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In the first week of December 2017, a very severe cyclonic storm, namely "OCKHI", made its landfall over the western coastline of the Indian peninsula. In a climatological perspective, this was one of the very rarest cyclonic storms that developed over the Comorin Sea with rapid intensification from a deep depression into a cyclonic storm within 6 h. Here, we present a case study on the performance evaluation of a regional numerical weather prediction model, Consortium for Small-scale Modelling (COSMO) during the passage of this cyclonic storm from 29 November 2017 to 6 December 2017 over the Arabian Sea by comparing the model-simulated fields against concurrent observations from the India Meteorological Department and European Centre for Medium-Range Weather Forecasts—Interim Reanalysis, respectively. Results obtained from this case study indicate good credentials to the COSMO in capturing the progression of OCKHI from its genesis as a deep depression in the early hours [0230 Indian Standard Time (IST)] of 30 November 2017 to a very severe cyclonic storm in the afternoon (1430 IST) of 01 December 2017 with a lead time of about 18 h. However, the intensity of the storm simulated by COSMO in terms of wind speed magnitudes and convective rainfall was found to be low in magnitudes as against the observations. The mean deviation between the model-simulated and observed trajectory of the storm was about 74 km for a lead time of 24 h, whereas it was below 41 km for a lead time of 18 h. The progression of OCKHI and the prevailing meteorological conditions for its intensification and subsequent weakening are also discussed in this article. [ABSTRACT FROM AUTHOR]