National weather agency, the India Meteorological Department (IMD), has forecast that southwest monsoon will arrive on the Kerala coast on June 6, “slightly delayed” than the normal onset date. But with a model error of four days, monsoon may hit the mainland any day between June 2 and June 10.
Monsoon performance during the past five years shows that a delay is not necessarily associated with deficient rains. The best rainfall year in the last five years was 2016, and though the monsoon arrived on June 8 that year, rains were 97 per cent of normal, though not well spread across regions.
IMD has been quite accurate in predicting the onset date. In the last 14 years, actual onset date was incorrect only in one instance.
Experts and scientists from the IMD said that a delayed monsoon arrival has no bearing on the advance of the southwest monsoon over the entire country or its regional spread. Progress of sowing or agricultural output cannot be predicted or surmised from onset date either.
Current thermal circulation and convective patterns over northern and northwest India are dominated by western disturbances among others. This condition is not conducive for formation of a pressure gradient pulls the winds flowing north of the equator to India from the southwest direction.
The met department has been issuing operational forecasts for the date of monsoon onset over Kerala from 2005. An indigenously developed statistical model with a model error of ± 4 days is used for the purpose.
The model uses six parameters to get the probable onset date. They are: minimum temperatures over northwest India, pre-monsoon rainfall peak over south peninsula, outgoing long wave radiation (OLR) over South China Sea, lower tropospheric zonal wind over southeast Indian ocean, upper tropospheric zonal wind over the east equatorial Indian Ocean, and OLR over the south-west Pacific region.
Delayed monsoon doesn’t mean deficient rains
|Year||Forecast Onset Date||Actual Onset Date||Seasonal rainfall as % of normal|
|2014||5th June||6th June||88|
|2015||30th May||5th June||86|
|2016||7th June||8th June||97|
|2017||30th May||30th May||95|
|2018||29th May||29th May||91|
|2019*||6th June||Yet to happen||96|
Note: * Forecast only