NEW DELHI: The southwest (summer) monsoon is likely to set in over Kerala on May 26 with a model error of ±4 days, the India Meteorological Department (IMD) announced on Friday, forecasting an early arrival of the rainy season in India. Monsoon normally sets in over Kerala on June 1 with a standard deviation of about seven days.
Significance of Monsoon Onset
The advance of the southwest monsoon over the Indian mainland is marked by monsoon onset over Kerala, serving as an important indicator of the transition from the hot and dry season to the rainy season. Last year, monsoon arrived over Kerala eight days in advance on May 24. However, an early or late arrival of monsoon has no bearing on the overall quantitative or spatial rainfall during the four-month rainy season.
Below-Normal Rainfall Expected
The Met department has already predicted 'below normal' monsoon rainfall this year, amid the growing risk of El Niño—a climatic pattern associated with warming of the ocean surface in the central and eastern tropical Pacific Ocean. El Niño is invariably linked with depressed monsoon rainfall in South Asia.
Forecast Methodology
IMD has been issuing operational forecasts for the date of monsoon onset over Kerala since 2005. An indigenously developed state-of-the-art statistical model with a model error of ±4 days is used for this purpose. The six predictors employed in the model for onset forecast include: minimum temperatures over north-west India, pre-monsoon rainfall peak over south Peninsula, outgoing longwave radiation (OLR) over the South China Sea, lower tropospheric zonal wind over the equatorial southeast Indian Ocean, OLR over the southwest Pacific Ocean, and upper tropospheric zonal wind over the equatorial northeast Indian Ocean.
Track Record of Forecasts
The Met department claimed that its operational forecasts of the date of monsoon onset over Kerala during the past 21 years (2005-2025) were correct, except in 2015.
