The monsoon’s arrival over Kerala has been delayed to June 3, according to an update by the India Meteorological Department (IMD) on Sunday. The agency had last month forecast that the onset would be on May 31.
“As per the latest meteorological indications, the south-westerly winds could strengthen further gradually from 01st June, resulting in likely enhancement in rainfall activity over Kerala. Hence the monsoon onset over Kerala is likely to take place by 03rd June 2021,” the IMD statement noted.
However, the IMD had, until Saturday evening, maintained that the monsoon would keep its May 31 schedule. All monsoon onset forecasts have a built-in error window of four days, and so, June 3 still falls in this window.
“The monsoon winds haven’t picked up. We expected that in the aftermath of the cyclone [Yaas] there would be a pickup in windspeeds but that hasn’t happened. We expect the wind flows to strengthen in the next few days,” M. Mohapatra, Director-General, IMD, told The Hindu .
Private weather forecast agency, Skymet, however, said that the monsoon had arrived. This was because two of the three criteria — as defined by the IMD — had been met, said meteorologist Mahesh Palawat.
The criteria are rain-bearing westerlies being at a minimum depth and speed; atleast 60% of the available 14 stations in Kerala and coastal Karnataka, namely Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore reporting rainfall of 2.5 mm or more for two consecutive days after May 10; a certain degree of clouding, indicated by a parameter called ‘outgoing longwave radiation’ (OLR), being below 200 W/square metre.
Mr. Palawat said IMD’s own data indicated that except for the OLR, the other criteria were met. “This is certainly a weak onset and the OLR was actually below 200 two days ago but has now increased to 250. There is an element of subjectivity in arrival and even IMD has, on previous occasions, relied on the two out of three criteria to declare monsoon onset.”
Skymet added that conditions were favourable for further advancement of the monsoon to more parts of south Peninsula and east central Bay of Bengal.
Last year, the IMD announced a monsoon onset date over Kerala of June 1, whereas the monsoon actually arrived on June 5. There is no correlation between the date of onset of the monsoon and the actual quantum of rain that is received during these months. The IMD as well as Skymet have forecast normal monsoon from June-September this year.
To herald the onset, initial rains first occur over south Andaman Sea and the monsoon winds then advance across the Bay of Bengal.
Since 2005, the monsoon has arrived within the error margin of the IMD’s weather models, except in 2015.
