IMD declares Onset of Southwest Monsoon over Kerala
1. Southwest Monsoon has set in over Kerala today, the 1st June, 2020, coinciding with its normal date.
2. The Northern Limit of Monsoon (NLM) passes through Cannur, Coimbatore, Kanyakumari.
3. Meteorological conditions leading to onset of southwest monsoon over Kerala are:
a) Widespread rainfall occurred over Kerala, during past 2 days. More than 70% of the 14 rainfall monitoring stations for Monsoon onset over Kerala have reported rainfall during last 48 hours
b) Westerly winds have strengthened in the lower levels (wind speed up to 20 knots) and deepened up to 4.5 km over the south Arabian Sea.
c) There is persistent convection (cloudiness indicated by Outgoing Long wave Radiation values <200 Wm-2) from satellite imagery and coastal Doppler Weather Radars from 27th May, 2020 onwards. d) In addition to the above features, a low pressure area has formed over Southeast & adjoining East-central Arabian Sea and Lakshadweep area on 31st May, 2020 which intensified into a depression over eastcentral & adjoining southeast Arabian Sea today morning, the 1st June, 2020. e) Criterion for declaration of southwest monsoon over Kerala is given below: i) Rainfall: If after 10th May, 60% of the available 14 stations enlisted*, viz. Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Alapuzha, Kottayam, Kochi, Vellanikkara (Thrissur), Kozhikode, Thalassery, Kannur, Kudulu (Kasargod) and Mangaluru report rainfall of 2.5 mm or more for two consecutive days, the onset over Kerala be declared on the 2nd day, provided the following criteria are also in concurrence. ii) Wind field: Depth of westerlies should be maintained up to 600 hPa, in the box equator to Lat. 10ºN and Long. 55ºE to 80ºE. 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 source of data can be RSMC wind analysis/satellite derived winds. iii) Outgoing Long wave Radiation (OLR): INSAT derived OLR value should be below 200 wm-2 in the box confined by Lat. 5-10ºN and Long. 70-75ºE.
