The southwest monsoon arrived over the Andaman Sea on 16THMay, a week before the scheduled arrival. In the past 24hr, it advanced further over few more parts of South and East Central Bay of Bengal (BoB). Conditions are becoming favorable for its further advancement over south and central BoB. The monsoon stream is likely to reach Sri Lanka anytime soon.
The announcement of arrival of monsoon over Andaman Sea is more or less subjective. Broad band of zonal westerlies extending to a depth of about 600 hPa, streaming cross equatorial flow and rainfall coverage over wide area of the islands become the decisive factors. Declaration of monsoon over Kerala need to fulfill stipulated criteria, a judicious mix of westerly winds with requisite speed and depth, out going long wave radiations over a specific region of Arabian Sea and the minimum rainfall over 14 nominated observatories over Lakshadweep, Kerala and Karnataka.
If after 10May, 60% of the available 14 stations, viz Minicoy, Amini Divi, Trivandrum, 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 following criteria are also in concurrence :
Wind Field : Depth of westerly winds should be maintained up to 600 hPa (12,00 feet) in the box , equator to 10°N and 55°E -80°E.
Wind Speed : The zonal wind speed over the area bounded by 5°N - 10°N and 70°E - 80°E should be of the order of 15-20 knots at 925 hPa (3000 feet).
OLR : The out going long wave radiation value should be below 200 watt/ sq. meter in the box confined by 5°N -10°N and 70°E -75°E.
Monsoon has not arrived ever over Andaman Sea prior to 10thMay. Monsoon onset over mainland is generally not attempted before 15thMay. If at all conditions find favor, it might as well be a temporary surge which retreats to get back again in the next attempt. Arrival over Andaman Sea has got no connection with its likely arrival over mainland Kerala. Similarly, announcement over Kerala has got no bearing with its further advancement over other parts. Invariably, Intensity and speed of monsoon is not so smooth to find linear progression. Sometimes, it halts to take a long break and on some other occasions may arrive with military punctuality over many parts. Decoding monsoon has always remained a difficult task and it is not going to be any different, this season.