“The southwest monsoon onset is likely to set over Kerala on 6th June with a model error of ± 4 days.”
The normal onset date for monsoon over Kerala was June 1, which also marks the start of the rainy season over the region and relief from scorching summer temperatures. The IMD has been issuing operational forecasts for the date of monsoon onset over Kerala from 2005 onwards. An indigenously developed state of the art statistical model with a model error of ± 4 days is used for the purpose.
The 6 Predictors used in the models are; i) Minimum Temperatures over North-west India, ii) Pre-monsoon rainfall peak over south Peninsula, iii) Outgoing Longwave Radiation (OLR) over South China Sea, (iv) Lower tropospheric zonal wind over southeast Indian ocean, (v) upper tropospheric zonal wind over the east equatorial Indian Ocean, and (vi) Outgoing Longwave Radiation (OLR) over the south-west Pacific region.
IMD’s operational forecasts of the date of monsoon onset during the past 14 years (2005-2018) were proved to be correct except in 2015.
“Conditions are becoming favourable for the advance of southwest monsoon over the southern part of Andaman Sea, Nicobar Islands and adjoining the southeast Bay of Bengal during May 18-19th 2019,” said the IMD press release.
The Skymet had said on Tuesday that the monsoon would hit the Kerala coast on June 4, with an error margin of two days.
If the monsoon arrives late, it will be third such instance since 2014 when it arrived on June 5, followed by June 6 in 2015 and June 8 in 2016. The delay in the arrival of monsoon may not necessarily have an impact on the overall rainfall. In 2018, it had hit Kerala on May 29, three days before the normal onset date. Yet, the country received ‘below-normal’ rainfall.
Similarly, in 2017, the monsoon arrived in Kerala on May 30, but the overall rainfall was 95 per cent of the long period average (LPA), which falls under the below normal category.
In its initial forecast released in April, the IMD had predicted a near-normal rainfall with an LPA of 96 per cent, which falls on the border of ‘below-normal’ and ‘normal’ rainfall category. On the other hand, the Skymet has predicted a ‘below-normal’ rainfall with an LPA of 93 per cent.
Prasanta Patnaik