has predicted a two-day delay beyond that date.

IMD said in a special bulletin on Wednesday that the monsoon may enter Kerala ‘slightly delayed’ on June 6 with a model error of +/- four days. It sees a ‘near normal’ season this year, with its second long-range forecast update due in early June.

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Conditions are becoming favourable for the advance of the monsoon over the southern part of the Andaman Sea, Nicobar, and adjoining South-East Bay of Bengal by the weekend (May 18/19), it said. 

In contrast, Skymet had announced that the onset over the Andamans, the monsoon’s first port of call in the Indian territorial waters, would materialise by May 22. Both agencies seem to agree that the onset of seasonal rains in the extreme South-East Bay would happen within or just around the normal window, which is May 15 to 20. 

BusinessLine  had already reported the possibility of the onsets over both the Bay Islands and over the mainland in Kerala materialising on dates now suggested officially by the IMD. 

Forecast comparison

There is not much to differentiate between the IMD and Skymet Weather with respect to the element of accuracy since both have their own assessments of both the forecast and the actual dates (see table). A comparison of the last five years suggests that, except on two occasions (2017 and 2018), they have mostly agreed to disagree with the choice of the respective dates. 

The IMD said its operational forecasts of the date of monsoon onset during the past 14 years (2005-2018) have proved to be correct except in 2015. 

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