Track Forecast Error Analysis for Landfall of Chanthu under Effects of a Northeast Cold Vortex in 2021

Typhoon Chanthu was an intense tropical cyclone during the 2021 western North Pacific typhoon season, undergoing rapid intensification from a tropical storm to a super typhoon within 24 h. After skirting Taiwan, China, and turning northward, a significant track forecast error was observed after Typhoon Chanthu passed near the eastern coast of Taiwan, China, without making landfall and turned northward. Most official forecasts and ensemble members predicted that Chanthu would make landfall in Shanghai. However, the typhoon remained offshore, lingered near Shanghai for several days before recurving northeastward toward Japan. Subsequent analyses reveal that a mid-level Northeast cold vortex merged with a trough near 150°E, which led to the intensification and southward extent of the trough. This synoptic feature remained quasi-stationary for several days, contributing to the forecast inaccuracy. Forecast models that failed to capture this interaction struggled to produce an accurate track forecast. Additionally, the subtropical high (SH) to the south of Chanthu played a critical role in steering its movement. Beyond environmental steering, the initial intensity and structure of the storm within the forecast models also had a significant influence on its subsequent evolution and interaction with surrounding environmental flow.