The India Meteorological Department (IMD) declared that the monsoon had arrived over Kerala on Wednesday, a day later than its forecast date of June 7.
Though it has been raining heavily in Kerala for well over a week — prompting private weather forecasters such as Skymet, to declare that the monsoon set in on May 30 — these rains have only qualified as ‘pre monsoon’ showers.
The meteorological department has an elaborate set of criteria to be fulfilled before it declares the onset of the monsoon over Kerala.
After May 10, at least eight of 14 stations across Kerala and Karnataka have to register at least 2.5 mm of rain, and monsoon-bearing winds have to be at least of 30-40 kmph velocity and below 600 hPa (approximately upto 4.5km) over the south Arabian Sea.
According to a statement from the Indian Meteorological Department (IMD), the monsoon has further advanced into remaining parts of south Arabian Sea, Maldives, Comorin area, most parts of Kerala and Tamil Nadu, some parts of south interior Karnataka, remaining parts of south Bay of Bengal and some more parts of central Bay of Bengal. It is expected to cover the whole country by July 15.
The IMD has predicted monsoon rains to be 6 per cent more than the historical average of 89 cm and spread out quite evenly over the country. This would be the first time in at least five years that the IMD has predicted above-average monsoon rains since April and on the back of two years of consecutive droughts.
The IMD has an elaborate set of conditions to be fulfilled before the monsoon can be declared to have set in. All of these must be consecutively met for two days for the IMD to officially declare the monsoon.
A key parameter is the Outgoing Longwave Radiation — an indicator of convection — has to be below 200 W/m2.
“The Outgoing Longwave Radiation was the only value that was not being met for so far and used to vary,” D.S. Pai, chief forecaster, IMD, told The Hindu . “That has now been met, based on latest data available on June 7. Hence we declared it [arrival of the monsoon] today.”
A private forecaster, who didn’t want to be identified, said that IMD’s declaration was “merely technical” and that monsoon had already advanced near the coast of western Maharashtra.
