How the onset of the monsoon over Kerala is declared: fourteen stations and three criteria

Background

Kerala has been through a deficient southwest monsoon: India Meteorological Department data for the period from 1st June to 17th September showed the State with a 28 per cent rainfall deficit, Wayanad with the highest shortfall at 54 per cent and Idukki at 47 per cent. The season whose shortfall is counted that way has a precisely defined beginning. The India Meteorological Department's guidelines for the onset of monsoon over Kerala say that if after 10th May, 60% of the available 14 stations enlisted report rainfall of 2.5 mm or more for two consecutive days, the onset over Kerala is declared on the 2nd day, provided two further criteria are in concurrence. The enlisted stations are Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore. The second criterion is the wind field: over defined boxes north of the equator, the depth of westerlies should be maintained upto 600 hPa and the zonal wind speed should be of the order of 15 to 20 knots at 925 hPa. The third is Outgoing Longwave Radiation: the INSAT derived OLR value should be below 200 wm-2 over a defined box in the ocean west of Kerala. Southwest monsoon normally sets in over Kerala around 1st June, advances northwards usually in surges, and covers the entire country around 15th July.