Real-Time Objective Extended-Range Prediction of the 2025 Monsoon Onset over Kerala

Abstract

Since the declaration of the Monsoon Onset over Kerala (MOK) involves subjective assessment by forecasters, an objective approach based on a dynamical model can provide forecasters with appropriate guidance to reduce errors in MOK declarations. In this study, four objective indices derived from the real-time Extended Range Forecast (ERF)—representing the strength and depth of westerlies over the Arabian Sea, rainfall over the Kerala coast, and the meridional pressure gradient along the west coast—are used to predict the actual onset date of the 2025 monsoon. The MOK is defined as the first day when, for five consecutive days: (i) rainfall over the Kerala coast exceeds 80 percent of its climatological mean, (ii) 850 hPa zonal wind over the Arabian Sea exceeds 70 percent of its mean, (iii) 600 hPa zonal wind becomes westerly, and (iv) the south–north pressure difference along the west coast exceeds 2 hPa in the ERF.

          Applying these criteria to the 2025 monsoon, the week-4 ERF forecast (valid for 22–28 May) based on the initial condition of 30 April indicated favourable circulation and rainfall, suggesting an early onset of the monsoon. Subsequent ERFs initialised on 7 May and 14 May also showed consistent signals of an early MOK during the same period. The objective method predicted onset dates of 22 May and 23 May, respectively, based on 7th and 14th May ERFs—closely matching the actual onset declared by IMD on 24 May 2025.