Southwest monsoon onset criteria are strictly objective. Quite a few times, it is challenging to achieve copybook alignment of all the stipulated conditions, more so for the weak monsoon current, like the one observed this season. The changes taking place are not stable across the range of parameters, so they are essential for the desired onset. Nevertheless, the process of alignment has commenced and is expected to conclude shortly for the entry of monsoon currents over mainland India.
Precisely, there are 4 meteorological conditions to be satisfied for the announcement of the onset of monsoon. The rainfall criteria remain the most visible manifestation of the arrival of the monsoon stream. Dense and sheet clouds have to necessarily precede over the stipulated region marked between 5°N-10°N and 70°E-75°E. The cloud cover over this region decides the prevalence of OLR ( outgoing long-wave radiations). Actually, the rainfall and OLR parameters are the byproduct of the rest and therefore mandatorily need to be met.
Rainfall: Out of the available 14 stations, viz Minicoy, Amini Devi, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Cochin, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu, and Mangalore, at least 60% of these should record rainfall of 2.5mm or more on 2 successive days. Subject to the other parameters being concurrent, the onset of monsoon may be declared on the 2nd day. On 05th June, only one location (Thrissur) recorded 2.5mm of rainfall and the remaining stations absolutely dry. Today morning, on 06th June, 7 out of 14 stations met the criteria. Therefore, the stipulated condition remains unsatisfied.
OLR: INSAT-derived OLR value should be below 200 wm~2 in the box confined by latitude 5°-10°N and 70°-75°E. The persistence of thick clouds spread over the equatorial region, particularly in this window will ensure the OLR value is satisfied. Currently, the OLR value has been maintained above the mark of 250 for the last many days.
A depression has formed in the Southeast Arabian Sea and is likely to intensify into a tropical storm shortly. This cyclone will move northward initially, keeping a safe distance from the coastline. Such powerful systems in the Arabian Sea around the time of onset do not augur well for streaming in of monsoon current forcefully.