Onset of Monsoon over Kerala

• Southwest monsoon has set in over Kerala, today, the 29^th May 2018.
• Today, the southwest monsoon has further advanced into remaining parts of southeast Arabian Sea, Comorin – Maldives area, entire Lakshadweep, most parts of Kerala, some parts of Tamil Nadu and some more parts of southwest, central and northeast Bay of Bengal.
• Thus the southwest monsoon has set in over Kerala, today, the 29^th May 2018, 3 days ahead of its normal date.
• The Northern Limit of Monsoon (NLM) passes through Lat. 12⁰N/ Long 60⁰E, Lat 12⁰N/ Long 65⁰E, Lat.12⁰N/ Long 70⁰E, Kannur, Coimbatore, Kodaikanal, Tuticorin, Lat. 09⁰N/ Long 80⁰E, Lat. 13⁰N/ Long 85⁰E, 18⁰N/ Long 90⁰E, Lat. 21⁰N/ Long 93⁰E. Figure-1 shows the Northern Limit of Monsoon (NLM) as on today.
• Prevailing meteorological conditions leading to onset of monsoon over Kerala.
• Widespread rainfall occurred over Kerala, during the past 3-4 days. The 14 rainfall monitoring stations for Monsoon onset over Kerala have reported more than 60% rainfall from 25^th (Fig-2).
• Westerly winds have strengthened in the lower levels (more than 30 knots) and deepened with Westerly/ West-southwesterly winds upto 600 hPa (approximately upto 4.5km) over the south Arabian Sea (equator to Lat. 10^oN and Long. 55^oE to 80^oE) from today morning.
• There is persistent convection (cloudiness indicated by Outgoing Long-wave Radiation values <200 Wm-1) from 23^rd The Satellite (INSAT-3D) derived Outgoing Long wave Radiation value in the box confined by Lat. 5-10^oN, Long. 70-80^oE is 187 W/m². (Fig-2). ü In addition to the above features, the following developments are also noticed:
• An east-west shear zone runs roughly along latitude 12⁰N (across south peninsula) at 3.1 km above mean sea level.
• A well marked low pressure area lies over southeast and adjoining eastcentral Arabian Sea off north Kerala- Karnataka coasts.
• Another well marked low pressure area lies over eastcentral and adjoining northeast Bay of Bengal. It is likely to concentrate into a Depression during next 12 hours.

Ø Further advance during next 48 hours

Conditions are favorable for further advance of Southwest Monsoon into some parts of central Arabian Sea, remaining parts of Kerala, some parts of coastal and south interior Karnataka, some more parts of eastcentral and northeast Bay of Bengal, and some parts of  northeastern states during next 48 hours.

Fig 1: Northern Limit of Monsoon 2018 as on 29^th

Fig.2: Parameters for declaring onset of monsoon over Kerala