Nowcast guidance of afternoon convection initiation for Taiwan

Focusing on afternoon thunderstorms in Taiwan during the warm season (May-October) under weak synoptic forcing, this study applied the TaiwanAuto-NowCaster (TANC) to produce 1-h likelihood nowcasts of afternoon convection initiation (ACI) using a fuzzy logic approach. The primary objective is to design more useful forecast products with uncertainty regions of predicted thunderstorms to provide nowcast guidance of ACI for forecasters. Four sensitivity tests on forecast performance were conducted to improve the usefulness of nowcasts for forecasters. The optimal likelihood threshold (Lt) for ACIs, which is the likelihood value that best corresponds to the observed ACIs, was determined to be 0.6. Because of the high uncertainty on the exact location or timing of ACIs in nowcasts, location displacement and temporal shifting of ACIs should be considered in operational applications. When a spatial window of 5 km and a temporal window of 18 min are applied, the TANC displays moderate accuracy and satisfactory discrimination with an acceptable degree of overforecasting. The non-parametric Mann-Whitney test indicated that the performance of the TANC substantially surpasses the competing Space and Time Multiscale Analysis System-Weather Research and Forecasting Model, which serves as a pertinent reference for short-range (0-6 h) forecasts at the Central Weather Bureau in Taiwan.