Kerala has been reporting heavy rainfall over the past few days, but the IMD maintains some criteria, all of which, when satisfactorily met, declare the monsoon onset.(Express file photo)
The India Meteorological Department (IMD) on Thursday announced the onset of the southwest monsoon over Kerala, marking the beginning of the four-month-long season.
The normal onset date is June 1, with a deviation of seven days.
The IMD has issued a ‘yellow’ alert warning of continuing heavy rainfall (70 – 110mm in 24 hours) for all districts of Kerala till June 7.
This year, the Met department has forecast below-normal rainfall over the country. Quantitatively, it is expected to be 90 per cent of the Long Period Average, which is 870 mm (1971-2020). This subdued rainfall forecast is mainly in view of the developing El Nino condition — a global, naturally occurring ocean-atmospheric phenomenon along the equatorial Pacific Ocean. El Nino, during which the ocean surface along the Pacific is warmer than usual, is known to suppress summer monsoon and drive extreme temperatures over India.
Kerala has been reporting heavy rainfall over the past few days, but the IMD maintains some criteria, all of which, when satisfactorily met, declare the monsoon onset. These criteria include (i) after May 10, if there is rainfall of 2.5 mm or more lasting two consecutive days recorded over stipulated 14 stations within Kerala, Lakshadweep and Karnataka; (ii) strengthening and deepening of the westerly winds to 15-20 knots and at least 600 hecta Pascal, respectively; and (iii) the Outgoing Long Range (OLR) value (or the heat) remains below 200 watts/sq metre over 5-10ºN latitude and 70-75ºE latitude area.
Once the southwest monsoon sets in over Kerala, it advances northwards in multiple surges depending on the monsoon wind pulses and other factors. Normally, the monsoon covers the entire country by mid-July, after which it enters active and break phases from time to time.
Curated by Aisha Patel
