The IMD has been issuing operational forecasts for the date of monsoon onset over Kerala from 2005 onwards. It claimed that the operational forecasts of the date of monsoon onset over Kerala during the past 16 years (2005-2020) were proved to be correct except in 2015.
The IMD will come out with the monsoon probabilistic forecast for the month of June in the last week of this month.
In the Indian monsoon region, initial monsoon rains are experienced over south Andaman Sea and the monsoon winds then advance north-westwards across the Bay of Bengal. As per the new normal dates of monsoon onset/progress, the southwest monsoon advances over the Andaman Sea around May 22. This year, the monsoon advance over Andaman & Nicobar Islands is very likely around May 21.
“However, the past data suggest that there is no association of the date of monsoon advance over the Andaman Sea either with the date of monsoon onset over Kerala or with the seasonal monsoon rainfall over the country,” said the IMD.
The IMD had last month predicted overall ‘normal’ monsoon rainfall in the country during the June-September period. It had said the monsoon seasonal rainfall was likely to be 98% of the Long Period Average (LPA) with a model error of +/- 5%. The LPA of the season rainfall over the country as a whole for the period 1961-2010 is 88 cm. The rainfall to an extent of 96-104% of the LPA is considered ‘normal’.
The IMD had released new normal dates of onset and withdrawal of southwest monsoon based on recent data on April 15 last year. The normal dates of onset were revised based on data during 1961-2019 and normal dates of withdrawal were revised based on data during 1971-2019.
As per the new normal, the monsoon sets over Kerala around June 1, nearly same as the earlier normal date and covers the entire country one week before the previous normal. However, monsoon withdrawal from northwest India is delayed by more than 2 weeks compared to the earlier normal date (September 1).
Monsoon retreats from most parts of the country except south Peninsula and some parts of neighbouring central India by October 15, coinciding with the earlier normal, and subsequently northeast monsoon gets established over south Peninsula.