The onset of southwest monsoon over Kerala — which marks the beginning of monsoon season in India — is likely to be delayed by four days as forecast by the weather agency, Indian Meteorological Department (IMD). The southwest monsoon usually sets over Kerala by June 1, with a standard deviation of seven days.
“This year, the onset of southwest monsoon over Kerala this year is likely to be slightly delayed as compared to normal date of onset. The monsoon onset over Kerala this year is likely on June 5 with a model error of ± 4 days,” said IMD in its forecast for the 2020 southwest monsoon.
In 2019, the southwest monsoon was delayed by a week and hit the state on June 8, while in 2018, the monsoon began on May 29, three days earlier than the scheduled arrival. The delay in monsoon arrival in 2019 had led to a rainfall deficit of 38% in India in June.
The onset of southwest monsoon over Kerala is an important marker, which indicates the transition from hot season to rainy season.
From Kerala, the monsoon covers states like Tamil Nadu, Rayalaseema (Andhra Pradesh), parts of Karnataka, south Maharashtra and advances to northeastern states, including Tripura and Mizoram.
The SW monsoon starts retreating by the first week of September, marking the onset of the northeast monsoon. In 2019, like several years in the last decade, the SE monsoon withdrawal was delayed by two weeks.
The IMD has released new normal dates of the onset and withdrawal of southwest monsoon. The normal dates of onset are revised based on the new rainfall criteria for 1961-2019 and the withdrawal dates are based on operational data for 1971-2019.
According to IMD, the onset over Kerala by June 1 is nearly the same as the existing normal date. “Monsoon covers most parts of the country except extreme northwest India by end of June, close to the existing normal date. But covers the entire country by July 8, exactly one week before the existing normal date of July 15. Monsoon onset/progress over most parts of central India between the second week of June and early July in the new normal dates is delayed by 1-8 days, compared to existing normal dates,” said IMD’s Climate Research and Services (CRS) report.
Per the new normal, the SW monsoon will start withdrawing from northwest India around September 17, which is a delay of more than two weeks compared to the existing normal date, that is, September 1. “After October 15, the southwest monsoon retreats from the country and northeast monsoon gets established over the south Peninsula,” it read.