Southwest monsoon has not kept its date over mainland Kerala, this season. It has overshot its scheduled date of 01st June and is lingering over the Bay of Bengal Islands. Earlier, it arrived just on time over the extreme southern tip of the Bay of Bengal covering Nancowry Island on the 19th of May 2023. It literally halted for the next 10 days and is now waiting on the outskirts of Maldives, Comorin and Sri Lanka. The onset of Monsoon has been delayed over the major island Port Blair( BoB) and the Island nation Sri Lanka by over one week. It is likely to reach Sri Lanka and Maldives in the next 48hr. The next stop over Kerala will happen only in the next week.
The onset of the Southwest Monsoon over Kerala has stringent criteria. It is a combination of winds, clouds and rainfall over specified areas over the equatorial Indian Ocean, Lakshadweep Islands, the state of Kerala and coastal Karnataka. The onset of Monsoon is not attempted before 10th May. Since 1971, the earliest onset was on 18th May 2004. Coincidentally, 2014 happened to be a drought year. The precise criterion for announcing onset is complex enough to find a copybook declaration.
Rainfall: If after 10th May, 60% of the available 14 stations, viz Minicoy, Amini Devi, Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore report rainfall of 2.5mm or more for 2 consecutive days, the onset over Kerala be declared on the 2nd day, provided the other criteria of winds and clouds also find concurrence.
Wind Field: The depth of westerly winds should be maintained up to 600 hectopascal (hPa) which is equivalent to a height of 12,000 feet, in the box between the equator to 10°N and longitude 55°-80°E. The zonal wind speed over the area bounded by 5°-10°N and 70°-80°E should be of the order of 15-20 Knots at 925 hPa (3000’).
Outgoing Long Wave Radiations (OLR): INSAT-derived OLR value should be below 200 wm~2 in the box confined by latitude 5°-10°N and longitude 70°-75°E.
The onset of monsoon over the various parts is depicted by the Northern Limit of Monsoon (NLM). The first burst of monsoon over the mainland mostly covers a major portion of Kerala, nearly half of Tamil Nadu, central & northeast BoB and stopping short of the northeastern states of Nagaland, Manipur, Mizoram and Tripura. As per normal dates, the monsoon reaches Mumbai and Kolkata between the 10th and 12th of June. The monsoon arrival date for Delhi has now been revised to 27th June from its earlier landing on 29th June.
