New Delhi: The Southwest monsoon is expected to make an onset over Kerala around 31 May, the India Meteorological Department (IMD) announced Wednesday.
IMD director general M. Mohapatra said that this year, the onset of Southwest monsoon over Kerala is likely to be “near normal”.
According to past records maintained by the Met department, the usual onset date for the state is 1 June. “This year, the Southwest monsoon is likely to set in over Kerala on May 31, with a model error of +/- four days,” the Met department said in a statement.
The advancement of Southwest monsoon over India is marked by its onset over Kerala. It marks the beginning of the four-month season when the country receives nearly 80 percent of its annual rainfall.
It is significant not only for farmers to harvest their crops and prepare for the next crop cycle, but also for the general public who anticipate the arrival of the monsoon to get some relief from the scorching temperatures.
Over the last few years, the Southwest monsoon has arrived slightly late in Kerala. Apart from 2022 — when the onset happened 29 May — in all other years since 2019, the monsoon has set in over Kerala in the first week of June.
It is also important to note that monsoon onset does not mean the first shower of the season. Certain parameters are monitored by the IMD before the southwest monsoon onset is declared.
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What are the onset criteria?
There are six predictors in the models used by the Met department while forecasting the monsoon onset. These are minimum temperatures over northwest India, pre-monsoon rainfall peak over the southern Peninsula, outgoing longwave radiation (OLR) over the South China Sea, lower tropospheric zonal winds over the equatorial southeast Indian Ocean, outgoing longwave radiation over southwest the Pacific Ocean and the upper tropospheric zonal wind over equatorial northeast Indian Ocean.
Simply put, before announcing the onset, the Met officials analyse if consistent rainfall has been received over a specific geographical region. The intensity and wind speed during this period also need to be over a defined limit.
According to weather forecasters, the onset is declared if a significant region in Kerala and Lakshadweep records at least 2.5mm of rainfall for two consecutive days anytime after 10 May. While the minimum rainfall criteria need to be fulfilled, the depth of westerly winds also needs to be around 600 hectopascals.
The OLR, which is the energy generated from the Earth’s surface, also needs to be below 200wt/sqm over a certain region.
Once these conditions are attained, the Met department declares the onset of the Southwest monsoon over Kerala. In the next few weeks, the monsoon progresses northward and covers the rest of India between June and September.
“There is always an error margin of (+/-) four days in our forecasts. In the past few years, we have always been accurate with southwest monsoon forecasts, except in 2015,” the IMD chief said.
The Met department’s seasonal outlook, released in April, forecasts “normal” rain over most parts of the country.
(Edited by Tony Rai)
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