Evaluation of operational extended range forecast of cyclogenesis over the north Indian Ocean

The performance of the operational extended range forecast issued by the India Meteorological Department in the nil, low, moderate and high categories of probability of cyclogenesis has been evaluated based on 868 forecasts issued every Thursday for week 1 and week 2 for the Arabian Sea (AS), Bay of Bengal (BoB) and north Indian Ocean (NIO) as a whole during April 2018 to December 2023. The forecast is biased towards under-warning for low and moderate categories over the NIO, BoB & AS and towards over-warning for high categories over NIO and BoB in week 1. It is biased towards over-warning for moderate & high categories and under-warning for low category forecast over NIO and BoB for week 2. It is biased towards under-warning for low and high categories and over-warning for moderate category forecasts over AS in week 2. The Brier score (Brier skill score) for week 1 and week 2 are 0.051 (48.7%) and 0.087 (8.6%) over NIO respectively. The association of Madden Julian Oscillation (MJO), equatorial Rossby wave (ERW) and Kelvin wave (KW) with genesis increases and that of low-frequency background waves (LW) and inter-tropical convergence zone (ITCZ) decreases with an increase in the intensity of storms from depression to very severe cyclonic storms (VSCS). About 100%, 92%, 92%, 92% and 100% of the cases of the genesis of VSCS & above category storms over the NIO are associated with stronger westerlies to the south, stronger easterlies to the north, convective phase of MJO, ERW and KW over the region of genesis.