Exploring Hafs Forecast Challenges with Extreme Intensification and Weakening during 2023 Hurricanes Lee and Otis

Despite skillful composite statistics during the 2023 hurricane season, newly-operational HAFS faced notable forecast challenges, especially for Hurricane Lee in the North Atlantic and Hurricane Otis in the eastern North Pacific. Hurricane Lee was a unique tropical cyclone (TC) that experienced a period of rapid intensification (RI) followed immediately by rapid weakening (RW). The combination of 75 kt/24h of intensification followed by 45 kt/24h of weakening proved challenging for operational forecast models, including HAFS. In this study, we examine both operational and experimental forecasts using the HAFS-B configuration to examine the physical reasons for the abrupt changes in Lee’s intensity and also to better understand why this RI/RW combination was not well forecast in HAFS. As part of this evaluation, we examine HAFS forecasts conducted using different convective schemes, including the Tiedtke scheme and two versions of the Scale-Aware Simplified Arakawa-Schubert (sa-SAS) scheme, to see how the model physics affect both the inner-core structure as well as the large-scale environment surrounding the TC. Hurricane Otis was a forecast bust in the opposite direction of Lee: all forecast models, including operational and experimental versions of HAFS, mostly missed the extreme RI into a Category 5 hurricane prior to landfall near Acapulco, Mexico. We explore the degree to which this poorly-forecast RI was a consequence of issues with model resolution, initialization, and physics (including the convective scheme). The detailed examination of these two cases will help to inform future upgrades to HAFS to improve forecasts of both RI and RW.