It is that time of the year when almost the entire Indian farming fraternity wait eagerly for the IMD announcement about Onset of Monsoon of Kerala. Traditionally June 1 is considered the Monsoon Onset. So what are the probabilities of Southwest Monsoon Kerala Onset happening on time?
IMD has a set of parameters that govern the onset announcement, while rains area major criteria, there are others like Wind etc that also decide whether Monsoon has actually arrived over the Indian Sub Continent. The parameters are listed in this link https://www.imd.gov.in/section/nhac/dynamic/MOK.htm.
Let us examine the criteria and see if they are expected to fulfill before June 1st for the Monsoon onset.
OLR – INSAT derived OLR value should be below 200 wm-2 in the box confined by Lat. 5-10ºN and Long. 70- 75ºE. This looks fairly done going by the OLR map of 27th May.
Winds at 925 hPa : The zonal wind speed over the area bounded by Lat. 5-10ºN, Long. 70-80ºE should be of the order of 15 – 20 Kts. at 925 hPa. The below image is of 4th June 2015. Even as on 4th June models are not expecting 925 hPa winds to be consistently above 15 – 20 knots.
Westerly Wind Depth upto 600 hPa : Depth of westerlies should be maintained upto 600 hPa, in the box equator to Lat. 10ºN and Long. 55 ºE to 80ºE. In the image below at 700 hPa levels the winds are not completely westerlies yet as of 4th June 2015,
Rainfall Criteria : Rainfall criteria which will be fulfilled when 60% of the 14 select stations over the entire state and Mangalore register 2.5 mm rains for two days. The Monsoon onset is declared on the second day of rains. As things stand the rainfall criteria is nowhere near fulfilled yet as well.
That is 3 out of 4 factors not in favor of a timely onset of Southwest Monsoon over Kerala. In all probability the declaration of onset would be delayed by may be upto a week.