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New Delhi, June 1, 2020
The India Meteorological Department (IMD) said the southwest monsoon had set in over Kerala today, coinciding with its normal date of arrival over the Indian mainland.
The Northern Limit of Monsoon (NLM) passes through lat. 120N/ Long. 600E, Lat. 120N/Long. 700E, Cannur, Coimbatore, Kanyakumari, Lat. 80N/ Long. 860E, Lat. 110N/ Long. 900E, Lat. 140N/ Long. 930E and Lat. 160N/ Long. 950E, a press release from IMD said.
Giving details of the meteorological conditions leading to onset of southwest monsoon over Kerala, it said widespread rainfall had occurred over the state during the past two days. More than 70% of the 14 rainfall monitoring stations for monsoon onset over Kerala have reported rainfall during the last 48 hours.
The release said westerly winds had strengthened in the lower levels (wind speed upto 20 knots) and deepened upto 4.5 km over the south Arabian Sea.
There is persistent convection (cloudiness indicated by Outgoing Longwave Radiation values <200 Wm-2) from satellite imagery and coastal Doppler Weather Radars from May 27 onwards.
In addition to the above features, a low pressure area has formed over Southeast and adjoining Eastcentral Arabian Sea and Lakshadweep area on May 31, which intensified into a depression over eastcentral & adjoining southeast Arabian Sea today morning.
The declaration of the onset of the southwest monsoon over Kerala is made if, after May 10, 60 per cent of the available 14 stations -- 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 declaration is made on the second day, provided the following criteria are also in concurrence:
-- Wind field Depth of westerlies should be maintained upto 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.
-- Outgoing Longwave Radiation (OLR) INSAT derived OLR value should be below 200 wm-2 in the box confined by Lat. 5-10ºN and Long. 70-75ºE.
The IMD had, on May 15, said that the onset of the southwest monsoon over Kerala was likely to be slightly delayed till June 5 as compared to the normal date of onset of June 1.
The IMD prediction also gave a model error of plus or minus four days for the onset of the monsoon.
The advance of the southwest monsoon over the Indian mainland is marked by its onset over Kerala and is an important indicator characterizing the transition from hot and dry season to the rainy season. As the monsoon progresses northward, relief from scorching summer temperatures is experienced over the areas.
Southwest monsoon sets over Kerala on June 1 with a standard deviation of about 7 days.
In the Indian monsoon region, initial monsoon rains are experienced over South Andaman Sea and the monsoon winds then advance in north-westwards across the Bay of Bengal.
As per the new normal dates of monsoon onset/progress, the southwest monsoon advances over the Andaman Sea around May 22.
Past data suggest that there is no association of the date of monsoon advance over the Andaman Sea either with the date of monsoon onset over Kerala or with the seasonal monsoon rainfall over the country.