IMD Criteria For On Set Of Southwest Monsoon Over Kerala
a) Rainfall
If after 10th May, 60% of the available 14 stations enlisted*, viz. Minicoy,
Amini,Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur,
Kozhikode, Thalassery,Kannur, Kasargode and Mangalore 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.
b) 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.
c). 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.
Further Advance of Monsoon over the Country
a) Further advance be declared based on the occurrence of rainfall over
parts/sectors of the sub-divisions and maintaining the spatial continuity of the
northern limit of monsoon, further advance be declared.
The following auxiliary features may also be looked into:
b) Along the west coast, position of maximum cloud zone, as inferred from the
satellite imageries may be taken into account.
c) The satellite water vapour imageries may be monitored to assess the extent
of moisture incursion.